Morphology and biogenesis of endosomal compartments in epithelial cells

Abstract

Epithelial tissues which line the body cavities and tubules typically face two environments that are biochemically and physically different from each other. Consequently, these cells possess two structurally and functionally distinct plasma membrane domains; an apical domain facing the lumenal or the free surface and a basolateral domain facing the basement membrane and the intercellular space. Furthermore, via processes such as domain selective absorption, secretion and transcytosis, epithelial cells not only maintain these trans-epithelial differences but also actively contribute to their generation. Therefore, maintenance of high fidelity in polarized protein sorting and membrane trafficking aee of cardinal importance in all epithelia for their proper function. Although the central role membrane trafficking plays in generation and maintenance of epithelial cell polarity is clear, nature of the endosomal compartment(s) involved and the molecular determinants employed in this process remains ill-defined. In this study, using a unique apical endosomal marker, endotubin, and a model polarized epithelial cell line, Madin-Darby canine kidney (MDCK), we characterize the endotubin-positive endosomes as a subset of apical early endosomes which can be reached with an endocytic marker only when it is internalized apically. Furthermore, we show that endotubin-positive endosomes do not contain basolaterally recycling transferrin or the small GTPase Rab 11 and therefore they are distinct from the previously described apical recycling endosomes (ARE) in MDCK cells. In addition, using a panel of endotubin mutants we characterize two cytoplasmic sorting signals, a hydrophobic cluster and a casein kinase II phosphorylation site, as the molecular determinants required for polarized sorting and endosomal targeting of this molecule. Also, using a panel of domain exchange chimeras we show that endotubin cytoplasmic domain is sufficient to mediate apical sorting and early endosomal targeting of an unrelated protein in MDCK cells. Nevertheless, overexpression of these chimeras but not a mutant form defective in endosomal targeting results in missorting of the construct to the basolateral domain. These results indicate that, the endotubin-positive apical endosomes possess a saturable sorting machinery capable of recognizing the cytoplasmic sorting determinants here we characterize.