© 2018. Published by The Company of Biologists Ltd | Journal of Cell Science (2018) 131, jcs208884. doi:10.1242/jcs.208884 CELL SCIENCE AT A GLANCE Galectins at a glance Ludger Johannes1,*, Ralf Jacob2,* and Hakon Leffler3,* ABSTRACT or nuclear targets and carbohydrate-dependent interactions with Galectins are carbohydrate-binding proteins that are involved in many extracellular glycoconjugates. We discuss how these intra- and physiological functions, such as inflammation, immune responses, extracellular activities might be linked and point out the importance of cell migration, autophagy and signalling. They are also linked to unravelling molecular mechanisms of galectin function to gain a true diseases such as fibrosis, cancer and heart disease. How such a understanding of their contributions to the physiology of the cell. We small family of only 15 members can have such widespread effects close with a short outlook on the organismal functions of galectins remains a conundrum. In this Cell Science at a Glance article, we and a perspective on the major challenges in the field. summarise recent literature on the many cellular activities that have KEY WORDS: Endocytosis, Galectin, Glycosylation been ascribed to galectins. As shown on the accompanying poster, these include carbohydrate-independent interactions with cytosolic Introduction Galectins were discovered based on their galactoside binding 1Institut Curie, PSL Research University, Cellular and Chemical Biology unit, U1143 activity, in a quest to find proteins that decode complex cell-surface INSERM, UMR3666 CNRS, 26 rue d’Ulm, 75248 Paris Cedex 05, France. 2Philipps- glycans. They were defined as a protein family based on conserved Universität Marburg, Institut für Zytobiologie, Robert-Koch-Str. 6, 35037 Marburg, β-galactoside-binding sites found within their characteristic ∼130 Germany. 3Sect. MIG (Microbiology, Immunology, Glycobiology), Dept Laboratory Medicine, Lund University, POB 117, 22100 Lund, Sweden. amino acid (aa) carbohydrate recognition domains (CRDs) (Barondes et al., 1994). However, it was soon realised that *Authors for correspondence ([email protected]; [email protected]; galectins are synthesised as cytosolic proteins, reside in the [email protected]) cytosol or nucleus for much of their lifetime and reach their L.J., 0000-0002-2168-0004 galactoside ligands only after non-classical secretion that bypasses Journal of Cell Science 1 CELL SCIENCE AT A GLANCE Journal of Cell Science (2018) 131, jcs208884. doi:10.1242/jcs.208884 the Golgi complex (Cummings et al., 2015). Indeed, galectins also with the goal of stimulating interest in the interplay between intra- interact with various non-galactose-containing binding partners, and extracellular galectin activities, and on the best studied and their CRDs (or other parts) also have non-carbohydrate binding galectins, mainly in human and mouse. We highlight how the sites (Kim et al., 2013; Li et al., 2013) (see poster). While all cells understanding of molecular mechanisms will be critical to obtain express galectins, sometimes at cytosolic concentrations as high as confirmed models of galectin function. The organismal functions of 5 µM (Lindstedt et al., 1993), the expression pattern varies between galectins are only briefly summarised, including their tissue cell types and tissues (Table 1). distribution (Table 1) and small molecule inhibitors (Box 1). A Galectins are small soluble proteins that contain one or two CRDs more detailed description of the carbohydrate structures to which but no other types of folded protein domains (Hirabayashi and galectins bind (Boxes S1 and S2) and of the main defects that are Kasai, 1993). A core set of mammalian galectins are conserved from observed in galectin knockout mice (Table S1) are presented in the earlier vertebrate species, these comprise the ‘prototype’ galectin-1, supplementary material. -2 and -7, which contain one CRD, the chimeric galectin-3, and galectin-4, -8, -9 and -12, which are tandem repeat galectins with Binding activities of galectins two CRDs (Houzelstein et al., 2004). Other galectins and galectin- The galectin CRD folds as a slightly bent β-sandwich (Di Lella related proteins are listed in Table 1. et al., 2011; Leffler et al., 2004) (see poster). The concave S side Here, and in the accompanying poster, we exemplify the binding forms a shallow groove that can approximately hold a activities of galectins and their cytosolic and nuclear functions, as tetrasaccharide (labelled A-B-C-D on the poster and Fig. S1). The well as their capacity to: (1) recognise disrupted endosomal defining galactose-binding site (C) that is conferred by a conserved membranes; (2) be secreted via the non-classical pathway; and (3) sequence motif of about seven amino acids is localised within this modulate the activity of glycoproteins at the cell surface and inside groove. Weaker sites on either side of the galactose-binding site the lumen of intracellular compartments. Because of space (including site E shown on the poster) enhance or decrease binding, limitations, the focus of this article is on their cellular activities thereby giving each galectin CRD its unique specificity and higher Table 1. Tissue and species distribution of galectins and galectin-related proteins Galectin Tissue distribution in human and rodents Species (closest relativea) Reference Galectin-1 Expressed in many tissues and cell types Vertebrates Cooper, 2002 Galectin-2 Gastrointestinal tract, placenta Birds and mammals, frogs, Hutter et al., 2015; Oka et al., 1999 lizards (galectin-1) Galectin-3 Expressed in many tissues and cell types, Vertebrates Cooper, 2002 especially prominent in macrophages and epithelial cells Galectin-4 Intestine and stomach, uterine epithelial cells, blood Vertebrates, except most birds Froehlich et al., 2012; Huflejt and Leffler, 2004; vessel walls, hippocampal and cortical neurons Velasco et al., 2013 Galectin-5 Lung, kidney, reticulocytes, bone marrow, Rat (galectin-9, C-CRD) Cerra et al., 1985; Gitt et al., 1995; Lensch erythrocytes et al., 2006 Galectin-6 Gastrointestinal tract, liver, kidney, spleen, heart Some mouse strains (galectin-4) Gitt et al., 1998; Houzelstein et al., 2008; Houzelstein et al., 2013 Galectin-7 Gastrointestinal tract, stratified epithelia, skin, fetal Mammals (galectin-4, N-CRD) Magnaldo et al., 1998; Sato et al., 2002; heart Timmons et al., 1999 Galectin-8 Liver, kidney, cardiac muscle, lung, and brain Vertebrates Hadari et al., 1995 Galectin-9 T-lymphocytes, small intestine, liver, uterine Vertebrates Froehlich et al., 2012; Lensch et al., 2006; epithelial cells, skin epidermis and oesophageal Matsumoto et al., 1998; Wada et al., 1997 epithelium Galectin-10 Eosinophils Primates, bats (galectin-4, N-CRD) Abedin et al., 2003 Galectin-12 Adipose tissue Vertebrates (galectin-8) Hotta et al., 2001; Yang et al., 2004 Galectin-11b Abomasal mucosa (ruminants), epithelial cells of Ruminants (galectin-4, N-CRD) Dunphy et al., 2000; Young et al., 2012 bile ducts Galectin-14b Eosinophils Ruminants (galectin-4, N-CRD) Young et al., 2009 Galectin-15b Uterus Ruminants (galectin-4, N-CRD) Gray et al., 2004 Galectin-11 Placenta Primates (galectin-4, N-CRD) Than et al., 2004; Than et al., 2009 and othersb GRPc B-lymphocytes, bursa of Fabricius Vertebrates (galectin-8 or -12, Houzelstein et al., 2004; Manning et al., 2018 C-CRD) GRIFINd Eye lens Vertebrates (galectin-2) Caballero et al., 2018; Manning et al., 2018 aThe main group within mammals is shown for galectins with limited species distribution. The closest related CRD among those conserved in most vertebrates is given. bIn the human placenta, a cluster of genes on chromosome 19 encode single-CRD galectins, all related to galectin-7, -10 and the N-terminal CRD of galectin-4 (also on chromosome 19); these include galectin-11, -13, -15 and potentially others. In ruminants, galectins with closely related sequences have been given overlapping names, despite their different tissue expression. Thus, it cannot easily be determined which are orthologues and which are paralogues. cThis galectin-related protein, termed HSPC159, was discovered as a clone from haematopoietic stem/progenitor cells (HSPCs). GRP does not have galactose binding activity and has a highly conserved sequence throughout vertebrates; it is best characterised in chicken. dThis protein was discovered in rat eye lens. It is found throughout vertebrates, with a highly variable sequence. The 7 amino acid galectin motif is disrupted in mammals, but almost complete in birds, and complete in fish and amphibians, suggesting it is a functional galectin in some species, as shown for chicken and zebrafish. C-CRD, C-terminal carbohydrate recognition domain; N-CRD, N-terminal carbohydrate recognition domain. Journal of Cell Science 2 CELL SCIENCE AT A GLANCE Journal of Cell Science (2018) 131, jcs208884. doi:10.1242/jcs.208884 encounter. Galectin-3 is monomeric up to high concentrations Box 1. Galectin inhibitors (100 µM), but can aggregate with ligands and agglutinate cells at The roles of galectins in cellular and pathophysiological processes has much lower concentrations, which is promoted by its N-terminal inspired the search for galectin inhibitors, both as potential drug leads, non-CRD region (Hsu et al., 1992; Massa et al., 1993) via an and as experimental tools in cell biology. A wide array of small-molecule oligomerisation mechanism that is not yet completely understood inhibitors has been reviewed recently (Campo et al., 2016). Among (Ahmad et al., 2004; Barboni et al., 1999; Birdsall et al., 2001; these, the most potent inhibitors are based on disaccharide derivatives that fit into the core site (C-D), and with aromatic extensions that project Kuklinski and Probstmeier, 1998; Lepur et al., 2012; Nieminen into sites B and E, thereby reaching affinities in the low nM range. The et al., 2007; Yang et al., 1998) (see poster and Box S3). Galectins synthetic disaccharide thiodigalactoside (Galβ1-S-1βGal) projects a with two different CRDs may form non-covalent dimers to achieve glycotope for site C-D (see Box S1 and poster), which is very similar to an increase in valency (Kamili et al., 2016).