X-ray structure of a protease-resistant mutant form of human galectin-8 with two carbohydrate recognition domains

CUREs for high-level Galectin-3 expression

Galectins are a large and diverse protein family defined by the presence of a carbohydrate recognition domain (CRD) that binds β-galactosides. They play important roles in early development, tissue regeneration, immune homeostasis, pathogen recognition, and cancer. In many cases, studies that examine galectin biology and the effect of manipulating galectins are aided by, or require the ability to express and purify, specific members of the galectin family. In many cases, E. coli is employed as a heterologous expression system, and galectin expression is induced with isopropyl β-galactoside (IPTG). Here, we show that galectin-3 recognizes IPTG with micromolar affinity and that as IPTG induces expression, newly synthesized galectin can bind and sequester cytosolic IPTG, potentially repressing further expression. To circumvent this putative inhibitory feedback loop, we utilized an autoinduction protocol that lacks IPTG, leading to significantly increased yields of galectin-3. Much of this work was done within the context of a course-based undergraduate research experience, indicating the ease and reproducibility of the resulting expression and purification protocols.

Glycomimetic inhibitors of tandem-repeat galectins: Simple and efficient

The binding of human galectins by glycomimetic inhibitors is a promising therapeutic approach. The structurally distinct group of tandem-repeat galectins has scarcely been studied so far, and there is hardly any knowledge on their ligand specificity or their inhibitory potential, particularly concerning non-natural carbohydrates. Here, we present the synthesis of a library of seven 3-O-disubstituted thiodigalactoside-derived glycomimetics and their affinity to two tandem-repeat galectins, Gal-8 and Gal-9. The straightforward synthesis of these glycomimetics involved dibutyltin oxide-catalyzed 3,3́-O-disubstitution of commercially available unprotected thiodigalactoside, and conjugation of various aryl substituents by copper-catalyzed Huisgen azide-alkyne cycloaddition (CuAAC). The inhibitory potential of the prepared glycomimetics for Gal-8 and Gal-9 was assessed, and compared with the established galectins Gal-1 and Gal-3. The introduction of C-3 substituents resulted in an over 40-fold increase in affinity compared with unmodified TDG. The structure-affinity relations within the studied series were discussed using molecular modeling. Furthermore, the prepared glycomimetics were shown to scavenge Gal-8 and Gal-9 from the surface of cancer cells. This pioneering study on the synthetic inhibitors especially of Gal-9 identified lead compounds that may be used in further biomedical research.

Discovery of N-Arylsulfonyl-Indole-2-Carboxamide Derivatives as Galectin-3 and Galectin-8 C-Terminal Domain Inhibitors

Both galectin-3 and galectin-8 are involved in cell adhesion, migration, apoptosis, angiogenesis, and inflammatory processes by recognizing galactose-containing glycoproteins. Inhibiting galectin-3/8 activities is a potential treatment for cancer and tissue fibrosis. Herein, a series of novel N-arylsulfonyl-5-aryloxy-indole-2-carboxamide derivatives was disclosed as dual inhibitors toward galectin-3 and galectin-8 C-terminal domain with Kd values of low micromolar level (Cpd53, gal-3: Kd= 4.12 μM, gal-8C: Kd= 6.04 μM; Cpd57, gal-3: Kd= 12.8 μM, gal-8C: Kd= 2.06 μM), which are the most potent and selective noncarbohydrate-based inhibitors toward gal-3/8 isoforms to date. The molecular docking investigations suggested that the unique amino acids Arg144 in galectin-3 and Ser213 in galectin-8C could contribute to their potency and selectivity. The scratch wound assay demonstrated that Cpd53 and Cpd57 were able to inhibit the MRC-5 lung fibroblast cells migration as well. This class of inhibitors could serve as a new starting point for further discovering structurally distinct gal-3 and gal-8C inhibitors to be used in cancer and tissue fibrosis treatment.

Molecular characterization and expression profiling of tandem-repeat galectin-8 from red-spotted grouper (Epinephelus akaara): Potential antibacterial, antiviral, and wound healing activities

Galectin-8 is a typical β-galactoside binding lectin, which primarily functions as a pattern recognition receptor and/or danger receptor that is engaged in pathogen recognition by the host innate immune system. Although several fish galectins have been identified, the role of galectin-8 in teleost immunity is still not fully understood. In this study, molecular, transcriptional, and immune-related functions of galectin-8 (EaGal8) from red-spotted grouper (Epinephelus akaara) were analyzed. The open reading frame of EaGal8 comprised 960 bp encoding 319 amino acids of a ∼35 kDa protein, composed of the N- and C-terminal carbohydrate recognition domains joined by a short hinge peptide. Phylogenetic analysis revealed that EaGal8 was closely related to the Epinephelus lanceolatus galectin-8-like protein. Although EaGal8 showed ubiquitous tissue expression, the highest expression level was observed in the blood. Immunostimulants, including lipopolysaccharide, poly(I:C), and nervous necrosis virus, significantly upregulated the EaGal8 transcription level in a time-dependent manner (p < 0.05). Furthermore, recombinant EaGal8 (rEaGal8) showed a binding affinity toward seven different carbohydrates in a concentration-dependent manner. In addition, rEaGal8 caused strong agglutination of fish red blood cells and several gram-positive and gram-negative bacteria, including Streptococcus iniae, Streptococcus parauberis, Lactococcus garvieae, Escherichia coli, Edwardsiella tarda, Vibrio alginolyticus, Vibrio parahaemolyticus, and Pseudomonas aeruginosa. For the first time in teleosts, we report the wound healing ability of galectin-8 in this study. At low concentrations, rEaGal8 showed potential wound healing responses in FHM cells, in vitro. Thus, this study reinforces the role of EaGal8 in innate immune responses against bacterial and viral infections and wound healing in red-spotted grouper.

Selective Galectin-8N Ligands: The Design and Synthesis of Phthalazinone-d-Galactals

Ligand selectivity among the highly conserved galectins has been an ever-challenging objective. For galectin-8, a protein prevalent in both pathology and tissue distribution, we report phthalazinone-galactals that show excellent selectivity for the galectin-8N-terminal domain. A dissection of structure-activity relationships of the phthalazinone and an extensive molecular dynamics meta-analysis accompany the discovery of the selective galectin-8N ligands presented here. These selective compounds will facilitate the study of galectin-8 biology and may have pharmaceutical relevance in the wide range of galectin-8 associated pathologies.

Selective Monovalent Galectin-8 Ligands Based on 3-Lactoylgalactoside

Galectin-8 has gained attention as a potential new pharmacological target for the treatment of various diseases, including cancer, inflammation, and disorders associated with bone mass reduction. To that end, new molecular probes are needed in order to better understand its role and its functions. Herein we aimed to improve the affinity and target selectivity of a recently published galectin-8 ligand, 3-O-[1-carboxyethyl]-β-d-galactopyranoside, by introducing modifications at positions 1 and 3 of the galactose. Affinity data measured by fluorescence polarization show that the most potent compound reached a K_D of 12 μM. Furthermore, reasonable selectivity versus other galectins was achieved, making the highlighted compound a promising lead for the development of new selective and potent ligands for galectin-8 as molecular probes to examine the protein's role in cell-based and in vivo studies.

The two domains of human galectin-8 bind sialyl- and fucose-containing oligosaccharides in an independent manner. A 3D view by using NMR

The interaction of human galectin-8 and its two separate N-terminal and C-terminal carbohydrate recognition domains (CRD) to their natural ligands has been analysed using a synergistic combination of experimental NMR and ITC methods, and molecular dynamics simulations. Both domains bind the minimal epitopes N-acetyllactosamine (1) and Galβ1–3GalNAc (2) in a similar manner. However, the N-terminal and C-terminal domains show exquisite and opposing specificity to bind either Neu5Ac- or Fuc-containing ligands, respectively. Moreover, the addition of the high-affinity ligands specific for one of the CRDs does not make any effect on the binding at the alternative one. Thus, the two CRDs behave independently and may simultaneously target different molecular entities to promote clustering through the generation of supramolecular assemblies.

NMR, ITC, and MD data show that the two domains of human galectin-8 independently recognize sialyl- and fucosyl-containing glycans.

Full-length galectin-8 and separate carbohydrate recognition domains: the whole is greater than the sum of its parts?

Galectin-8 (Gal-8) is a tandem-repeat type galectin with affinity for β-galactosides, bearing two carbohydrate recognition domains (CRD) connected by a linker peptide. The N- and C-terminal domains (Gal-8N and Gal-8C) share 35% homology, and their glycan ligand specificity is notably dissimilar: while Gal-8N shows strong affinity for α(2-3)-sialylated oligosaccharides, Gal-8C has higher affinity for non-sialylated oligosaccharides, including poly-N-acetyllactosamine and/ or A and B blood group structures. Particularly relevant for understanding the biological role of this lectin, full-length Gal-8 can bind cell surface glycoconjugates with broader affinity than the isolated Gal-8N and Gal-8C domains, a trait also described for other tandem-repeat galectins. Herein, we aim to discuss the potential use of separate CRDs in modelling tandem-repeat galectin-8 and its biological functions. For this purpose, we will cover several aspects of the structure-function relationship of this protein including crystallographic structures, glycan specificity, cell function and biological roles, with the ultimate goal of understanding the potential role of each CRD in predicting full-length Gal-8 involvement in relevant biological processes.

Galectin-8 binds to the Farnesylated C-terminus of K-Ras4B and Modifies Ras/ERK Signaling and Migration in Pancreatic and Lung Carcinoma Cells

K-Ras is the most prominent driver of oncogenesis and no effective K-Ras inhibitors have been established despite decades of intensive research. Identifying new K-Ras-binding proteins and their interaction domains offers the opportunity for defining new approaches in tackling oncogenic K-Ras. We have identified Galectin-8 as a novel, direct binding protein for K-Ras4B by mass spectrometry analyses and protein interaction studies. Galectin-8 is a tandem-repeat Galectin and it is widely expressed in lung and pancreatic carcinoma cells. siRNA-mediated depletion of Galectin-8 resulted in increased K-Ras4B content and ERK1/2 activity in lung and pancreatic carcinoma cells. Moreover, cell migration and cell proliferation were inhibited by the depletion of Galectin-8. The K-Ras4B–Galectin-8 interaction is indispensably associated with the farnesylation of K-Ras4B. The lysine-rich polybasic domain (PBD), a region that is unique for K-Ras4B as compared to H- and N-Ras, stabilizes the interaction and accounts for the specificity. Binding assays with the deletion mutants of Galectin-8, comprising either of the two carbohydrate recognition domains (CRD), revealed that K-Ras4B only interacts with the N-CRD, but not with the C-CRD. Structural modeling uncovers a potential binding pocket for the hydrophobic farnesyl chain of K-Ras4B and a cluster of negatively charged amino acids for interaction with the positively charged lysine residues in the N-CRD. Our results demonstrate that Galectin-8 is a new binding partner for K-Ras4B and it interacts via the N-CRD with the farnesylated PBD of K-Ras, thereby modulating the K-Ras effector pathways as well as cell proliferation and migration.

Molecular, transcriptional and functional delineation of Galectin-8 from black rockfish (Sebastes schlegelii) and its potential immunological role

Galectins are β-galactoside-binding lectins, which are involved in pattern recognition, cell adhesion, and stimulation of the host innate immune responses against microbial pathogens. In spite of several functional studies on different galectins isolated from vertebrates and invertebrates, this is the first report to present functional studies for galectin-8 from the marine teleost tissues. In the present study, we characterized galectin-8 homolog from black rockfish (Sebastes schlegelii), in molecular and functional aspects. Rockfish galectin-8 (SsGal8) was found to consist of a 969 bp long open reading frame (ORF), encoding a protein of 322 amino acids and the predicted molecular weight was 35.82 kDa. In silico analysis of SsGal8 revealed the presence of two carbohydrate binding domains (CRDs), at both N and C-termini and a linker peptide of 40 amino acids, in between the two domains. As expected, the phylogenetic tree categorized SsGal8 as a tandem-repeat galectin, and ultimately positioned it in the sub-clade of fish galectin-8. rSsGal8 was able to strongly agglutinate fish erythrocytes and the inhibition of agglutination was successfully exhibited by lactose and d-galactose. Bacterial agglutination assay resulted in agglutination of both Gram (+) and Gram (-) bacteria, including Escherichia coli, Vibrio harveyi, Vibrio parahaemolyticus, Streptococcus parauberis, Lactococcus garvieae, Streptococcus iniae and Vibrio tapetis. The tissue distribution analysis based on qPCR assays, revealed a ubiquitous tissue expression of SsGal8 for the examined rockfish tissues, with the most pronounced expression in blood, followed by brain, intestine, head kidney and kidney. Furthermore, the mRNA transcription level of SsGal8 was significantly up-regulated in spleen, liver and head kidney, upon immune challenges with Streptococcus iniae, LPS and poly I:C, in a time dependent manner. Taken together, these findings strongly suggest the contribution of SsGal8 in regulating innate immune responses to protect the rockfish from bacterial infections.

The Impact of Natural Selection on the Evolution and Function of Placentally Expressed Galectins

Immunity genes have repeatedly experienced natural selection during mammalian evolution. Galectins are carbohydrate-binding proteins that regulate diverse immune responses, including maternal-fetal immune tolerance in placental pregnancy. Seven human galectins, four conserved across vertebrates and three specific to primates, are involved in placental development. To comprehensively study the molecular evolution of these galectins, both across mammals and within humans, we conducted a series of between- and within-species evolutionary analyses. By examining patterns of sequence evolution between species, we found that primate-specific galectins showed uniformly high substitution rates, whereas two of the four other galectins experienced accelerated evolution in primates. By examining human population genomic variation, we found that galectin genes and variants, including variants previously linked to immune diseases, showed signatures of recent positive selection in specific human populations. By examining one nonsynonymous variant in Galectin-8 previously associated with autoimmune diseases, we further discovered that it is tightly linked to three other nonsynonymous variants; surprisingly, the global frequency of this four-variant haplotype is ∼50%. To begin understanding the impact of this major haplotype on Galectin-8 protein structure, we modeled its 3D protein structure and found that it differed substantially from the reference protein structure. These results suggest that placentally expressed galectins experienced both ancient and more recent selection in a lineage- and population-specific manner. Furthermore, our discovery that the major Galectin-8 haplotype is structurally distinct from and more commonly found than the reference haplotype illustrates the significance of understanding the evolutionary processes that sculpted variants associated with human genetic disease.

Dual knockdown of Galectin-8 and its glycosylated ligand, the activated leukocyte cell adhesion molecule (ALCAM/CD166), synergistically delays in vivo breast cancer growth

Galectin-8 (Gal-8), a 'tandem-repeat'-type galectin, has been described as a modulator of cellular functions including adhesion, spreading, growth arrest, apoptosis, pathogen recognition, autophagy, and immunomodulation. We have previously shown that activated leukocyte cell adhesion molecule (ALCAM), also known as CD166, serves as a receptor for endogenous Gal-8. ALCAM is a member of the immunoglobulin superfamily involved in cell-cell adhesion through homophilic (ALCAM-ALCAM) and heterophilic (i.e. ALCAM-CD6) interactions in different tissues. Here we investigated the physiologic relevance of ALCAM-Gal-8 association and glycosylation-dependent mechanisms governing these interactions. We found that silencing of ALCAM in MDA-MB-231 triple negative breast cancer cells decreases cell adhesion and migration onto Gal-8-coated surfaces in a glycan-dependent fashion. Remarkably, either Gal-8 or ALCAM silencing also disrupted cell-cell adhesion, and led to reduced tumor growth in a murine model of triple negative breast cancer. Moreover, structural characterization of endogenous ALCAM N-glycosylation showed abundant permissive structures for Gal-8 binding. Importantly, we also found that cell sialylation controls Gal-8-mediated cell adhesion. Altogether, these findings demonstrate a central role of either ALCAM or Gal-8 (or both) in controlling triple negative breast cancer.

Sialic acid as a target for the development of novel antiangiogenic strategies

Sialic acid is associated with glycoproteins and gangliosides of eukaryotic cells. It regulates various molecular interactions, being implicated in inflammation and cancer, where its expression is regulated by sialyltransferases and sialidases. Angiogenesis, the formation of new capillaries, takes place during inflammation and cancer, and represents the outcome of several interactions occurring at the endothelial surface among angiogenic growth factors, inhibitors, receptors, gangliosides and cell-adhesion molecules. Here, we elaborate on the evidences that many structures involved in angiogenesis are sialylated and that their interactions depend on sialic acid with implications in angiogenesis itself, inflammation and cancer. We also discuss the possibility to exploit sialic acid as a target for the development of novel antiangiogenic drugs.

A Brief History of Charcot-Leyden Crystal Protein/Galectin-10 Research

Eosinophils are present in tissues, such as the respiratory tract, spleen, lymph nodes and blood vessels. The significant presence of eosinophils in these tissues are associated with various diseases, including asthma, allergies, acute myeloid leukemia, etc. Charcot-Leyden crystal protein/galectin-10 is overexpressed in eosinophils and has also been identified in basophils and macrophages. In human body, this protein could spontaneously form Charcot-Leyden crystal in lymphocytes or in the lysates of lymphocytes. At present, the role of Charcot-Leyden crystal protein/galectin-10 in lymphocytes is not fully understood. This review summarizes research progress on Charcot-Leyden crystal protein/galectin-10, with emphasis on its history, cellular distributions, relations to diseases, structures and ligand binding specificity.

Role of N-glycosylation in activation of proMMP-9. A molecular dynamics simulations study

Human matrix metalloproteinase proMMP-9 is secreted as latent zymogen, which requires two-steps proteolytic activation. The secreted proMMP-9 is glycosylated at two positions: Asn38 and Asn120 located in the prodomain and catalytic domain, respectively. It has been demonstrated that glycosylation at Asn120 is required for secretion of the enzyme, while the role of Asn38 glycosylation is not well understood, but is usually linked to the activation process. One hypothesis stated that the Asn38 glycosylation might protect against proteolytic activation. However, the activation process occurs with or without the presence of this glycosylation. We conducted molecular dynamics (MD) simulations on the glycosylated and non-glycosylated proMMP-9 to elucidate the effect of Asn38 glycosylation on this two-step activation process. The simulation results suggest that Asn38 glycosylation does not hinder the activation process directly, but induces conformational changes in the vicinity of the first proteolytic region in such a way that E59-M60 cleavage is processed before R106-F107. These results correlate with analysis provided by Boon et al. and experimental data from Ogata et al. who attempted to determine the order of events in activation of proMMP-9. Results from additional MD simulations for the model of glycosylated proMMP-9 bound to galectin-8 N-domain suggest that Gal-8 by interacting with Asn38 glycan might further facilitate processing of the first cleavage between E59-M60. Thus, our simulation results suggest that both Asn38 glycosylation and interaction with Gal-8N may be involved in facilitating and the temporal order of the activation process of pro-MMP9. The aim of this report is to provide an inspiration for future detailed experiments aimed at explaining the role of N-glycosylation in the activation process of prodomain of MMP-9.

The evolutionary origin of digit patterning

The evolution of tetrapod limbs from paired fins has long been of interest to both evolutionary and developmental biologists. Several recent investigative tracks have converged to restructure hypotheses in this area. First, there is now general agreement that the limb skeleton is patterned by one or more Turing-type reaction–diffusion, or reaction–diffusion–adhesion, mechanism that involves the dynamical breaking of spatial symmetry. Second, experimental studies in finned vertebrates, such as catshark and zebrafish, have disclosed unexpected correspondence between the development of digits and the development of both the endoskeleton and the dermal skeleton of fins. Finally, detailed mathematical models in conjunction with analyses of the evolution of putative Turing system components have permitted formulation of scenarios for the stepwise evolutionary origin of patterning networks in the tetrapod limb. The confluence of experimental and biological physics approaches in conjunction with deepening understanding of the developmental genetics of paired fins and limbs has moved the field closer to understanding the fin-to-limb transition. We indicate challenges posed by still unresolved issues of novelty, homology, and the relation between cell differentiation and pattern formation.

Studying the Structural Significance of Galectin Design by Playing a Modular Puzzle: Homodimer Generation from Human Tandem-Repeat-Type (Heterodimeric) Galectin-8 by Domain Shuffling

Tissue lectins are emerging (patho)physiological effectors with broad significance. The capacity of adhesion/growth-regulatory galectins to form functional complexes with distinct cellular glycoconjugates is based on molecular selection of matching partners. Engineering of variants by changing the topological display of carbohydrate recognition domains (CRDs) provides tools to understand the inherent specificity of the functional pairing. We here illustrate its practical implementation in the case of human tandem-repeat-type galectin-8 (Gal-8). It is termed Gal-8 (NC) due to presence of two different CRDs at the N- and C-terminal positions. Gal-8N exhibits exceptionally high affinity for 3'-sialylated/sulfated β-galactosides. This protein is turned into a new homodimer, i.e., Gal-8 (NN), by engineering. The product maintained activity for lactose-inhibitable binding of glycans and glycoproteins. Preferential association with 3'-sialylated/sulfated (and 6-sulfated) β-galactosides was seen by glycan-array analysis when compared to the wild-type protein, which also strongly bound to ABH-type epitopes. Agglutination of erythrocytes documented functional bivalency. This result substantiates the potential for comparative functional studies between the variant and natural Gal-8 (NC)/Gal-8N.

X-ray structure of a protease-resistant mutant form of human galectin-9 having two carbohydrate recognition domains with a metal-binding site

Galectin-9 (G9) is a tandem-repeat type β-galactoside-specific animal lectin having N-terminal and C-terminal carbohydrate recognition domains (N-CRD and C-CRD, respectively) joined by a linker peptide that is involved in the immune system. G9 is divalent in glycan binding, and structural information about the spatial arrangement of the two CRDs is very important for elucidating its biological functions. As G9 is protease sensitive due to the long linker, the protease-resistant mutant form of G9 (G9Null) was developed by modification of the linker peptide, while retaining its biological functions. The X-ray structure of a mutant form of G9Null with the replacement of Arg221 by Ser (G9Null_R221S) having two CRDs was determined. The structure of G9Null_R221S was compact to associate the two CRDs in the back-to-back orientation with a large interface area, including hydrogen bonds and hydrophobic interactions. A metal ion was newly found in the galectin structure, possibly contributing to the stable structure of protein. The presented X-ray structure was thought to be one of the stable structures of G9, which likely occurs in solution. This was supported by structural comparisons with other tandem-repeated galectins and the analyses of protein thermostability by CD spectra measurements.

Teaming up synthetic chemistry and histochemistry for activity screening in galectin-directed inhibitor design

A hallmark of endogenous lectins is their ability to select a few distinct glycoconjugates as counterreceptors for functional pairing from the natural abundance of cellular glycoproteins and glycolipids. As a consequence, assays to assess inhibition of lectin binding should necessarily come as close as possible to the physiological situation, to characterize an impact of a synthetic compound on biorelevant binding with pharmaceutical perspective. We here introduce in a proof-of-principle manner work with sections of paraffin-embedded tissue (jejunum, epididymis) and labeled adhesion/growth-regulatory galectins, harboring one (galectin-1 and galectin-3) or two (galectin-8) types of lectin domain. Six pairs of synthetic lactosides from tailoring of the headgroup (3'-O-sulfation) and the aglycone (β-methyl to aromatic S- and O-linked extensions) as well as three bi- to tetravalent glycoclusters were used as test compounds. Varying extents of reduction in staining intensity by synthetic compounds relative to unsubstituted/free lactose proved the applicability and sensitivity of the method. Flanking cytofluorimetric assays on lectin binding to native cells gave similar grading, excluding a major impact of tissue fixation. The experiments revealed cell/tissue binding of galectin-8 preferentially via one domain, depending on the cell type so that the effect of an inhibitor in a certain context cannot be extrapolated to other cells/tissues. Moreover, the work with the other galectins attests that this assay enables comprehensive analysis of the galectin network in serial tissue sections to determine overlaps and regional differences in inhibitory profiles.

Full-length model of the human galectin-4 and insights into dynamics of inter-domain communication

Galectins are proteins involved in diverse cellular contexts due to their capacity to decipher and respond to the information encoded by β-galactoside sugars. In particular, human galectin-4, normally expressed in the healthy gastrointestinal tract, displays differential expression in cancerous tissues and is considered a potential drug target for liver and lung cancer. Galectin-4 is a tandem-repeat galectin characterized by two carbohydrate recognition domains connected by a linker-peptide. Despite their relevance to cell function and pathogenesis, structural characterization of full-length tandem-repeat galectins has remained elusive. Here, we investigate galectin-4 using X-ray crystallography, small- and wide-angle X-ray scattering, molecular modelling, molecular dynamics simulations, and differential scanning fluorimetry assays and describe for the first time a structural model for human galectin-4. Our results provide insight into the structural role of the linker-peptide and shed light on the dynamic characteristics of the mechanism of carbohydrate recognition among tandem-repeat galectins.

Deep phylogenomics of a tandem-repeat galectin regulating appendicular skeletal pattern formation

BACKGROUND

A multiscale network of two galectins Galectin-1 (Gal-1) and Galectin-8 (Gal-8) patterns the avian limb skeleton. Among vertebrates with paired appendages, chondrichthyan fins typically have one or more cartilage plates and many repeating parallel endoskeletal elements, actinopterygian fins have more varied patterns of nodules, bars and plates, while tetrapod limbs exhibit tandem arrays of few, proximodistally increasing numbers of elements. We applied a comparative genomic and protein evolution approach to understand the origin of the galectin patterning network. Having previously observed a phylogenetic constraint on Gal-1 structure across vertebrates, we asked whether evolutionary changes of Gal-8 could have critically contributed to the origin of the tetrapod pattern.

RESULTS

Translocations, duplications, and losses of Gal-8 genes in Actinopterygii established them in different genomic locations from those that the Sarcopterygii (including the tetrapods) share with chondrichthyans. The sarcopterygian Gal-8 genes acquired a potentially regulatory non-coding motif and underwent purifying selection. The actinopterygian Gal-8 genes, in contrast, did not acquire the non-coding motif and underwent positive selection.

CONCLUSION

These observations interpreted through the lens of a reaction-diffusion-adhesion model based on avian experimental findings can account for the distinct endoskeletal patterns of cartilaginous, ray-finned, and lobe-finned fishes, and the stereotypical limb skeletons of tetrapods.

Characterization of a double-CRD-mutated Gal-8 recombinant protein that retains co-stimulatory activity on antigen-specific T-cell response

Galectins (Gals) constitute a family of mammalian lectins with affinity for β-galactosides, characterized by the presence of conserved CRDs (carbohydrate-recognition domains). We have found previously that Gal-8, from the tandem-repeat group with two linked CRDs, exerts two separate actions on CD4(+)T-cells: antigen-independent proliferation and, at lower concentration, antigen-specific co-stimulation. Whereas proliferation can be ascribed to the pro-inflammatory role of Gal-8, the co-stimulatory activity of borderline T-cell-specific responses allows the proposal of Gal-8 as an adjuvant in vaccination. To study the relevance of glycan-lectin interaction to these T-cell activities, we generated a double-mutated protein (Gal-8mut) by replacing canonical arginine residues on each CRD, so as to abolish sugar-binding capacity. As expected, Gal-8mut was unable to bind to lactosyl-Sepharose, confirming that lactose recognition was precluded; however, preservation of lectin activity was still evident since Gal-8mut displayed haemoagglutinatory effects and binding capacity to the T-cell surface. To search for glycan affinity, a glycan microarray analysis was conducted which revealed that Gal-8mut lost most low- and intermediate-, but retained high-, affinity interactions, mainly to polylactosamines and blood group antigens. These findings were supported further by molecular modelling. Regarding biological activity, Gal-8mut was unable to induce T-cell proliferation, but efficiently co-stimulated antigen-specific responses, bothin vitroandin vivo.Therefore Gal-8mut represents a useful tool to dissect the specificities of lectin-glycan interactions underlying distinctive Gal-8 activities on T-cell biology. Moreover, given its distinguishing properties, Gal-8mut could be used to enhance borderline immune responses without the non-specific pro-inflammatory activity or other potential adverse effects.

Heterologous expression of newly identified galectin-8 from sea urchin embryos produces recombinant protein with lactose binding specificity and anti-adhesive activity

Galectin family members specifically bind beta-galactoside derivatives and are involved in different cellular events, including cell communication, signalling, apoptosis, and immune responses. Here, we report a tandem-repeat type galectin from the Paracentrotus lividus sea urchin embryo, referred to as Pl-GAL-8. The 933nt sequence encodes a protein of 34.73 kDa, containing the conserved HFNPRF and WGxExR motifs in the two highly similar carbohydrate-recognition domains (CRD). The three-dimensional protein structure model of the N-CRD confirms the high evolutionary conservation of carbohydrate binding sites. The temporal gene expression is regulated during development and transcripts localize at the tip of the archenteron at gastrula stage, in a subset of the secondary mesenchyme cells that differentiate into blastocoelar (immune) cells. Functional studies using a recombinant Pl-GAL-8 expressed in bacteria demonstrate its hemo-agglutinating activity on human red blood cells through the binding to lactose, as well as its ability in inhibiting the adhesion of human Hep-G2 cells to the substrate. The recent implications in autoimmune diseases and inflammatory disorders make Gal-8 an attractive candidate for therapeutic purposes. Our results offer a solid basis for addressing the use of the new Pl-GAL-8 in functional and applicative studies, respectively in the developmental and biomedical fields.

NMR assignments of the C-terminal domain of human galectin-8

Galectins recognize β-galectosides to promote a variety of cellular functions. Despite their sequence variations, all galectins share the same carbohydrate recognition domains (CRD) and their modes of ligand recognition at a structural level are essentially identical. Human galectin 8 plays an important role in numerous cancer and immune responses. It consists of two CRDs that are connected via a flexible linker. The substrate affinities and specificities of the N- and C-terminal domains are quite different. In order to investigate the structural basis of their substrate specificities, we complete the NMR (1)H, (13)C, and (15)N chemical shift assignments of C-terminal domain of human galectin-8 (hG8C).

Structural significance of galectin design: impairment of homodimer stability by linker insertion and partial reversion by ligand presence

Lectins translate information encoded in glycan chains of cellular glycoconjugates into bioeffects. The topological presentation of contact sites for cognate sugar binding is a crucial factor toward this end. To dissect the significance of such phylogenetically conserved properties, the design and engineering of non-natural variants are attractive approaches. Here, a homodimeric human lectin, i.e. adhesion/growth-regulatory galectin-1, is converted into a tandem-repeat display by introducing the 33-amino-acid linker of another family member (i.e. galectin-8). The yield of variant was reduced by about a third. This protein had ∼10-fold higher activity in hemagglutination. Nearly complete sequence determination by mass-spectrometric in-source decay and fingerprinting excluded the presence of any modifications. When (1)H-(15)N heteronuclear single-quantum coherence data on the (15)N-labeled variant and wild-type protein were compared, changes in chemical shifts, signal intensities and resonance multiplicities revealed reduction of stability of interfacial contacts between the lectin domains and an increase in inter-domain flexibility. When both binding sites in the variant were loaded with ligand, association of the two carbohydrate recognition domains was enhanced, corroborated by gel filtration. Dynamic changes in the spatial presentation of the two lectin domains in the context of a tandem-repeat display can alter counterreceptor targeting relative to the fixed positions found in the proto-type galectin homodimer.

Galectin-8: a matricellular lectin with key roles in angiogenesis

Galectin-8 (gal-8) is a "tandem-repeat"-type galectin, containing two carbohydrate recognition domains connected by a linker peptide. gal-8 is expressed both in the cytoplasm and nucleus in vascular endothelial cells (ECs) from normal and tumor-associated blood vessels, and in lymphatic endothelial cells. Herein, we describe a novel role for gal-8 in the regulation of vascular and lymphatic angiogenesis and provide evidence of its critical implications in tumor biology. Functional assays revealed central roles for gal-8 in the control of capillary-tube formation, EC migration and in vivo angiogenesis. So far, two endothelial ligands have been described for gal-8, namely podoplanin in lymphatic vessels and CD166 (ALCAM, activated leukocyte cell adhesion molecule) in vascular ECs. Other related gal-8 functions are also summarized here, including cell adhesion and migration, which collectively demonstrate the multi-functionality of this complex lectin. Thus, gal-8 is an important component of the angiogenesis network, and an essential molecule in the extracellular matrix by providing molecular anchoring to this surrounding matrix. The implications of gal-8 in tumor angiogenesis remain to be further explored, but it is exciting to speculate that modulating gal-8-glycan interactions could be used to block lymphatic-vascular connections vital for metastasis.

Three-dimensional structural aspects of protein-polysaccharide interactions

Linear polysaccharides are typically composed of repeating mono- or disaccharide units and are ubiquitous among living organisms. Polysaccharide diversity arises from chain-length variation, branching, and additional modifications. Structural diversity is associated with various physiological functions, which are often regulated by cognate polysaccharide-binding proteins. Proteins that interact with linear polysaccharides have been identified or developed, such as galectins and polysaccharide-specific antibodies, respectively. Currently, data is accumulating on the three-dimensional structure of polysaccharide-binding proteins. These proteins are classified into two types: exo-type and endo-type. The former group specifically interacts with the terminal units of polysaccharides, whereas the latter with internal units. In this review, we describe the structural aspects of exo-type and endo-type protein-polysaccharide interactions. Further, we discuss the structural basis for affinity and specificity enhancement in the face of inherently weak binding interactions.

Structural basis for recognition of autophagic receptor NDP52 by the sugar receptor galectin-8

Infectious bacteria are cleared from mammalian cells by host autophagy in combination with other upstream cellular components, such as the autophagic receptor NDP52 and sugar receptor galectin-8. However, the detailed molecular basis of the interaction between these two receptors remains to be elucidated. Here, we report the biochemical characterization of both NDP52 and galectin-8 as well as the crystal structure of galectin-8 complexed with an NDP52 peptide. The unexpected observation of nicotinamide adenine dinucleotide located at the carbohydrate-binding site expands our knowledge of the sugar-binding specificity of galectin-8. The NDP52-galectin-8 complex structure explains the key determinants for recognition on both receptors and defines a special orientation of N- and C-terminal carbohydrate recognition domains of galectin-8. Dimeric NDP52 forms a ternary complex with two monomeric galectin-8 molecules as well as two LC3C molecules. These results lay the groundwork for understanding how host cells target bacterial pathogens for autophagy.

Understanding the specificity of human Galectin-8C domain interactions with its glycan ligands based on molecular dynamics simulations

Human Galectin-8 (Gal-8) is a member of the galectin family which shares an affinity for β-galactosides. The tandem-repeat Gal-8 consists of a N- and a C-terminal carbohydrate recognition domain (N- and C-CRD) joined by a linker peptide of various length. Despite their structural similarity both CRDs recognize different oligosaccharides. While the molecular requirements of the N-CRD for high binding affinity to sulfated and sialylated glycans have recently been elucidated by crystallographic studies of complexes with several oligosaccharides, the binding specificities of the C-CRD for a different set of oligosaccharides, as derived from experimental data, has only been explained in terms of the three-dimensional structure for the complex C-CRD with lactose. In this study we performed molecular dynamics (MD) simulations using the recently released crystal structure of the Gal-8C-CRD to analyse the three-dimensional conditions for its specific binding to a variety of oligosaccharides as previously defined by glycan-microarray analysis. The terminal β-galactose of disaccharides (LacNAc, lacto-N-biose and lactose) and the internal β-galactose moiety of blood group antigens A and B (BGA, BGB) as well as of longer linear oligosaccharide chains (di-LacNAc and lacto-N-neotetraose) are interacting favorably with conserved amino acids (H53, R57, N66, W73, E76). Lacto-N-neotetraose and di-LacNAc as well as BGA and BGB are well accommodated. BGA and BGB showed higher affinity than LacNAc and lactose due to generally stronger hydrogen bond interactions and water mediated hydrogen bonds with α1-2 fucose respectively. Our results derived from molecular dynamics simulations are able to explain the glycan binding specificities of the Gal-8C-CRD in comparison to those of the Gal-8N -CRD.