Galectins in teleost fish: Zebrafish (Danio rerio) as a model species to address their biological roles in development and innate immunity.

Cell surface glycans, such as glycocoproteins and glycolipids, encode information that modulates interactions between cells, or between cells and the extracellular matrix, by specifically regulating the binding to cell surface-associated or soluble carbohydrate-binding receptors, such as lectins. Rapid modifications of exposed carbohydrate moieties by glycosidases and glycosyltransferases, and the equally dynamic patterns of expression of their receptors during early development, suggest that both play important roles during embryogenesis. Among a variety of biological roles, galectins have been proposed to mediate developmental processes, such as embryo implantation and myogenesis. However, the high functional "redundancy" of the galectin repertoire in mammals has hindered the rigorous characterization of their specific roles by gene knockout approaches in murine models. In recent years, the use of teleost fish as alternative models for addressing developmental questions in mammals has expanded dramatically, and we propose their use for the elucidation of biological roles of galectins in embryogenesis and innate immunity. All three major galectin types, proto, chimera, and tandem-repeat, are present in teleost fish, and phylogenetic topologies confirm the expected clustering with their mammalian orthologues. As a model organism, the zebrafish (Danio rerio) may help to overcome limitations imposed by the murine models because it offers substantial advantages: external fertilization, transparent embryos that develop rapidly in vitro, a diverse toolbox of established methods to manipulate early gene expression, a growing collection of mutations that affect early embryonic development, availability of cell lines, and most importantly, an apparently less diversified galectin repertoire.