Protein zero, a myelin IgCAM, induces physiologically operative tight junctions in nonadhesive carcinoma cells.

In the peripheral nervous system, protein zero (P0), a homophilic immunoglubulin cell adhesion molecule, mediates adhesion of Schwann cell membranes as they enwrap axons and generate compact myelin. Although P0 is naturally only expressed in peripheral myelin, it can behave as a vigorous adhesion molecule in a variety of cell types (Filbin et al. [1990] Nature 344:871-872; Schneider-Schaulies et al. [1990] J Neurosci Res 27:286-297; Doyle et al. [1995] J Cell Biol 131:465-482) and can thus be characterized as an obligatory adhesion molecule. Previously, we showed that when HeLa, a cervical carcinoma cell line devoid of epithelial junctions, is forced to express P0, strong cell-cell adhesion is induced, proteins associated with junctional elements are upregulated, and ultrastructurally tight junctions, adherens junctions, and desmosomes become apparent (Doyle et al., 1995). In this report, we assessed whether the tight junctions were physiologically operative in P0 HeLa expressors. Consistent with the presence of operative tight junctions, we found that P0 expressors in monolayers maintained endogenous proteins in their apical and basolateral plasma membrane subdomains. Furthermore, these cells generated a higher transepithelial resistance than did control HeLa cells, which is indicative of the formation of an effective intercellular permeability barrier.