Galectin-9 trafficking regulates apical-basal polarity in Madin-Darby canine kidney epithelial cells.

Galectins are unconventionally secreted lectins that participate in the formation of glycoprotein lattices that perform a variety of cell surface functions. Galectins also bind glycosphingolipid headgroups with as yet unclear implications for cellular physiology. We report a specific interaction between galectin-9 and the Forssman glycosphingolipid (FGL) that is important for polarizing Madin-Darby canine kidney epithelial cells. Galectin-9 knockdown leads to a severe loss of epithelial polarity that can be rescued by addition of the recombinant protein. The FGL glycan is identified as the surface receptor that cycles galectin-9 to the Golgi apparatus from which the protein is recycled back to the apical surface. Together our results suggest a model wherein such glycosphingolipid-galectin couples form a circuit between the Golgi apparatus and the cell surface that in an epithelial context facilitates the apical sorting of proteins and lipids.