Truncation mutants of the tight junction protein ZO-1 disrupt corneal epithelial cell morphology

The tight junction is the most apical intercellular junction of epithelial cells and regulates transepithelial permeability through the paracellular p athway. To examine possible functions for the tight junction-associated pro tein ZO-1, C-terminally truncated mutants and a deletion mutant of ZO-1 wer e epitope tagged and stably expressed in corneal epithelial cell lines. Onl y full-length ZO-1 and one N-terminal truncation mutant targeted to cell bo rders; other mutants showed variable cytoplasmic distributions. None of the mutants initially disrupted the localization of endogenous ZO-1. However, long-term stable expression of two of the N-terminal mutants resulted in a dramatic change in cell shape and patterns of gene expression. An elongated fibroblast-like shape replaced characteristic epithelial cobblestone morph ology. In addition, vimentin and smooth muscle actin expression were up-reg ulated, although variable cytokeratin expression remained, suggesting a par tial transformation to a mesenchymal cell type. Concomitant with the morpho logical change, the expression of the integral membrane tight junction prot ein occludin was significantly down-regulated. The localizations of endogen ous ZO-1 and another family member, ZO-2, were disrupted. These findings su ggest that ZO-1 may participate in regulation of cellular differentiation.