Galectin-3 modulates the polarized surface delivery of ß1-integrin in epithelial cells.

Epithelial cells require a precise intracellular transport and sorting machinery to establish and maintain their polarized architecture. This machinery includes ß-galactoside-binding galectins for targeting of glycoprotein to the apical membrane. Galectin-3 sorts cargo destined for the apical plasma membrane into vesicular carriers. After delivery of cargo to the apical milieu, galectin-3 recycles back into sorting organelles. We analysed the role of galectin-3 in the polarized distribution of ß1-integrin in MDCK cells. Integrins are located primarily at the basolateral domain of epithelial cells. We demonstrate that a minor pool of ß1-integrin interacts with galectin-3 at the apical plasma membrane. Knockdown of galectin-3 decreases apical delivery of ß1-integrin. This loss is restored by supplementation with recombinant galectin-3 and galectin-3 overexpression. Our data suggest that galectin-3 targets newly synthesized ß1-integrin to the apical membrane and promotes apical delivery of ß1-integrin internalized from the basolateral membrane. In parallel, knockout of galectin-3 results in a reduction in cell proliferation and an impairment in proper cyst development. Our results suggest that galectin-3 modulates the surface distribution of ß1-integrin and affects the morphogenesis of polarized cells.

Recycling of galectin-3 in epithelial cells.

Galectins, a family of ß-galactoside binding proteins, do not possess a signalling sequence to enter the endoplasmic reticulum as a starting point for the classical secretory pathway. They use a so-called unconventional secretion mechanism for translocation across the plasma membrane and/or into the lumen of transport vesicles. The ß-galactoside binding protein galectin-3 is highly expressed in a variety of epithelial cell lines. Polarized MDCK cells secrete this lectin predominantly into the apical medium. The lectin re-enters the cell by non-clathrin mediated endocytosis and passages through endosomal organelles. This internalized galectin-3 plays an important role in apical protein trafficking by directing the subcellular targeting of apical glycoproteins via oligomerization into high molecular weight clusters, a process that can be fine-tuned by changes in the environmental pH. Following release at the apical plasma membrane, the lectin can reenter the cell for another round of recycling and apical protein sorting. This review will briefly address galectin-3-functions in epithelia and focus on distinct phases in apical recycling of the lectin.

The large GTPase Mx1 is involved in apical transport in MDCK cells.

In epithelial cells apical proteins are transported by specific transport carriers to the correct membrane domain. The composition of these carriers is heterogeneous and comprises components such as motor proteins, annexins, lectins, Rab GTPases and cargo molecules. Here, we provide biochemical and fluorescence microscopic data to show that the dynamin-related large GTPase Mx1 is a component of post-Golgi vesicles carrying the neurotrophin receptor p75(NTR) . Moreover, siRNA-mediated depletion of Mx1 significantly decreased the transport efficiency of apical proteins in MDCK cells. In conclusion, Mx1 plays a crucial role in the delivery of cargo molecules to the apical membrane of epithelial cells.

pH-dependent recycling of galectin-3 at the apical membrane of epithelial cells.

The ß-galactoside binding protein galectin-3 is highly expressed in a variety of epithelial cell lines. Polarized MDCK cells secrete this lectin predominantly into the apical medium by non-classical secretion. Once within the apical extracellular milieu, galectin-3 can re-enter the cell followed by passage through endosomal organelles and modulate apical protein sorting. Here, we could show that galectin-3 is internalized by non-clathrin mediated endocytosis. Within endosomal organelles this pool associates with newly synthesized neurotrophin receptor in the biosynthetic pathway and assists in its membrane targeting. This recycling process is accompanied by transient interaction of galectin-3 with detergent insoluble membrane microdomains in a lactose- and pH-dependent manner. Moreover, in the presence of lactose, apical sorting of the neurotrophin receptor is affected following endosomal deacidification. Taken together, our results suggest that internalized galectin-3 directs the subcellular targeting of apical glycoproteins by membrane recycling.