The role of galectins in protein trafficking

Hyperactivation of Nrf2 in early tubular development induces nephrogenic diabetes insipidus

NF-E2-related factor-2 (Nrf2) regulates cellular responses to oxidative and electrophilic stress. Loss of Keap1 increases Nrf2 protein levels, and Keap1-null mice die of oesophageal hyperkeratosis because of Nrf2 hyperactivation. Here we show that deletion of oesophageal Nrf2 in Keap1-null mice allows survival until adulthood, but the animals develop polyuria with low osmolality and bilateral hydronephrosis. This phenotype is caused by defects in water reabsorption that are the result of reduced aquaporin 2 levels in the kidney. Renal tubular deletion of Keap1 promotes nephrogenic diabetes insipidus features, confirming that Nrf2 activation in developing tubular cells causes a water reabsorption defect. These findings suggest that Nrf2 activity should be tightly controlled during development in order to maintain renal homeostasis. In addition, tissue-specific ablation of Nrf2 in Keap1-null mice might create useful animal models to uncover novel physiological functions of Nrf2.

Perdeuteration, crystallization, data collection and comparison of five neutron diffraction data sets of complexes of human galectin-3C

Galectin-3 is an important protein in molecular signalling events involving carbohydrate recognition, and an understanding of the hydrogen-bonding patterns in the carbohydrate-binding site of its C-terminal domain (galectin-3C) is important for the development of new potent inhibitors. The authors are studying these patterns using neutron crystallography. Here, the production of perdeuterated human galectin-3C and successive improvement in crystal size by the development of a crystal-growth protocol involving feeding of the crystallization drops are described. The larger crystals resulted in improved data quality and reduced data-collection times. Furthermore, protocols for complete removal of the lactose that is necessary for the production of large crystals of apo galectin-3C suitable for neutron diffraction are described. Five data sets have been collected at three different neutron sources from galectin-3C crystals of various volumes. It was possible to merge two of these to generate an almost complete neutron data set for the galectin-3C-lactose complex. These data sets provide insights into the crystal volumes and data-collection times necessary for the same system at sources with different technologies and data-collection strategies, and these insights are applicable to other systems.

SPRED1 Interferes with K-ras but Not H-ras Membrane Anchorage and Signaling

The Ras/mitogen-activated protein kinase (MAPK) signaling pathway is tightly controlled by negative feedback regulators, such as the tumor suppressor SPRED1. The SPRED1 gene also carries loss-of-function mutations in the RASopathy Legius syndrome. Growth factor stimulation translocates SPRED1 to the plasma membrane, triggering its inhibitory activity. However, it remains unclear whether SPRED1 there acts at the level of Ras or Raf. We show that pharmacological or galectin-1 (Gal-1)-mediated induction of B- and C-Raf-containing dimers translocates SPRED1 to the plasma membrane. This is facilitated in particular by SPRED1 interaction with B-Raf and, via its N terminus, with Gal-1. The physiological significance of these novel interactions is supported by two Legius syndrome-associated mutations that show diminished binding to both Gal-1 and B-Raf. On the plasma membrane, SPRED1 becomes enriched in acidic membrane domains to specifically perturb membrane organization and extracellular signal-regulated kinase (ERK) signaling of active K-ras4B (here, K-ras) but not H-ras. However, SPRED1 also blocks on the nanoscale the positive effects of Gal-1 on H-ras. Therefore, a combinatorial expression of SPRED1 and Gal-1 potentially regulates specific patterns of K-ras- and H-ras-dependent signaling output. More broadly, our results open up the possibility that related SPRED and Sprouty proteins act in a similar Ras and Raf isoform-specific manner.

Galectin-1 circumvents lysolecithin-induced demyelination through the modulation of microglial polarization/phagocytosis and oligodendroglial differentiation

Galectin-1 (Gal-1), a member of a highly conserved family of animal lectins, binds to the common disaccharide [Galβ(1-4)-GlcNAc] on both N- and O-glycans decorating cell surface glycoconjugates. Current evidence supports a role for Gal-1 in the pathophysiology of multiple sclerosis (MS), one of the most prevalent chronic inflammatory diseases. Previous studies showed that Gal-1 exerts neuroprotective effects by promoting microglial deactivation in a model of autoimmune neuroinflammation and induces axonal regeneration in spinal cord injury. Seeking a model that could link demyelination, oligodendrocyte (OLG) responses and microglial activation, here we used a lysolecithin (LPC)-induced demyelination model to evaluate the ability of Gal-1 to preserve myelin without taking part in T-cell modulation. Gal-1 treatment after LPC-induced demyelination promoted a significant decrease in the demyelinated area and fostered more efficient remyelination, concomitantly with an attenuated oligodendroglial progenitor response reflecting less severe myelination damage. These results were accompanied by a decrease in the area of microglial activation with a shift toward an M2-polarized microglial phenotype and diminished astroglial activation. In vitro studies further showed that, mechanistically, Gal-1 targets activated microglia, promoting an increase in their myelin phagocytic capacity and their shift toward an M2 phenotype, and leads to oligodendroglial differentiation. Therefore, this study supports the use of Gal-1 as a potential treatment for demyelinating diseases such as MS.

Galectin-3 interacts with components of the nuclear ribonucleoprotein complex

Background

The multifunctional β-galactoside-binding protein galectin-3 is found in many distinct subcellular compartments including the cell nucleus. Expression and distribution of galectin-3 between the cell nucleus and the cytosol changes during cell differentiation and cancer development. Nuclear functions of galectin-3 and how they contribute to tumorigenesis are not understood.

Methods

In order to identify nuclear galectin-3 interaction partners, we used affinity chromatography and co-immunoprecipitation. Spatial proximity in the nucleus was assessed by immunofluorescence and proximity ligation assay. We also investigated the function of galectin-3 on mRNA-export by fluorescence in situ hybridization and on mRNA-processing by RNA-sequencing.

Results

The heterogeneous ribonucleoprotein particle component hnRNPA2B1 was identified as a novel galectin-3 binding protein that associates with the lectin in a lactose-dependent manner in the cell nucleus. Specific individual depletion of galectin-3 does not affect the mRNA distribution between cytoplasm and nucleus. A significant alteration of this distribution was observed after combined depletion of galectin-1 and −3. However, silencing of galectin-3 was sufficient to alter the splicing patterns of several genes.

Conclusions

Galectin-3 and hnRNPA2B1 interact as members of the early splicing machinery. Galectin-3 and −1 have redundant functions in mRNA transport and at least in part in mRNA splicing. RNA-sequencing data points to a specific function of the hnRNPA2B1/galectin-3 interaction in the processing of transcripts coding for the nuclear oncoprotein SET.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2546-0) contains supplementary material, which is available to authorized users.

Iterative sorting of apical and basolateral cargo in Madin-Darby canine kidney cells

A novel assay quantitatively distinguishes different cargo pairs by their degree of colocalization at the TGN and the evolution of colocalization during their TGN-to-surface transport. Apical NTRp75 and basolateral VSVG in MDCK cells undergo continuous sorting between TGN exit and surface arrival.

For several decades, the trans-Golgi network (TGN) was considered the most distal stop and hence the ultimate protein-sorting station for distinct apical and basolateral transport carriers that reach their respective surface domains in the direct trafficking pathway. However, recent reports of apical and basolateral cargoes traversing post-Golgi compartments accessible to endocytic ligands before their arrival at the cell surface and the post-TGN breakup of large pleomorphic membrane fragments that exit the Golgi region toward the surface raised the possibility that compartments distal to the TGN mediate or contribute to biosynthetic sorting. Here we describe the development of a novel assay that quantitatively distinguishes different cargo pairs by their degree of colocalization at the TGN and by the evolution of colocalization during their TGN-to-surface transport. Keys to the high resolution of our approach are 1) conversion of perinuclear organelle clustering into a two-dimensional microsomal spread and 2) identification of TGN and post-TGN cargo without the need for a TGN marker that universally cosegregates with all cargo. Using our assay, we provide the first evidence that apical NTRp75 and basolateral VSVG in Madin–Darby canine kidney cells still undergo progressive sorting after they exit the TGN toward the cell surface.

Identification of VPS13C as a Galectin-12-Binding Protein That Regulates Galectin-12 Protein Stability and Adipogenesis

Galectin-12, a member of the galectin family of β-galactoside-binding animal lectins, is preferentially expressed in adipocytes and required for adipocyte differentiation in vitro. This protein was recently found to regulate lipolysis, whole body adiposity, and glucose homeostasis in vivo. Here we identify VPS13C, a member of the VPS13 family of vacuolar protein sorting-associated proteins highly conserved throughout eukaryotic evolution, as a major galectin-12-binding protein. VPS13C is upregulated during adipocyte differentiation, and is required for galectin-12 protein stability. Knockdown of Vps13c markedly reduces the steady-state levels of galectin-12 by promoting its degradation through primarily the lysosomal pathway, and impairs adipocyte differentiation. Our studies also suggest that VPS13C may have a broader role in protein quality control. The regulation of galectin-12 stability by VPS13C could potentially be exploited for therapeutic intervention of obesity and related metabolic diseases.

Structural characterisation of human galectin-4 N-terminal carbohydrate recognition domain in complex with glycerol, lactose, 3'-sulfo-lactose, and 2'-fucosyllactose

Galectin-4 is a tandem-repeat galectin with two distinct carbohydrate recognition domains (CRD). Galectin-4 is expressed mainly in the alimentary tract and is proposed to function as a lipid raft and adherens junction stabilizer by its glycan cross-linking capacity. Galectin-4 plays divergent roles in cancer and inflammatory conditions, either promoting or inhibiting each disease progression, depending on the specific pathological condition. The study of galectin-4's ligand-binding profile may help decipher its roles under specific conditions. Here we present the X-ray structures of human galectin-4 N-terminal CRD (galectin-4N) bound to different saccharide ligands. Galectin-4's overall fold and its core interactions to lactose are similar to other galectin CRDs. Galectin-4N recognises the sulfate cap of 3'-sulfated glycans by a weak interaction through Arg45 and two water-mediated hydrogen bonds via Trp84 and Asn49. When galectin-4N interacts with the H-antigen mimic, 2'-fucosyllactose, an interaction is formed between the ring oxygen of fucose and Arg45. The extended binding site of galectin-4N may not be well suited to the A/B-antigen determinants, α-GalNAc/α-Gal, specifically due to clashes with residue Phe47. Overall, galectin-4N favours sulfated glycans whilst galectin-4C prefers blood group determinants. However, the two CRDs of galectin-4 can, to a less extent, recognise each other's ligands.

Immunohistochemical Studies on Galectin Expression in Colectomised Patients with Ulcerative Colitis

Introduction. The aetiology and pathogenesis of ulcerative colitis (UC) are essentially unknown. Galectins are carbohydrate-binding lectins involved in a large number of physiological and pathophysiological processes. Little is known about the role of galectins in human UC. In this immunohistochemical exploratory study, both epithelial and inflammatory cell galectin expression were studied in patients with a thoroughly documented clinical history and were correlated with inflammatory activity. Material and Methods. Surgical whole intestinal wall colon specimens from UC patients (n = 22) and controls (n = 10) were studied. Clinical history, pharmacological treatment, and modified Mayo-score were recorded. Tissue inflammation was graded, and sections were stained with antibodies recognizing galectin-1, galectin-2, galectin-3, and galectin-4. Results. Galectin-1 was undetectable in normal and UC colonic epithelium, while galectin-2, galectin-3, and galectin-4 were strongly expressed. A tendency towards diminished epithelial expression with increased inflammatory grade for galectin-2, galectin-3, and galectin-4 was also found. In the inflammatory cells, a strong expression of galectin-2 and a weak expression of galectin-3 were seen. No clear-cut correlation between epithelial galectin expression and severity of the disease was found. Conclusion. Galectin expression in patients with UC seems to be more dependent on disease focality and individual variation than on degree of tissue inflammation.

Galactosylated Polymer Nano-objects by Polymerization-Induced Self-Assembly, Potential Drug Nanocarriers

Glycopolymer-based nanostructures are invaluable tools to both study biological phenomena and to design future targeted drug delivery systems. Polymerization-induced self-assembly, especially RAFT aqueous dispersion polymerization is a unique method to prepare such polymer nanostructures, as it enables the preparation of very-well-defined morphologies at very high concentrations. Here we describe the implementation of PISA to the synthesis of galactosylated spheres, wormlike micelles and vesicles, and the preliminary results of cell toxicity, cell uptake, and cargo delivering capacity of galactose-decorated vesicles.

The Diverse Forms of Lactose Intolerance and the Putative Linkage to Several Cancers

Lactase-phlorizin hydrolase (LPH) is a membrane glycoprotein and the only β-galactosidase of the brush border membrane of the intestinal epithelium. Besides active transcription, expression of the active LPH requires different maturation steps of the polypeptide through the secretory pathway, including N- and O-glycosylation, dimerization and proteolytic cleavage steps. The inability to digest lactose due to insufficient lactase activity results in gastrointestinal symptoms known as lactose intolerance. In this review, we will concentrate on the structural and functional features of LPH protein and summarize the cellular and molecular mechanism required for its maturation and trafficking. Then, different types of lactose intolerance are discussed, and the molecular aspects of lactase persistence/non-persistence phenotypes are investigated. Finally, we will review the literature focusing on the lactase persistence/non-persistence populations as a comparative model in order to determine the protective or adverse effects of milk and dairy foods on the incidence of colorectal, ovarian and prostate cancers.

Extracellular Vesicles from Ovarian Carcinoma Cells Display Specific Glycosignatures

Cells release vesicles to the extracellular environment with characteristic nucleic acid, protein, lipid, and glycan composition. Here we have isolated and characterized extracellular vesicles (EVs) and total cell membranes (MBs) from ovarian carcinoma OVMz cells. EVs were enriched in specific markers, including Tsg101, CD63, CD9, annexin-I, and MBs contained markers of cellular membrane compartments, including calnexin, GRASP65, GS28, LAMP-1, and L1CAM. The glycoprotein galectin-3 binding protein (LGALS3BP) was strongly enriched in EVs and it contained sialylated complex N-glycans. Lectin blotting with a panel of lectins showed that EVs had specific glycosignatures relative to MBs. Furthermore, the presence of glycoproteins bearing complex N-glycans with α2,3-linked sialic acid, fucose, bisecting-GlcNAc and LacdiNAc structures, and O-glycans with the T-antigen were detected. The inhibition of N-glycosylation processing from high mannose to complex glycans using kifunensine caused changes in the composition of EVs and induced a decrease of several glycoproteins. In conclusion, the results showed that glycosignatures of EVs were specific and altered glycosylation within the cell affected the composition and/or dynamics of EVs release. Furthermore, the identified glycosignatures of EVs could provide novel biomarkers for ovarian cancer.

[Galectins, a class of unconventional lectins]

Galectins constitute a family of soluble animal lectins defined by their evolutionary conserved carbohydrate recognition domain and their affinity for β-galactosides containing glycoconjugates. Each galectin is characterized by a specific spatio-temporal distribution and a unique set of ligands and molecular partners. Interestingly, galectins are found both extracellularly and intracellularly and modulate various cellular processes. Knock-out mutant mice for galectins-1, 3 or 7 are viable but display a wide range of defects under various stress conditions. Indeed, galectins are multifunctional proteins involved in cell-cell and cell-extracellular matrix interactions, organization of membrane domains, cell signalling and also in intracellular trafficking, apoptosis, regulation of cell cycle. Galectins represent potential therapeutic targets, especially in the context of cancer and inflammatory diseases.

Recycling of galectin-3 in epithelial cells

Galectins, a family of β-galactoside binding proteins, do not possess a signalling sequence to enter the endoplasmic reticulum as a starting point for the classical secretory pathway. They use a so-called unconventional secretion mechanism for translocation across the plasma membrane and/or into the lumen of transport vesicles. 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. The lectin re-enters the cell by non-clathrin mediated endocytosis and passages through endosomal organelles. This internalized galectin-3 plays an important role in apical protein trafficking by directing the subcellular targeting of apical glycoproteins via oligomerization into high molecular weight clusters, a process that can be fine-tuned by changes in the environmental pH. Following release at the apical plasma membrane, the lectin can reenter the cell for another round of recycling and apical protein sorting. This review will briefly address galectin-3-functions in epithelia and focus on distinct phases in apical recycling of the lectin.

Analysis of the intracellular role of galectins in cell growth and apoptosis

Galectins are a family of animal lectins with conserved carbohydrate-recognition domains that recognize β-galactosides. Despite structural similarities, these proteins have diverse functions in a variety of cellular processes. While a large number of extracellular functions have been demonstrated for galectins, the existence of intracellular functions has been clearly shown for a number of galectins, including regulation of cell growth and apoptosis; these latter functions may not involve glycan binding. There is considerable interest in intracellular regulation by galectins of cell growth and apoptosis, as these are fundamental cellular processes in normal homeostasis. Their dysregulation can cause pathologies such as autoimmune disorders, cancer, and neural degenerative diseases. Here we describe methods that we routinely perform in the laboratory to investigate the role of galectins in cell growth and apoptosis. These include methods for cell isolation, cell maintenance, and genetic manipulations to perturb galectin gene expression, as well as assays for cell growth and apoptosis.

Klotho up-regulates renal calcium channel transient receptor potential vanilloid 5 (TRPV5) by intra- and extracellular N-glycosylation-dependent mechanisms

The anti-aging protein Klotho is a type 1 membrane protein produced predominantly in the distal convoluted tubule. The ectodomain of Klotho is cleaved and secreted into the urine to regulate several ion channels and transporters. Secreted Klotho (sKL) up-regulates the TRPV5 calcium channel from the cell exterior by removing sialic acids from N-glycan of the channel and inhibiting its endocytosis. Because TRPV5 and Klotho coexpress in the distal convoluted tubule, we investigated whether Klotho regulates TRPV5 action from inside the cell. Whole-cell TRPV5-mediated channel activity was recorded in HEK cells coexpressing TRPV5 and sKL or membranous Klotho (mKL). Transfection of sKL, but not mKL, produced detectable Klotho protein in cell culture media. As for sKL, mKL increased TRPV5 current density. The role of sialidase activity of mKL acting inside is supported by findings that mutations of putative sialidase activity sites in sKL and mKL abrogated the regulation of TRPV5 but that the extracellular application of a sialidase inhibitor prevented the regulation of TRPV5 by sKL only. Mechanistically, coexpression with a dominant-negative dynamin II prevented the regulation of TRPV5 by sKL but not by mKL. In contrast, blocking forward trafficking by brefeldin A prevented the effect with mKL but not with sKL. Therefore, Klotho up-regulates TRPV5 from both the inside and outside of cells. The intracellular action of Klotho is likely due to enhanced forward trafficking of channel proteins, whereas the extracellular action is due to inhibition of endocytosis. Both effects involve putative Klotho sialidase activity. These effects of Klotho may play important roles regarding calcium reabsorption in the kidney.

Structural basis of multivalent galactose-based dendrimer recognition by human galectin-7

Galectins are evolutionarily conserved and ubiquitously present animal lectins with a high affinity for β-galactose-containing oligosaccharides. To date, 15 mammalian galectins have been identified. Their involvement in cell-cell and cell-matrix interactions has highlighted their importance in signal transduction and other intracellular processes. Human galectin-7 (hGal-7) is a 15 kDa proto type galectin that forms a dimer in solution and its involvement in the stimulation and development of tumour growth has been reported. Previously, we reported the crystal structure of hGal-7 and its complex with galactose and lactose which provided insight into its molecular recognition and detailed interactions. Here, we present newly obtained high-resolution structural data on carbohydrate-based dendrons in complex with hGal-7. Our crystallographic data reveal how multivalent ligands interact with and form cross-links with these galectin molecules. Understanding how these dendrimeric compounds interact with hGal-7 would help in the design of new tools to investigate the recognition of carbohydrates by lectins.

The role of Galectin-3 in α-synuclein-induced microglial activation

BACKGROUND

Parkinson's disease (PD) is the most prevalent neurodegenerative motor disorder. The neuropathology is characterized by intraneuronal protein aggregates of α-synuclein and progressive degeneration of dopaminergic neurons within the substantia nigra. Previous studies have shown that extracellular α-synuclein aggregates can activate microglial cells, induce inflammation and contribute to the neurodegenerative process in PD. However, the signaling pathways involved in α-synuclein-mediated microglia activation are poorly understood. Galectin-3 is a member of a carbohydrate-binding protein family involved in cell activation and inflammation. Therefore, we investigated whether galectin-3 is involved in the microglia activation triggered by α-synuclein.

RESULTS

We cultured microglial (BV2) cells and induced cell activation by addition of exogenous α-synuclein monomers or aggregates to the cell culture medium. This treatment induced a significant increase in the levels of proinflammatory mediators including the inducible Nitric Oxide Synthase (iNOS), interleukin 1 Beta (IL-1β) and Interleukin-12 (IL-12). We then reduced the levels of galectin-3 expression using siRNA or pharmacologically targeting galectin-3 activity using bis-(3-deoxy-3-(3-fluorophenyl-1H-1,2,3-triazol-1-yl)-β-D-galactopyranosyl)-sulfane. Both approaches led to a significant reduction in the observed inflammatory response induced by α-synuclein. We confirmed these findings using primary microglial cells obtained from wild-type and galectin-3 null mutant mice. Finally, we performed injections of α-synuclein in the olfactory bulb of wild type mice and observed that some of the α-synuclein was taken up by activated microglia that were immunopositive for galectin-3.

CONCLUSIONS

We show that α-synuclein aggregates induce microglial activation and demonstrate for the first time that galectin-3 plays a significant role in microglia activation induced by α-synuclein. These results suggest that genetic down-regulation or pharmacological inhibition of galectin-3 might constitute a novel therapeutic target in PD and other synucleinopathies.

Complex N-linked glycans serve as a determinant for exosome/microvesicle cargo recruitment

Exosomes, also known as microvesicles (EMVs), are nano-sized membranous particles secreted from nearly all mammalian cell types. These nanoparticles play critical roles in many physiological processes including cell-cell signaling, immune activation, and suppression and are associated with disease states such as tumor progression. The biological functions of EMVs are highly dependent on their protein composition, which can dictate pathogenicity. Although some mechanisms have been proposed for the regulation of EMV protein trafficking, little attention has been paid to N-linked glycosylation as a potential sorting signal. Previous work from our laboratory found a conserved glycan signature for EMVs, which differed from that of the parent cell membranes, suggesting a potential role for glycosylation in EMV biogenesis. In this study, we further explore the role of glycosylation in EMV protein trafficking. We identify EMV glycoproteins and demonstrate alteration of their recruitment as a function of their glycosylation status upon pharmacological manipulation. Furthermore, we show that genetic manipulation of the glycosylation levels of a specific EMV glycoprotein, EWI-2, directly impacts its recruitment as a function of N-linked glycan sites. Taken together, our data provide strong evidence that N-linked glycosylation directs glycoprotein sorting into EMVs.

Melanocytic galectin-3 is associated with tyrosinase-related protein-1 and pigment biosynthesis

Galectin-3 is a family member of the carbohydrate-binding proteins widely expressed by many cell types and exhibits multiple cellular functions. We demonstrate that melanocytes express galectin-3, which is predominantly localized to the cell body peripherally along the Golgi zone. Downregulation of galectin-3 in human melanocytes using short hairpin RNA technology resulted in the reduction of both melanin synthesis and expression/activity of tyrosinase-related protein-1 (Tyrp-1). In the cell body, galectin-3 colocalizes with melanosome-destined cargo, specifically tyrosinase and Tyrp-1. We studied melanocytes cultured from patients with forms of Hermansky-Pudlak syndrome (HPS) containing defects in trafficking steps governed by biogenesis of lysosome-related organelle complex-2 (BLOC-2) (HPS-5), BLOC-3 (HPS-1), and adaptin-3 (HPS-2). We found that galectin-3 expression mimicked the defective expression of the tyrosinase cargo in dendrites of HPS-5 melanocytes, but it was not altered in HPS-1 or HPS-2 melanocytes. In addition, galectin-3 colocalized predominantly with the HPS-5 component of BLOC-2 in normal human melanocytes. These data indicate that galectin-3 is a regulatory component in melanin synthesis affecting the expression of Tyrp-1.

Galectin-3 drives glycosphingolipid-dependent biogenesis of clathrin-independent carriers

Several cell surface molecules including signalling receptors are internalized by clathrin-independent endocytosis. How this process is initiated, how cargo proteins are sorted and membranes are bent remains unknown. Here, we found that a carbohydrate-binding protein, galectin-3 (Gal3), triggered the glycosphingolipid (GSL)-dependent biogenesis of a morphologically distinct class of endocytic structures, termed clathrin-independent carriers (CLICs). Super-resolution and reconstitution studies showed that Gal3 required GSLs for clustering and membrane bending. Gal3 interacted with a defined set of cargo proteins. Cellular uptake of the CLIC cargo CD44 was dependent on Gal3, GSLs and branched N-glycosylation. Endocytosis of β1-integrin was also reliant on Gal3. Analysis of different galectins revealed a distinct profile of cargoes and uptake structures, suggesting the existence of different CLIC populations. We conclude that Gal3 functionally integrates carbohydrate specificity on cargo proteins with the capacity of GSLs to drive clathrin-independent plasma membrane bending as a first step of CLIC biogenesis.

Galectins in epithelial functions

Galectins are a family of animal lectins comprising 15 members in vertebrates. These proteins are involved in many biological processes including epithelial homeostasis and tumor progression by displaying intracellular and extracellular activities. Hence Galectins can be found either in the cytoplasm or the nucleus, associated with membranes or in the extracellular matrix. Current studies aim at understanding the roles of Galectins in cell-cell and cell-matrix adhesion, cellular polarity and motility. This review discusses recent progress in defining the specificities and mechanisms of action of Galectins as cell regulators in epithelial cells. Physiological, cellular and molecular aspects of Galectin specificities will be treated successively.

Trafficking to the apical and basolateral membranes in polarized epithelial cells

Renal epithelial cells must maintain distinct protein compositions in their apical and basolateral membranes in order to perform their transport functions. The creation of these polarized protein distributions depends on sorting signals that designate the trafficking route and site of ultimate functional residence for each protein. Segregation of newly synthesized apical and basolateral proteins into distinct carrier vesicles can occur at the trans-Golgi network, recycling endosomes, or a growing assortment of stations along the cellular trafficking pathway. The nature of the specific sorting signal and the mechanism through which it is interpreted can influence the route a protein takes through the cell. Cell type-specific variations in the targeting motifs of a protein, as are evident for Na,K-ATPase, demonstrate a remarkable capacity to adapt sorting pathways to different developmental states or physiologic requirements. This review summarizes our current understanding of apical and basolateral trafficking routes in polarized epithelial cells.

Modulation of ionotropic glutamate receptor function by vertebrate galectins

AMPA and kainate receptors are glutamate-gated ion channels whose function is known to be altered by a variety of plant oligosaccharide-binding proteins, or lectins, but the physiological relevance of this activity has been uncertain because no lectins with analogous allosteric modulatory effects have been identified in animals. We report here that members of the prototype galectin family, which are β-galactoside-binding lectins, exhibit subunit-specific allosteric modulation of desensitization of recombinant homomeric and heteromeric AMPA and kainate receptors. Galectin modulation of GluK2 kainate receptors was dependent upon complex oligosaccharide processing of N-glycosylation sites in the amino-terminal domain and downstream linker region. The sensitivity of GluA4 AMPA receptors to human galectin-1 could be enhanced by supplementation of culture media with uridine and N-acetylglucosamine (GlcNAc), precursors for the hexosamine pathway that supplies UDP-GlcNAc for synthesis of complex oligosaccharides. Neuronal kainate receptors in dorsal root ganglia were sensitive to galectin modulation, whereas AMPA receptors in cultured hippocampal neurons were insensitive, which could be a reflection of differential N-glycan processing or receptor subunit selectivity. Because glycan content of integral proteins can be modified dynamically, we postulate that physiological or pathological conditions in the CNS could arise in which galectins alter excitatory neurotransmission or neuronal excitability through their actions on AMPA or kainate receptors.

Inhibitors of Galectins and Implications for Structure-Based Design of Galectin-Specific Therapeutics

Galectins are a family of galactoside-specific lectins that are involved in a myriad of metabolic and disease processes. Due to roles in cancer and inflammatory and heart diseases, galectins are attractive targets for drug development. Over the last two decades, various strategies have been used to inhibit galectins, including polysaccharide-based therapeutics, multivalent display of saccharides, peptides, peptidomimetics, and saccharide-modifications. Primarily due to galectin carbohydrate binding sites having high sequence identities, the design and development of selective inhibitors targeting particular galectins, thereby addressing specific disease states, is challenging. Furthermore, the use of different inhibition assays by research groups has hindered systematic assessment of the relative selectivity and affinity of inhibitors. This review summarises the status of current inhibitors, strategies, and novel scaffolds that exploit subtle differences in galectin structures that, in conjunction with increasing available data on multiple galectins, is enabling the feasible design of effective and specific inhibitors of galectins.

Ligand binding and complex formation of galectin-3 is modulated by pH variations

The observations of the present study provide new evidence for the idea that the formation and composition of galectin-3 networks can be fine-tuned by changes in the environmental pH value.

Polymerization-induced self-assembly of galactose-functionalized biocompatible diblock copolymers for intracellular delivery

Recent advances in polymer science are enabling substantial progress in nanobiotechnology, particularly in the design of new tools for enhanced understanding of cell biology and for smart drug delivery formulations. Herein, a range of novel galactosylated diblock copolymer nano-objects is prepared directly in concentrated aqueous solution via reversible addition-fragmentation chain transfer polymerization using polymerization-induced self-assembly. The resulting nanospheres, worm-like micelles, or vesicles interact in vitro with galectins as judged by a turbidity assay. In addition, galactosylated vesicles are highly biocompatible and allow intracellular delivery of an encapsulated molecular cargo.

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.

Expression and functions of galectin-7 in human and murine melanomas

The identification of galectin-7 as a p53-induced gene and its ability to induce apoptosis in many cell types support the hypothesis that galectin-7 has strong antitumor activity. This has been well documented in colon cancer. However, in some cases, such as breast cancer and lymphoma, its high expression level correlates with aggressive subtypes of cancer, suggesting that galectin-7 may have a dual role in cancer progression. In fact, in breast cancer, overexpression of galectin-7 alone is sufficient to promote metastasis to the bone and lung. In the present work, we investigated the expression and function of galectin-7 in melanoma. An analysis of datasets obtained from whole-genome profiling of human melanoma tissues revealed that galectin-7 mRNA was detected in more than 90% of biopsies of patients with nevi while its expression was more rarely found in biopsies collected from patients with malignant melanoma. This frequency, however, was likely due to the presence of normal epidermis tissues in biopsies, as shown our studies at the protein level by immunohistochemical analysis. Using the experimental melanoma B16F1 cell line, we found that melanoma cells can express galectin-7 at the primary tumor site and in lung metastasis. Moreover, we found that overexpression of galectin-7 increased the resistance of melanoma cells to apoptosis while inducing de novo egr-1 expression. Overexpression of galectin-7, however, was insufficient to modulate the growth of tumors induced by the subcutaneous injection of B16F1 cells. It also failed to modulate the dissemination of B16F1 cells to the lung.

Multiple motifs regulate apical sorting of p75 via a mechanism that involves dimerization and higher-order oligomerization

The sorting signals that direct proteins to the apical surface of polarized epithelial cells are complex and can include posttranslational modifications, such as N- and O-linked glycosylation. Efficient apical sorting of the neurotrophin receptor p75 is dependent on its O-glycosylated membrane proximal stalk, but how this domain mediates targeting is unknown. Protein oligomerization or clustering has been suggested as a common step in the segregation of all apical proteins. Like many apical proteins, p75 forms dimers, and we hypothesized that formation of higher-order clusters mediated by p75 dimerization and interactions of the stalk facilitate its apical sorting. Using fluorescence fluctuation techniques (photon-counting histogram and number and brightness analyses) to study p75 oligomerization status in vivo, we found that wild-type p75-green fluorescent protein forms clusters in the trans-Golgi network (TGN) but not at the plasma membrane. Disruption of either the dimerization motif or the stalk domain impaired both clustering and polarized delivery. Manipulation of O-glycan processing or depletion of multiple galectins expressed in Madin-Darby canine kidney cells had no effect on p75 sorting, suggesting that the stalk domain functions as a structural prop to position other determinants in the lumenal domain of p75 for oligomerization. Additionally, a p75 mutant with intact dimerization and stalk motifs but with a dominant basolateral sorting determinant (Δ250 mutant) did not form oligomers, consistent with a requirement for clustering in apical sorting. Artificially enhancing dimerization restored clustering to the Δ250 mutant but was insufficient to reroute this mutant to the apical surface. Together these studies demonstrate that clustering in the TGN is required for normal biosynthetic apical sorting of p75 but is not by itself sufficient to reroute a protein to the apical surface in the presence of a strong basolateral sorting determinant. Our studies shed new light on the hierarchy of polarized sorting signals and on the mechanisms by which newly synthesized proteins are segregated in the TGN for eventual apical delivery.

Apical sorting of lysoGPI-anchored proteins occurs independent of association with detergent-resistant membranes but dependent on their N-glycosylation

Overexpression of C-terminally tagged PGAP3 causes predominant production of lysoGPI-APs in MDCK cells. In these cells, produced lysoGPI-APs are not incorporated into detergent-resistant membranes but are still delivered apically. The apical transport of both fully remodeled and lysoGPI-APs is sensitive to cholesterol depletion and ablation of N-glycosylation.

Most glycosylphosphatidylinositol-anchored proteins (GPI-APs) are located at the apical surface of epithelial cells. The apical delivery of GPI-APs is believed to result from their association with lipid rafts. We find that overexpression of C-terminally tagged PGAP3 caused predominant production of lysoGPI-APs, an intermediate precursor in the GPI lipid remodeling process in Madin–Darby canine kidney cells. In these cells, produced lysoGPI-APs are not incorporated into detergent-resistant membranes (DRMs) but still are delivered apically, suggesting that GPI-AP association with DRMs is not necessary for apical targeting. In contrast, apical transport of both fully remodeled and lyso forms of GPI-APs is dependent on N-glycosylation, confirming a general role of N-glycans in apical protein transport. We also find that depletion of cholesterol causes apical-to-basolateral retargeting not only of fully remodeled GPI-APs, but also of lysoGPI-APs, as well as endogenous soluble and transmembrane proteins that would normally be targeted to the apical membrane. These findings confirm the essential role for cholesterol in the apical protein targeting and further demonstrate that the mechanism of cholesterol-dependent apical sorting is not related to DRM association of GPI-APs.

iTRAQ-Based and Label-Free Proteomics Approaches for Studies of Human Adenovirus Infections

Both isobaric tags for relative and absolute quantitation (iTRAQ) and label-free methods are widely used for quantitative proteomics. Here, we provide a detailed evaluation of these proteomics approaches based on large datasets from biological samples. iTRAQ-label-based and label-free quantitations were compared using protein lysate samples from noninfected human lung epithelial A549 cells and from cells infected for 24 h with human adenovirus type 3 or type 5. Either iTRAQ-label-based or label-free methods were used, and the resulting samples were analyzed by liquid chromatography (LC) and tandem mass spectrometry (MS/MS). To reduce a possible bias from quantitation software, we applied several software packages for each procedure. ProteinPilot and Scaffold Q+ software were used for iTRAQ-labeled samples, while Progenesis LC-MS and ProgenesisF-T2PQ/T3PQ were employed for label-free analyses. R² correlation coefficients correlated well between two software packages applied to the same datasets with values between 0.48 and 0.78 for iTRAQ-label-based quantitations and 0.5 and 0.86 for label-free quantitations. Analyses of label-free samples showed higher levels of protein up- or downregulation in comparison to iTRAQ-labeled samples. The concentration differences were further evaluated by Western blotting for four downregulated proteins. These data suggested that the label-free method was more accurate than the iTRAQ method.

Nonclassically Secreted Regulators of Angiogenesis

Many secreted polypeptide regulators of angiogenesis are devoid of signal peptides. These proteins are released through nonclassical pathways independent of endoplasmic reticulum and Golgi. In most cases, the nonclassical protein export is induced by stress. It usually serves to stimulate repair or inflammation in damaged tissues. We review the secreted signal peptide-less regulators of angiogenesis and discuss the mechanisms and biological significance of their unconventional export.

Site-specific N-linked glycosylation of receptor guanylyl cyclase C regulates ligand binding, ligand-mediated activation and interaction with vesicular integral membrane protein 36, VIP36

Guanylyl cyclase C (GC-C) is a multidomain, membrane-associated receptor guanylyl cyclase. GC-C is primarily expressed in the gastrointestinal tract, where it mediates fluid-ion homeostasis, intestinal inflammation, and cell proliferation in a cGMP-dependent manner, following activation by its ligands guanylin, uroguanylin, or the heat-stable enterotoxin peptide (ST). GC-C is also expressed in neurons, where it plays a role in satiation and attention deficiency/hyperactive behavior. GC-C is glycosylated in the extracellular domain, and differentially glycosylated forms that are resident in the endoplasmic reticulum (130 kDa) and the plasma membrane (145 kDa) bind the ST peptide with equal affinity. When glycosylation of human GC-C was prevented, either by pharmacological intervention or by mutation of all of the 10 predicted glycosylation sites, ST binding and surface localization was abolished. Systematic mutagenesis of each of the 10 sites of glycosylation in GC-C, either singly or in combination, identified two sites that were critical for ligand binding and two that regulated ST-mediated activation. We also show that GC-C is the first identified receptor client of the lectin chaperone vesicular integral membrane protein, VIP36. Interaction with VIP36 is dependent on glycosylation at the same sites that allow GC-C to fold and bind ligand. Because glycosylation of proteins is altered in many diseases and in a tissue-dependent manner, the activity and/or glycan-mediated interactions of GC-C may have a crucial role to play in its functions in different cell types.

Sterol affinity for phospholipid bilayers is influenced by hydrophobic matching between lipids and transmembrane peptides

Lipid self-organization is believed to be essential for shaping the lateral structure of membranes, but it is becoming increasingly clear that also membrane proteins can be involved in the maintenance of membrane architecture. Cholesterol is thought to be important for the lateral organization of eukaryotic cell membranes and has also been implicated to take part in the sorting of cellular transmembrane proteins. Hence, a good starting point for studying the influence of lipid-protein interactions on membrane trafficking is to find out how transmembrane proteins influence the lateral sorting of cholesterol in phospholipid bilayers. By measuring equilibrium partitioning of the fluorescent cholesterol analog cholestatrienol between large unilamellar vesicles and methyl-β-cyclodextrin the effect of hydrophobic matching on the affinity of sterols for phospholipid bilayers was determined. Sterol partitioning was measured in 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers with and without WALP19, WALP23 or WALP27 peptides. The results showed that the affinity of the sterol for the bilayers was affected by hydrophobic matching. An increasing positive hydrophobic mismatch led to stronger sterol binding to the bilayers (except in extreme situations), and a large negative hydrophobic mismatch decreased the affinity of the sterol for the bilayer. In addition, peptide insertion into the phospholipid bilayers was observed to depend on hydrophobic matching. In conclusion, the results showed that hydrophobic matching can affect lipid-protein interactions in a way that may facilitate the formation of lateral domains in cell membranes. This could be of importance in membrane trafficking.

Alterations in the immunoexpression of galectins-1, -3 and -7 between different grades of oral epithelial dysplasia

INTRODUCTION

Oral epithelial dysplasia (OED) is a potentially malignant lesion characterized by a combination of cytological and architectural anomalies, which are essential for its diagnosis. Galectins are proteins that participate in cell cycle, adhesion and differentiation, apoptosis, and immune responses, as well as in cancer development and progression.

MATERIALS AND METHODS

The aim of this study was to analyze the immunohistochemical expression of galectins-1, -3, and -7 in the OED (21 low risk and 29 high risk) and normal oral mucosa (NOM). The binary grading system was used.

RESULTS

Galectin-1 was expressed in the middle/lower third in most OED cases. Nuclear/cytoplasmic staining was observed in most low-risk and high-risk OEDs. All cases of NOM were negative for galectin-1. Galectin-3 was expressed in the middle/lower third in most low-risk cases. Nuclear/cytoplasmic staining was noted in most low-risk and high-risk OEDs. Middle/lower third and in membrane staining was detected in four cases of NOM for galectin-3. Galectin-7 was expressed in the upper/middle third in most of OED cases. Nuclear/cytoplasmic staining predominated in low-risk and high-risk OEDs. Galectin-7 was detected in four cases of NOM, all of them presenting staining in the upper/middle third and in the membrane.

CONCLUSION

The differences in the immunoexpression of galactin-1, -3, and -7 between different grades of OEDs suggest the involvement of this protein in the progression of dysplasias.

Ligand induced galectin-3 protein self-association

Many functions of galectin-3 entail binding of its carbohydrate recognition site to glycans of a glycoprotein, resulting in cross-linking thought to be mediated by its N-terminal noncarbohydrate-binding domain. Here we studied interaction of galectin-3 with the model glycoprotein asialofetuin (ASF), using a fluorescence anisotropy assay to measure the concentration of free galectin carbohydrate recognition sites in solution. Surprisingly, in the presence of ASF, this remained low even at high galectin-3 concentrations, showing that many more galectin-3 molecules were engaged than expected due to the about nine known glycan-based binding sites per ASF molecule. This suggests that after ASF-induced nucleation, galectin-3 associates with itself by the carbohydrate recognition site binding to another galectin-3 molecule, possibly forming oligomers. We named this type-C self-association to distinguish it from the previously proposed models (type-N) where galectin-3 molecules bind to each other through the N-terminal domain, and all carbohydrate recognition sites are available for binding glycans. Both types of self-association can result in precipitates, as measured here by turbidimetry and dynamic light scattering. Type-C self-association and precipitation occurred even with a galectin-3 mutant (R186S) that bound poorly to ASF but required much higher concentration (∼50 μm) as compared with wild type (∼1 μm). ASF also induced weaker type-C self-association of galectin-3 lacking its N-terminal domains, but as expected, no precipitation. Neither a monovalent nor a divalent N-acetyl-d-lactosamine-containing glycan induced type-C self-association, even if the latter gave precipitates with high concentrations of galectin-3 (>∼50 μm) in agreement with published results and perhaps due to type-N self-association.

Galectin-3 endocytosis by carbohydrate independent and dependent pathways in different macrophage like cell types

BACKGROUND

Galectin-3 (the Mac-2 antigen) is abundantly expressed in both macrophage like cells and certain non-macrophage cells. We have studied endocytosis of galectin-3 as one important step relevant for its function, and compared it between variants of a macrophage like cell line, and non-macrophage cells.

METHODS

Endocytosis of galectin-3 was observed by fluorescence microscopy and measured by flow cytometry. The endocytosis mechanism was analysed using galectin-3 mutants, galectin-3 inhibitors and endocytic pathways inhibitors in the human leukaemia THP-1 cell line differentiated into naïve (M0), classical (M1) or alternatively activated (M2) macrophage like cells, and the non-macrophage cell lines HFL-1 fibroblasts and SKBR3 breast carcinoma.

RESULTS

Galectin-3 endocytosis in non-macrophage cells and M2 cells was blocked by lactose and a potent galectin-3 inhibitor TD139, and also by the R186S mutation in the galectin-3 carbohydrate recognition domain (CRD). In M1 cells galectin-3 endocytosis could be inhibited only by chlorpromazine and by interference with the non-CRD N-terminal part of galectin-3. In all the cell types galectin-3 entered early endosomes within 5-10 min, to be subsequently targeted mainly to non-degradative vesicles, where it remained even after 24 h.

CONCLUSIONS

Galectin-3 endocytosis in M1 cells is receptor mediated and carbohydrate independent, while in M2 cells it is CRD mediated, although the non-CRD galectin-3 domain is also involved. General significance The demonstration that galectin-3 endocytosis in M1 macrophages is carbohydrate independent and different from M2 macrophages and non-macrophage cells, suggests novel, immunologically significant interactions between phagocytic cells, galectin-3 and its ligands.

Galectins in acute and chronic inflammation

Galectins are animal lectins that bind to β-galactosides, such as lactose and N-acetyllactosamine, in free form or contained in glycoproteins or glycolipids. They are located intracellularly or extracellularly. In the latter they exhibit bivalent or multivalent interactions with glycans on cell surfaces and induce various cellular responses, including production of cytokines and other inflammatory mediators, cell adhesion, migration, and apoptosis. Furthermore, they can form lattices with membrane glycoprotein receptors and modulate receptor properties. Intracellular galectins can participate in signaling pathways and alter biological responses, including apoptosis, cell differentiation, and cell motility. Current evidence indicates that galectins play important roles in acute and chronic inflammatory responses, as well as other diverse pathological processes. Galectin involvement in some processes in vivo has been discovered, or confirmed, through studies of genetically engineered mouse strains, each deficient in a given galectin. Current evidence also suggests that galectins may be therapeutic targets or employed as therapeutic agents for these inflammatory responses.

Different fractions of human serum glycoproteins bind galectin-1 or galectin-8, and their ratio may provide a refined biomarker for pathophysiological conditions in cancer and inflammatory disease

BACKGROUND

Changes in glycosylation of serum proteins are common, and various glycoforms are being explored as biomarkers in cancer and inflammation. We recently showed that glycoforms detected by endogenous galectins not only provide potential biomarkers, but also have different functions when they encounter galectins in tissue cells. Now we have explored the use of a combination of two galectins with different specificities, to further increase biomarker sensitivity and specificity.

METHODS

Sera from 14 women with metastatic breast cancer, 12 healthy controls, 14 patients with IgA-nephritis (IgAN), and 12 patients with other glomerulonephritis were fractionated by affinity chromatography on immobilized human galectin-1 or galectin-8N, and the protein amounts of the bound and unbound fractions for each galectin were determined.

RESULTS

Each galectin bound largely different fractions of the serum glycoproteins, including different glycoforms of haptoglobin. In the cancer sera, the level of galectin-1 bound glycoproteins was higher and galectin-8N bound glycoproteins lower compared to the other patients groups, whereas in IgAN sera the level of galectin-8N bound glycoproteins were higher.

CONCLUSION

The ratio of galectin-1 bound/galectin-8N bound glycoproteins showed high discriminatory power between cancer patients and healthy, with AUC of 0.98 in ROC analysis, and thus provides an interesting novel cancer biomarker candidate.

GENERAL SIGNIFICANCE

The galectin-binding ability of a glycoprotein is not only a promising biomarker candidate but may also have a specific function when the glycoprotein encounters the galectin in tissue cells, and thus be related to the pathophysiological state of the patient. This article is part of a Special Issue entitled Glycoproteomics.

Glycoproteomic identification of galectin-3 and -8 ligands in bronchoalveolar lavage of mild asthmatics and healthy subjects

BACKGROUND

Galectins, a family of small carbohydrate binding proteins, have been implicated in regulation of inflammatory reactions, including asthma and fibrosis in the lungs. Galectins are found in cells of the airways and in airway secretions, but their glycoprotein ligands there have only been studied to a very limited extent.

METHODS

Bronchoalveolar lavage (BAL) fluid from mild asthmatics and healthy volunteers were fractionated by affinity chromatography on the immobilized galectins. Total (10-30 μg) and galectin bound (~1-10 μg) protein fractions were identified, quantified and compared using shot-gun proteomics and spectral counts.

RESULTS

About 175 proteins were identified in unfractionated BAL-fluid, and about 100 bound galectin-3 and 60 bound galectin-8. These included plasma glycoproteins, and typical airway proteins such as SP-A2, PIGR and SP-B. The concentration of galectin-binding proteins was 100-300 times higher than the concentration of galectins in BAL.

CONCLUSION

The low relative concentration of galectins in BAL makes it likely that functional interactions with glycoproteins occur at sites rich in galectin, such as cells of the airways, rather than the extracellular fluid itself. The profile of galectin bound proteins differed between samples from asthma patients and healthy subjects and correlated with the presence of fibroblasts or eosinophils. This included appearance of a specific galectin-8-binding glycoform of haptoglobin, previously shown to be increased in serum in other inflammatory conditions.

GENERAL SIGNIFICANCE

It is technically feasible to identify galectin-binding glycoproteins in low concentration patient samples such as BAL-fluid, to generate biomedically interesting results. This article is part of a Special Issue entitled Glycoproteomics.

Galectins and microenvironmental niches during hematopoiesis

PURPOSE OF REVIEW

Galectins, a family of evolutionarily conserved glycan-binding proteins, are involved in the regulation of multiple cellular processes (e.g. immunity, apoptosis, cellular signaling, development, angiogenesis and cellular growth) and diseases (e.g. chronic inflammation, autoimmunity, cancer, infection). We discuss here how galectins contribute to the development of specialized microenvironmental niches during hematopoiesis.

RECENT FINDINGS

An expanding set of data strengthens a role of galectins in hematopoietic differentiation, particularly by setting specific interactions between hematopoietic and stromal cells: galectin-5 is found in reticulocytes and erythroblastic islands suggesting a major role during erythropoiesis; galectin-1 and 3 are involved in thymocyte apoptosis, signaling and intrathymic migration; galectin-1 plays critical roles in pre-BII cells development. Moreover, expression of galectins-1 and 10 are differentially expressed during T-regulatory cell development. Various galectins (3, 4, 5, 9) have been reported to be regulated during myelopoiesis and traffic into intracellular compartments, dictating the cellular distribution of specific glycoproteins and glycosphingolipids.

SUMMARY

The abundance of galectins in both extracellular and intracellular compartments, their multifunctional properties and ability to form supramolecular signaling complexes with specific glycoconjugates, make these glycan-binding proteins excellent candidates to mediate interactions between hematopoietic cells and the stromal microenvironment. Their secretion by one of the cellular partners can modulate adhesive properties by cross-linking specific glycoconjugates present on stromal or hematopoietic cells, by favoring the formation of synapses or by creating glycoprotein lattices on the surface of different cell types. Their divergent specificities and affinities for various glycoproteins contribute to the multiplicity of their cellular interactions.

Inhibition mechanism of human galectin-7 by a novel galactose-benzylphosphate inhibitor

Galectins are involved in many cellular processes due to their ability to bind carbohydrates. Understanding their functions has shown the necessity for potent and specific galectin inhibitors. Human galectin-7 (hGal-7), in particular, has been highlighted as an important marker in many types of cancer by either inhibiting or promoting tumour growth. Producing ligands able to selectively target hGal-7 will offer promising tools for deciphering cancer processes in which hGal-7 is involved as well as present potential solutions for future therapeutics. Here we report the high resolution crystal structure of hGal-7 in complex with a synthetic 2-O-benzylphosphate-galactoside inhibitor (which is > 60-fold more potent than its parent galactoside). The high resolution crystallographic analysis highlights the validity of using saccharide derivatives, conserving properties of the galactose binding, while enhanced affinity and specificity is provided by the added phosphate group. This structural information will allow the design of further inhibitors with improved potency and specificity.

Galectin-8 in IgA nephritis: decreased binding of IgA by galectin-8 affinity chromatography and associated increased binding in non-IgA serum glycoproteins

Background

Immunoglobulin A nephritis (IgAN) is the most common primary glomerulonephritis worldwide. It is caused by accumulation of IgA1-containing immune complexes in the kidney resulting in renal failure, which is thought to be due to altered glycosylation of IgA with a decrease of 2–3-sialylated galactosides (NeuAcα2-3Gal).

Purpose

The purpose of this study was to analyze whether altered glycosylation of IgA would lead to an altered binding to galectin-8, an endogenous lectin with strong affinity for 2–3-sialylated galactosides. Galectins are a family of β-galactoside-binding proteins; by binding various glycoproteins, they play important roles in the regulation of cellular functions in inflammation and immunity. Hence, an altered binding of IgA to galectin-8 could lead to pathologic immune functions, such as glomerulonephritis.

Methods

Affinity chromatography of serum glycoproteins on the human sialogalactoside-binding lectin galectin-8N permitted quantitation of bound and unbound fractions, including IgA.

Results

Analysis of ∼100 IgA nephritis sera showed that the galectin-8N unbound fraction of IgA increased compared to ∼100 controls, consistent with the known loss of galactosylation. A subgroup of ∼15% of the IgAN patients had a ratio of galectin-8 bound/unbound IgA <0.09, not found for any of the controls. Unexpectedly, the galectin-8N-binding fraction of serum glycoproteins other than IgA increased in the sera of IgAN patients but not in controls, suggesting a previously unrecognized change in this disease.

Conclusion

This is the first study that relates a galectin, an endogenous lectin family, to IgA nephritis and thus should stimulate new avenues of research into the pathophysiology of the disease.

Electronic supplementary material

The online version of this article (doi:10.1007/s10875-011-9618-3) contains supplementary material, which is available to authorized users.

The carbohydrate-binding site in galectin-3 is preorganized to recognize a sugarlike framework of oxygens: ultra-high-resolution structures and water dynamics

The recognition of carbohydrates by proteins is a fundamental aspect of communication within and between living cells. Understanding the molecular basis of carbohydrate–protein interactions is a prerequisite for the rational design of synthetic ligands. Here we report the high- to ultra-high-resolution crystal structures of the carbohydrate recognition domain of galectin-3 (Gal3C) in the ligand-free state (1.08 Å at 100 K, 1.25 Å at 298 K) and in complex with lactose (0.86 Å) or glycerol (0.9 Å). These structures reveal striking similarities in the positions of water and carbohydrate oxygen atoms in all three states, indicating that the binding site of Gal3C is preorganized to coordinate oxygen atoms in an arrangement that is nearly optimal for the recognition of β-galactosides. Deuterium nuclear magnetic resonance (NMR) relaxation dispersion experiments and molecular dynamics simulations demonstrate that all water molecules in the lactose-binding site exchange with bulk water on a time scale of nanoseconds or shorter. Nevertheless, molecular dynamics simulations identify transient water binding at sites that agree well with those observed by crystallography, indicating that the energy landscape of the binding site is maintained in solution. All heavy atoms of glycerol are positioned like the corresponding atoms of lactose in the Gal3C complexes. However, binding of glycerol to Gal3C is insignificant in solution at room temperature, as monitored by NMR spectroscopy or isothermal titration calorimetry under conditions where lactose binding is readily detected. These observations make a case for protein cryo-crystallography as a valuable screening method in fragment-based drug discovery and further suggest that identification of water sites might inform inhibitor design.