Organization of vesicular trafficking in epithelia

Key Points At present, our understanding of the mechanisms that control the establishment and maintenance of polarized vesicular-transport routes in epithelial cells can be traced to the introduction of the Madin–Darby canine kidney (MDCK) model system almost three decades ago. Studies in this cell line have identified a multiplicity of sorting signals (hierarchically arranged, with basolateral usually dominant over apical signals) that guide proteins along biosynthetic, endocytic, recycling and transcytotic routes, to and from apical and basolateral membranes. The basolateral sorting mechanisms are better understood owing to the simpler nature of basolateral signals — short peptide motifs that are similar to endocytic motifs — and to the discovery of several potential interacting adaptors that function in this pathway. An epithelial-specific adaptor (AP1B) sorts basolateral proteins in a post-Golgi compartment at the crossroads of the biosynthetic and recycling routes. Recent work indicates that the clustering of small lipid rafts into larger lipid rafts, through protein oligomerization, might be an important determinant of apical targeting. The multiplicity of signals and adaptors probably accounts for the variability in polarized transport routes and in the final localization of proteins at the cell surface in different epithelial cell types (this is known as 'flexible epithelial phenotype'). Apical and basolateral routes, which were originally defined by biochemical approaches, are, at present, being more precisely defined by live-cell-imaging experiments using green fluorescent protein (GFP)-tagged apical and basolateral markers. These studies indicate that the junctional area is a 'hot spot' for the delivery of basolateral proteins and of some apical proteins that use the transcytotic route. Live-cell-imaging techniques are facilitating the study of the cytoskeleton and its contribution to polarized trafficking routes. Recent work indicates that the actin and microtubule cytoskeletons cooperate at various levels: first, by organizing the assembly of apical and basolateral vesicular and tubular transporters from intracellular sorting organelles ( trans -Golgi network, recycling endosomes); second, by facilitating their transport across the viscous cytoplasm; and third, by organizing the docking and fusion machinery at specific sites in the plasma membrane. Studies in Drosophila melanogaster and Caenorhabditis elegans have identified a number of 'polar