Immunohistochemical localization of six galectin subtypes in the mouse digestive tract

Galectin-4, a novel neuronal regulator of myelination

Myelination of axons by oligodendrocytes (OLGs) is essential for proper saltatory nerve conduction, i.e., rapid transmission of nerve impulses. Among others, extracellular matrix (ECM) molecules, neuronal signaling, and axonal adhesion regulate the biogenesis and maintenance of myelin membranes, driven by polarized transport of myelin-specific proteins and lipids. Galectin-4, a tandem-repeat-type lectin with affinity to sulfatide and nonsialylated termini of N-glycans, has the ability to regulate adhesion of cells to ECM components and is also involved in polarized membrane trafficking. We, therefore, anticipated that galectin-4 might play a role in myelination. Here, we show that in developing postnatal rat brains galectin-4 expression is downregulated just before the onset of myelination. Intriguingly, when immature OLGs were treated with galectin-4, OLG maturation was retarded, while a subset of the immature OLGs reverted to a morphologically less complex progenitor stage, displaying concomitantly an increase in proliferation. Similarly, myelination was inhibited when galectin-4 or anti-galectin-4 antibodies were added to co-cultures of dorsal root ganglion neurons and OLGs. Neurons and OLGs were identified as a possible source of galectin-4, both in vitro and in vivo. In culture, neurons but not OLGs released galectin-4. Interestingly, in co-cultures, a reduced release of endogenous galectin-4 correlated with the onset of myelination. Moreover, galectin-4-reactive sites are transiently expressed on processes of premyelinating primary OLGs, but not on neurons. Taken together, these results identify neuronal galectin-4 as a candidate for a soluble regulator of OLG differentiation and, hence, myelination. © 2012 Wiley Periodicals, Inc.

Galectin-9 induced by dietary synbiotics is involved in suppression of allergic symptoms in mice and humans

BACKGROUND

Prebiotic galacto- and fructo-oligosaccharides (scGOS/lcFOS) resembling non-digestible oligosaccharides in human milk reduce the development of atopic disorders. However, the underlying mechanisms are still unclear. Galectins are soluble-type lectins recognizing β-galactoside containing glycans. Galectin-9 has been shown to regulate mast cell degranulation and T-cell differentiation. In this study, the involvement of galectin-9 as a mechanism by which scGOS/lcFOS in combination with Bifidobacterium breve M-16V protects against acute allergic symptoms was investigated.

METHODS

Mice were sensitized orally to whey, while being fed with a diet containing scGOS/lcFOS and Bifidobacterium breve M-16V (GF/Bb) or a control diet. Galectin-9 expression was determined by immunohistochemistry in the intestine and measured in the serum by ELISA. T-cell differentiation was investigated in the mesenteric lymph nodes (MLN) as well as in galectin-9-exposed peripheral blood mononuclear cells (PBMC) cultures. Sera of the mice were evaluated for the capacity to suppress mast cell degranulation using a RBL-2H3 degranulation assay. In addition, in a double-blind, placebo-controlled multicenter trial, galectin-9 levels were measured in the sera of 90 infants with atopic dermatitis who received hydrolyzed formulae with or without GF/Bb.

RESULTS

Galectin-9 expression by intestinal epithelial cells and serum galectin-9 levels were increased in mice and humans following dietary intervention with GF/Bb and correlated with reduced acute allergic skin reaction and mast cell degranulation. In addition, GF/Bb enhanced T(h)1- and T(reg)-cell differentiation in MLN and in PBMC cultures exposed to galectin-9.

CONCLUSIONS

Dietary supplementation with GF/Bb enhances serum galectin-9 levels, which associates with the prevention of allergic symptoms.

Galectin-1-specific inhibitors as a new class of compounds to treat HIV-1 infection

Despite significant improvements, antiretroviral therapies against HIV-1 are plagued by a high frequency of therapeutic failures that have been associated with acquisition of drug resistance. We recently reported that HIV-1 exploits a host glycan binding protein, galectin-1, to increase its attachment to host cells, thereby increasing its overall infectivity in susceptible cells. This finding suggests that host molecules such as galectin-1 could reduce the expected efficiency of HIV-1 drugs targeting early steps of the replicative cycle, such as attachment and entry processes. Thus, new classes of drugs that would interfere with galectin-1/HIV-1 interactions could benefit the current antiretroviral therapy. To further explore this possibility, experiments were conducted to discover leading compounds showing specific inhibition of galectin-1 activity in a cellular model of HIV-1 infection. Three lactoside compounds were found to modestly inhibit the interaction of galectin-1 with primary human CD4(+) T cells. Interestingly, these same inhibitors reduced the galectin-1-mediated increase in HIV-1 attachment to target cells in a much more efficient manner. More important, the tested lactoside derivatives also significantly decreased the galectin-1-dependent enhancement of HIV-1 infection. These observations deserve further attention when considering that the development of new drugs to prevent and treat HIV-1 infection remains a priority.

Host-soluble galectin-1 promotes HIV-1 replication through a direct interaction with glycans of viral gp120 and host CD4

Sexual transmission of HIV-1 requires virus adsorption to a target cell, typically a CD4(+) T lymphocyte residing in the lamina propria, beneath the epithelium. To escape the mucosal clearance system and reach its target cells, HIV-1 has evolved strategies to circumvent deleterious host factors. Galectin-1, a soluble lectin found in the underlayers of the epithelium, increases HIV-1 infectivity by accelerating its binding to susceptible cells. By comparison, galectin-3, a family member expressed by epithelial cells and part of the mucosal clearance system, does not perform similarly. We show here that galectin-1 directly binds to HIV-1 in a β-galactoside-dependent fashion through recognition of clusters of N-linked glycans on the viral envelope gp120. Unexpectedly, this preferential binding of galectin-1 does not rely on the primary sequence of any particular glycans. Instead, glycan clustering arising from the tertiary structure of gp120 hinders its binding by galectin-3. Increased polyvalency of a specific ligand epitope is a common strategy for glycans to increase their avidity for lectins. In this peculiar occurrence, glycan clustering is instead exploited to prevent binding of gp120 by galectin-3, which would lead to a biological dead-end for the virus. Our data also suggest that galectin-1 binds preferentially to CD4, the host receptor for gp120. Together, these results suggest that HIV-1 exploits galectin-1 to enhance gp120-CD4 interactions, thereby promoting virus attachment and infection events. Since viral adhesion is a rate-limiting step for HIV-1 entry, modulation of the gp120 interaction with galectin-1 could thus represent a novel approach for the prevention of HIV-1 transmission.

Glycan recognition at the interface of the intestinal immune system: target for immune modulation via dietary components

The intestinal mucosa is constantly exposed to the luminal content, which includes micro-organisms and dietary components. Prebiotic non-digestible oligosaccharides may be supplemented to the diet to exert modulation of immune responses in the intestine. Short chain galacto- and long chain fructo-oligosaccharides (scGOS/lcFOS), functionally mimicking oligosaccharides present in human milk, have been reported to reduce the development of allergy through modulation of the intestinal microbiota and immune system. Nonetheless, the underlying working mechanisms of scGOS/lcFOS are unclear. Intestinal epithelial cells lining the mucosa are known to express carbohydrate (glycan)-binding receptors that may be involved in modulation of the mucosal immune response. This review aims to provide an overview of glycan-binding receptors, in particular galectins, which are expressed by intestinal epithelial cells and immune cells. In addition, their involvement in health and disease will be addressed, especially in food allergy and inflammatory bowel disease, diseases originating from the gastro-intestinal tract. Insight in the recognition of glycans in the intestinal tract may open new avenues for the treatment of intestinal inflammatory diseases by either nutritional concepts or pharmacological intervention.

Analysis of proteomic changes induced upon cellular differentiation of the human intestinal cell line Caco-2

The human intestinal cell line Caco-2 is a well-established model system to study cellular differentiation of human enterocytes of intestinal origin, because these cells have the capability to differentiate spontaneously into polarized cells with morphological and biochemical features of small intestinal enterocytes. Therefore, the cells are widely used as an in vitro model for the human intestinal barrier. In this study, a proteomic approach was used to identify the molecular marker of intestinal cellular differentiation. The proteome of proliferating Caco-2 cells was compared with that of fully differentiated cells. Two-dimensional gel analysis yielded 53 proteins that were differently regulated during the differentiation process. Pathway analysis conducted with those 34 proteins that were identified by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) analysis revealed subsets of proteins with common molecular and cellular function. It was shown that proteins involved in xenobiotic and drug metabolism as well as in lipid metabolism were upregulated upon cellular differentiation. In parallel, proteins associated with proliferation, cell growth and cancer were downregulated, reflecting the loss of the tumorigenic phenotype of the cells. Thus, the proteomic approach in combination with a literature-based pathway analysis yielded valuable information about the differentiation process of Caco-2 cells on the molecular level that contributes to the understanding of the development of colon cancer or inflammatory diseases such as ulcerative colitis--diseases associated with an imbalanced differentiation process of intestinal cells.

Novel gene expression responses in the ovine abomasal mucosa to infection with the gastric nematode Teladorsagia circumcincta

Infection of sheep with the gastric nematode Teladorsagia circumcincta results in distinct Th2-type changes in the mucosa, including mucous neck cell and mast cell hyperplasia, eosinophilia, recruitment of IgA/IgE producing cells and neutrophils, altered T-cell subsets and mucosal hypertrophy. To address the protective mechanisms generated in animals on previous exposure to this parasite, gene expression profiling was carried out using samples of abomasal mucosa collected pre- and post- challenge from animals of differing immune status, using an experimental model of T. circumcincta infection. Recently developed ovine cDNA arrays were used to compare the abomasal responses of sheep immunised by trickle infection with worm-naïve sheep, following a single oral challenge of 50 000 T. circumcincta L3. Key changes were validated using qRT-PCR techniques. Immune animals demonstrated highly significant increases in levels of transcripts normally associated with cytotoxicity such as granulysin and granzymes A, B and H, as well as mucous-cell derived transcripts, predominantly calcium-activated chloride channel 1 (CLCA1). Challenge infection also induced up-regulation of transcripts potentially involved in initiating or modulating the immune response, such as heat shock proteins, complement factors and the chemokine CCL2. In contrast, there was marked infection-associated down-regulation of gene expression of members of the gastric lysozyme family. The changes in gene expression levels described here may reflect roles in direct anti-parasitic effects, immuno-modulation or tissue repair. (Funding; DEFRA/SHEFC (VT0102) and the BBSRC (BB/E01867X/1)).

The glycan-binding protein galectin-1 controls survival of epithelial cells along the crypt-villus axis of small intestine

Intestinal epithelial cells serve as mechanical barriers and active components of the mucosal immune system. These cells migrate from the crypt to the tip of the villus, where different stimuli can differentially affect their survival. Here we investigated, using in vitro and in vivo strategies, the role of galectin-1 (Gal-1), an evolutionarily conserved glycan-binding protein, in modulating the survival of human and mouse enterocytes. Both Gal-1 and its specific glyco-receptors were broadly expressed in small bowel enterocytes. Exogenous Gal-1 reduced the viability of enterocytes through apoptotic mechanisms involving activation of both caspase and mitochondrial pathways. Consistent with these findings, apoptotic cells were mainly detected at the tip of the villi, following administration of Gal-1. Moreover, Gal-1-deficient (Lgals1^−/−) mice showed longer villi compared with their wild-type counterparts in vivo. In an experimental model of starvation, fasted wild-type mice displayed reduced villi and lower intestinal weight compared with Lgals1^−/− mutant mice, an effect reflected by changes in the frequency of enterocyte apoptosis. Of note, human small bowel enterocytes were also prone to this pro-apoptotic effect. Thus, Gal-1 is broadly expressed in mucosal tissue and influences the viability of human and mouse enterocytes, an effect which might influence the migration of these cells from the crypt, the integrity of the villus and the epithelial barrier function.

Glycosylation, galectins and cellular signaling

Glycosylation is a common posttranslational modification of proteins and lipids of the secretory pathway that generates binding sites for galactose-specific lectins or galectins. Branching of Asn-linked (N-)glycans by the N-acetylglucosaminyltransferases (Mgat genes) increases affinity for galectins. Both tissue-specific expression of the enzymes and the metabolic supply of sugar-nucleotides to the ER and Golgi regulate glycan distribution while protein sequences specify NXS/T site multiplicity, providing metabolic and genetic contributions to galectin-glycoprotein interactions. Galectins cross-link glycoproteins forming dynamic microdomains or lattices that regulate various mediators of cell adhesion, migration, proliferation, survival and differentiation. There are a similar number of galactose-specific galectins in C. elegans and humans, but expression of higher-affinity branched N-glycans are a more recent feature of vertebrate evolution. Galectins might be considered a reading code for repetition of the minimal units of binding [Gal(NAc)β1-3/4GlcNAc] and NXS/T site multiplicity in proteins. The rapidly evolving and structurally complex Golgi modifications to surface receptors are interpreted through affinity for the lattice, which regulates receptor levels as a function of the cellular environment, and thereby the probability of various cell fates. Many important questions remain concerning the regulation of the galectins, the glycan ligands and lattice interaction with other membrane domains and endocytic pathways.

Differential cellular localization of galectin-1 and galectin-3 in the regressing corpus luteum of mice and their possible contribution to luteal cell elimination

Galectin-1 and galectin-3, beta-galactoside-binding lectins, are predominantly expressed in the regressing corpus luteum (CL) of mouse ovary. This study revealed the expression patterns and cellular localizations of galectins during CL formation and regression by ISH and IHC. Galectin-1 mRNA expression temporarily increased in active CL, preceding the expression of progesterone degradation enzyme 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD), which represents functional luteolysis. The expressions of both galectin-1 and galectin-3 remarkably increased in the structurally regressing CL, which vigorously expressed 20alpha-HSD and contained abundant apoptotic luteal cells. Ultrastructurally, galectin-1- and galectin-3-immunoreactive cells were identified as fibroblasts and infiltrating macrophages, respectively. In addition, some populations of luteal cells themselves expressed galectin-3 in regressing CL and formed unique demarcation membranes in the cytoplasm, showing a non-typical apoptotic feature. Ovary of adult mice with repeated estrus cycles contained CL of three different generations. Among them, the old CL formed during previous estrus cycles consisted of galectin-3-positive luteal cells. The galectin-3-positive old CL was resistant to apoptosis and seemed to be eliminated by a mechanism different from apoptosis. The stage- and cell-specific expression of galectin in CL suggests its differential contribution to luteolysis, and this expression may be mediated by major regulatory molecules of CL function, prolactin and/or prostaglandin F2alpha.

Galectins in innate immunity: dual functions of host soluble beta-galactoside-binding lectins as damage-associated molecular patterns (DAMPs) and as receptors for pathogen-associated molecular patterns (PAMPs)

The glycocalyx is a glycan layer found on the surfaces of host cells as well as microorganisms and enveloped virus. Its thickness may easily exceed 50 nm. The glycocalyx does not only serve as a physical protective barrier but also contains various structurally different glycans, which provide cell- or microorganism-specific 'glycoinformation'. This information is decoded by host glycan-binding proteins, lectins. The roles of lectins in innate immunity are well established, as exemplified by collectins, dectin-1, and dendritic cell (DC)-specific intracellular adhesion molecule-3-grabbing non-integrin (DC-SIGN). These mammalian lectins are synthesized in the secretory pathway and presented on the cell surface to bind to specific glycan 'epitopes'. As they recognize non-self glycans presented by microorganisms, they can be considered as receptors for pathogen-associated molecular patterns (PAMPs), i.e. pattern recognition receptors (PRRs). One notable exception is the galectin family. Galectins are synthesized and stored in the cytoplasm, but upon infection-initiated tissue damage and/or following prolonged infection, cytosolic galectins are either passively released by dying cells or actively secreted by inflammatory activated cells through a non-classical pathway, the 'leaderless' secretory pathway. Once exported, galectins act as PRR, as well as immunomodulators (or cytokine-like modulators) in the innate response to some infectious diseases. As galectins are dominantly found in the lesions where pathogen-initiated tissue damage signals appear, this lectin family is also considered as potential damage-associated molecular pattern (DAMP) candidates that orchestrate innate immune responses alongside the PAMP system.

Galectin-2 at the enterocyte brush border of the small intestine

The brush border of pig small intestine is a local hotspot for beta-galactoside-recognizing lectins, as evidenced by its prominent labeling with fluorescent lectin PNA. Previously, galectins 3-4, intelectin, and lectin-like anti-glycosyl antibodies have been localized to this important body boundary. Together with the membrane glycolipids these lectins form stable lipid raft microdomains that also harbour several of the major digestive microvillar enzymes. In the present work, we identified a lactose-sensitive 14-kDa protein enriched in a microvillar detergent resistant fraction as galectin-2. Its release from closed, right-side-out microvillar membrane vesicles shows that at least some of the galectin-2 resides at the lumenal surface of the brush border, indicating that it plays a role in the organization/stabilization of the lipid raft domains. Galectin-2 was released more effectively from the membrane by lactose than was galectin-4, and surprisingly, it was also released by the noncanonical disaccharides sucrose and maltose. Furthermore, unlike galectin-4, galectin-2 was preferentially co-immunoisolated with sucrase-isomaltase rather than with aminopeptidase N. Together, these results show that the galectins are not simply redundant proteins competing for the same ligands but rather act in concert to ensure an optimal cross-linking of membrane glycolipids and glycoproteins. In this way, they offer a maximal protection of the brush border against exposure to bile, pancreatic enzymes and pathogens.