Galectins: major signaling modulators inside and outside the cell.

Galectins control cell behavior by acting on different signaling pathways. Most of the biological activities ascribed to these molecules rely upon recognition of extracellular glycoconjugates and establishment of multivalente interactions, which trigger adaptive biological responses. However, galectins are also detected within the cell in different compartments, where their regulatory functions still remain poorly understood. A deeper understanding of the entire galectin signalosome and its impact in cell behavior is therefore essential in order to delineate new strategies to specifically manipulate both galectin expression and function. This review summarizes our current knowledge of the signaling pathways activated by galectins, their glycan dependence and the cellular compartment where they become activated and are biologically relevant.

Galectins: matricellular glycan-binding proteins linking cell adhesion, migration, and survival.

Galectins are a taxonomically widespread family of glycan-binding proteins, defined by at least one conserved carbohydrate-recognition domain with a canonical amino acid sequence and affinity for beta-galactosides. Because of their anti-adhesive as well as pro-adhesive extracellular functions, galectins appear to be a novel class of adhesion-modulating proteins collectively known as matricellular proteins (which include thrombospondin, SPARC, tenascin, hevin, and disintegrins). Accordingly, galectins can display de-adhesive effects when presented as soluble proteins to cells in a strong adhesive state. In this context, the de-adhesive properties of galectins should be considered as physiologically relevant as the proadhesive effects of these glycan-binding proteins. This article focuses on the roles of mammalian galectins in cell adhesion, spreading, and migration, and the crossregulation of these functions. Although careful attention should be paid when examining individual galectin functions due to overlapping distributions, these intriguing glycan-binding proteins offer promising possibilities for the treatment and intervention of a wide variety of pathological processes, including cancer, inflammation, and autoimmunity.

Immunohistochemical localisation of a galectin from Bufo arenarum ovary.

Galectins are a group of soluble animal lectins that exhibit specificity for beta-galactosides and conserve sequence homology in the carbohydrate-recognition domain. The galectin from Bufo arenarum ovary showed a strong cross-reaction with the lectin of 14.5 kDa purified from embryos at early blastula stage. In this paper, we studied the immunohistochemical localisation of the galectin of 14.5 kDa from ovary of the toad B. arenarum in adult ovary sections. We also analysed the immunohistochemical localisation of the embryonic lectin during early development using the antiserum anti-ovary galectin. In the ovary, oocytes in the previtellogenic stage showed strong reactivity in the nucleus and the cortex but not in the cytoplasm. Oocytes in the stage of primary vitellogenesis exhibited a similar pattern in the nuclear and cortical areas but showed immunostaining in the cytoplasm. Intense nuclear staining was detected in oocytes in the stage of late vitellogenesis and in mature oocytes, which also presented strong reactions in the yolk platelets that completely covered the cytoplasm. In blastula embryos the staining was found in the blastomeres, the yolk platelets and the blastocoele. Each lectin localisation is discussed in relation to potential biological roles in the corresponding tissues.

Further studies on vertebrate S-type lectins: cross-reactivity between toad and human lectins.

Galectins (S-type or S-Lac lectins) are a well-defined family of beta-galactoside animal lectins characterized by a high sequence homology in the carbohydrate-binding domain. We have previously purified and characterized the S-type lectin from the ovary of the toad Bufo arenarum. In this study, we purified the S-type lectins from Bufo arenarum ovary and human spleen by an improved method which included ion exchange and affinity chromatography. Antibody cross-reactivities between both lectins and some other S-type lectins showed that they share epitopes. Glycosylation studies carried out with detection/differentiation kits suggested that both lectins are not glycosylated.

Purification and partial biochemical characterization of an S-type lectin from blastula embryos of Bufo arenarum.

1. S-type lectin from Bufo arenarum embryos at blastula stage was purified by affinity chromatography. The molecule is a dimer with equal-sized monomers and the apparent subunit molecular weight was found to be 14.5 kDa. 2. Analytical isoelectric focusing of the pure lectin showed an acidic pI of 4.7. 3. Inhibition of the hemagglutination by mono- and oligosaccharides revealed a specificity for sugars bearing a beta-galactoside configuration. 4. Crossreactivity studies between the blastula lectin and the one purified earlier from adult ovary performed by immunodotting, ELISA and immunoblotting showed that these lectins share many epitopes.

Animal lectins as self/non-self recognition molecules. Biochemical and genetic approaches to understanding their biological roles and evolution.

In recent years, the significant contributions from molecular research studies on animal lectins have elucidated structural aspects and provided clues not only to their evolution but also to their multiple biological functions. The experimental evidence has suggested that distinct, and probably unrelated, groups of molecules are included under the term "lectin." Within the invertebrate taxa, major groups of lectins can be identified: One group would include lectins that show significant homology to membrane-integrated or soluble vertebrate C-type lectins. The second would include those beta-galactosyl-specific lectins homologous to the S-type vertebrate lectins. The third group would be constituted by lectins that show homology to vertebrate pentraxins that exhibit lectin-like properties, such as C-reactive protein and serum amyloid P. Finally, there are examples that do not exhibit similarities to any of the aforementioned categories. Moreover, the vast majority of invertebrate lectins described so far cannot yet be placed in one or another group because of the lack of information regarding their primary structure. (See Table 1.) Animal lectins do not express a recombinatorial diversity like that of antibodies, but a limited diversity in recognition capabilities would be accomplished by the occurrence of multiple lectins with distinct specificities, the presence of more than one binding site, specific for different carbohydrates in a single molecule, and by certain "flexibility" of the binding sites that would allow the recognition of a range of structurally related carbohydrates. In order to identify the lectins' "natural" ligands, we have investigated the interactions between those proteins and the putative endogenous or exogenous glycosylated substances or cells that may be relevant to their biological function. Results from these studies, together with information on the biochemical properties of invertebrate and vertebrate lectins, including their structural relationships with other vertebrate recognition molecules, are discussed.

Interaction of ovary lectin with homologous sperm from Bufo arenarum.

A soluble beta-galactoside lectin purified from Bufo arenarum ovary agglutinated homologous neuraminidase-treated spermatozoa. Microscopic observations of sperm clusters showed that spermatozoa agglutinated in a random way, but the head-to-head type of sperm agglutination was the most common (94-98%). The lectin activity was specifically inhibited by D-galactose and its derivatives, thio-digalactoside being the most active saccharide inhibitor.

Purification and some characteristics of a beta-galactoside binding soluble lectin from amphibian ovary.

Soluble extracts of Bufo ovaries agglutinate sialidase-treated rabbit erythrocytes. Unlike other amphibian lectins this agglutination activity does not require the presence of calcium ions. It is specifically inhibited by D-galactose and its derivatives. Thiodi-D-galactoside is the most potent saccharide inhibitor followed by lactose and methyl-beta-D-galactoside, respectively. D-Fucose, D-glucose and D-mannose do not inhibit the activity at concentrations at or above 100 mM. The lectin has been purified 500-fold to apparent homogeneity from the ovaries by salt extraction and affinity chromatography on lactose-aminophenyl-agarose, with a yield of about 0.2%. The molecular mass determined by gel filtration under native conditions was 30 kDa; polyacrylamide gel electrophoresis in SDS gave a molecular mass of 15 kDa, suggesting that the lectin is a dimer. The lectin has an isoelectric point of 40 and contains a high proportion of acidic amino acids.

A developmentally regulated lectin in Bufo arenarum embryos.

We report the levels of an endogenous beta-galactoside lectin activity from Bufo arenarum whole embryos extracts and specific inhibition by saccharides at different developmental stages. Specific activity measured against trypsinized rabbit red blood cells showed relatively high and fluctuating levels during early stages (up to about 76 h post-fertilization) which fell to significantly lower and more constant values at late stages (77-264 h post-fertilization). Lactose is the most potent inhibitor of this lectin activity, and saccharides having alpha-galactoside configurations are weaker inhibitors. At the last embryonic stage, the agglutinating activity showed a different sugar specificity which suggests either the modification of the preexistent lectin or the synthesis of another type of lectin. The possible physiological roles of these lectins in the blockage of polyspermy or in embryonic cell-cell interactions are discussed.

A developmentally regulated lectin in Bufo arenarum embryos

We report the levels of an endogenous beta-galactoside lectin activity from Bufo arenarum whole embryos extracts and specific inhibition by saccharides at different developmental stages. Specific activity measured against trypsinized rabbit red blood cells showed relatively high and fluctualting levels during early stages (up to about 76 h post-fertilization) which fell to significantly lower and more constant values at late stages (77-264 h post-fertilization). Lactose is the most potent inhibitor of this lectin activithy, and saccharides having alpha-galactoside configurations are weaker inhibitors. At the last embryonic stage, the agglutinating activity showed a different sugar specificity which suggests either the modification of the preexistent lectin or the sybthesis of another type of lectin. The possible physiological roles of these in the blockage of polyspermy or in embryonic cell-cell interactions are discussed