Directional migration is important during embryonic development and wound healing. Maintenance of microtubule organization and cell polarization are important for regulating and directing epithelial cell movement. A few polarity protein complexes are implicated in tight junction formation and directional migration in epithelial cells; however, the mechanism of how these proteins are recruited to the leading edge is not clear. Zhengjun Chen and colleagues now report in Developmental Cell that the tight junction protein occludin is associated with and localizes the aPKC–Par3–PATJ polarity complex to the leading edge, and it also regulates actin organisation and directional migration during wound healing.

Immunofluorescence analysis in migrating epithelial cells demonstrated that occludin localizes at the leading edge along the actin cortex, as well as at cell junctions. Short interfering RNA (siRNA) depletion of occludin inhibited the ability of cells to close a wound, suggesting that it is important for leading edge dynamics. So, what is the mechanism activated by occludin during wound healing?

Occludin is known to interact with many signalling proteins such as p85α — the regulatory subunit of phosphoinositide 3-kinase (PI3K) — and aPKC, which forms a complex with Par3 and PATJ in polarised cells during migration. In support of this, Chen and colleagues demonstrate that the aPKC–Par3–PATJ complex is localized to the leading edge; knockdown of occludin inhibited this localization, as well as disrupting microtubule elongation. This suggests that occludin is involved in the accumulation of this complex at the leading edge and maintains microtubule organisation during wound healing. They further tested this hypothesis by immunoprecipitation analysis and immunofluorescence imaging, which supported an association between occludin and the aPKC–Par3–PATJ complex at the leading edge of migrating epithelial cells. This indicates that occludin recruits the aPKC–Par3–PATJ complex to the leading edge and thereby regulates polarised migration.

Lamellipodia formation is required for protrusion of cells during motility, and is regulated by the small GTPase Rac1. Occludin depletion disrupted Rac1 localization to the leading edge, and inhibited the formation of lamellipodia. Since PI3K regulates Rac1 activation, the authors tested whether occludin affects PI3K activity. Interestingly, knockdown of occludin interfered with Akt phosphorylation downstream of PI3K, and PI3K localization at the leading edge, indicating that occludin affects PI3K activation and protrusion during directional migration.

Next, the authors showed that stimulation of occludin tyrosine phosphorylation promotes association of occludin with p85α. In support of this, expression of mutant constructs mimicking phosphorylated and non-phosphorylated forms of occludin also demonstrated that tyrosine phosphorylation of occludin enhances PI3K activity at the leading edge, resulting in induced cell migration.

Finally, Chen and colleagues propose that occludin tyrosine phosphorylation is required for proper localization and activity of PI3K and aPKC–Par3–PATJ complex at the leading edge, which regulates lamellipodia formation and directional migration during wound healing.

Author: Iley Ozerlat - Copyright © 2010 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission