MOLECULAR AND FUNCTIONAL CHARACTERIZATION OF A 15 KDA GALECTIN FROM STRIPED BASS (MORONE SAXATILIS)

Abstract

I have employed biochemical and molecular techniques to investigate the role of galectin during bacterial challenge in teleost fish, using striped bass as an experimental model. Striped bass (Morone saxatilis) possesses a 15 kiloDalton Beta-galactoside binding protein, here after called MS15. It is a single polypeptide with an experimental mass of 15,000 Daltons. In vivo, MS15 exists as a non-covalently linked dimer, with two identical carbohydrate binding sites per protein. The organization of the gene coding for MS15 was the same as other vertebrate proto type galectins, with four exons the same size between organisms and three introns varying greatly in length. These investigations have shown that galectin is localized to the dermis and lamina propia (loose connective tissue) throughout the body, to smooth vascular muscle (veins, arteries), to large circulatory cells (heterophils, monocytes), to peripheral leukocytes (tissue resident macrophages, heterophils), to rodlet cells (possible immune cell in alimentary canal), and to leukocytes throughout the gills but not to mucus-producing cells. This is similar to what is found in mammals, but differs greatly from what has been observed in other teleosts. This finding led to the hypothesis that proto type galectin in striped bass was involved in innate immunity. To test this hypothesis, galectin was characterized based on stability, carbohydrate binding specificity, and native structure. Interactions between galectin and mucus, and galectin and bacteria were then tested. Results revealed galectin binds skin mucus and agglutinates selected bacteria. Striped bass were challenged with bacteria by intradermal and intramuscular injections. Results suggest an increase in the number of galectin-positive leukocytes observed in injection sites, independent of the presence of bacteria. Also, a novel observation was the strong galectin-positive nature of rodlet cells in the esophagus, stomach, and pyloric caeca of the striped bass. The function of rodlet cells is disputed, but in striped bass, galectin was detected most strongly in rodlet cells and both circulatory and peripheral leukocytes. Collectively, these data provide the foundation for three models to explain the biochemical events involved in bacterial clearance and/or wound healing in teleost fish, with potential relevance to innate immunity in other organisms.