Unpolarized morphology of a keratinocyte with impaired Par-Tiam1 polarity signalling illustrated by microtubule staining.

Image courtesy of Dr. John G. Collard, The Netherlands Cancer Institute, Amsterdam, The Netherlands.

Cell polarity is at the centre of many biological functions - from the maintenance of tissue integrity to directional cell migration. Conserved protein complexes such as the Par (partitioning-defective) polarity complex, which consists of Par3, Par6 and protein kinase PKC, function to establish and maintain cell polarity. Par and the guanine nucleotide exchange factor Tiam1 (T lymphoma invasion and metastasis 1) are known to control the apical-basal polarity of contacting epithelial cells. In Current Biology, John Collard and colleagues now report that the Par-Tiam1 complex also regulates polarized migration of free epithelial cells.

The authors observed that migrating Tiam1^-/- keratinocytes lost the front-rear polarity typical of wild-type (WT) cells, and that although motility per se was unaffected, mutant cells showed an altered pattern of migration. Tiam1^-/- cells changed direction frequently and migrated in a less linear (persistent) way than WT cells. Exposure to a gradient of epidermal growth factor substantially increased migration of WT keratinocytes but had no effect on Tiam1 knockout cells, suggesting that Tiam1 is required both for persistent and chemotactic epithelial cell migration.

As Tiam1 associates with the Par complex to regulate apical-basal polarity in non-migrating keratinocytes, Collard and colleagues tested whether this association also applied to directional migration. Downregulation of endogenous Par3 impaired front-rear polarization of WT cells, and reduced persistent migration as well as chemotaxis. A similar behaviour was observed upon inhibition of PKC signalling, the main effector of the Par polarity complex. Depletion of Par3 and/or PKC activity had no additional effect on Tiam1-deficient cells, indicating that Par3 and PKC function together with Tiam1.

The interaction of Tiam1 with both Par3 and PKC in migrating keratinocytes was confirmed by pull-down experiments, which also showed that PKC was activated upon association of Tiam1 with the Par complex. Furthermore, endogenous Par3 and PKC were enriched at the leading edge of polarized cells, and co-localized with green fluorescent protein (GFP)-tagged Tiam1, suggesting that these proteins all play a role in establishing front-rear cellular asymmetry.

Collard and colleagues went on to study how the cytoskeleton may affect persistent migration. Actin depolymerization resulted in complete abrogation of motility, whereas anti-microtubule drugs induced a loss of polarity, causing cells to migrate in a similar manner to when Par-Tiam1 signalling was impaired. WT keratinocytes were less susceptible to microtubule-depolymerizing drugs and contained more stable acetylated microtubules compared with Tiam1^-/- or PKC-inhibited cells, suggesting that the Par-Tiam1 complex controls front-rear polarity and persistence by affecting microtubule stability.

Taken together, these results indicate that the Par-Tiam1 complex functions to maintain front-rear polarity in migrating epithelial cells via the microtubule cytoskeleton. Thus the Par-Tiam1 complex controls two different polarization processes in the same cell type according to the context: it stabilizes front-rear polarization in free keratinocytes, thereby stimulating persistent migration, and it is also required to establish long-lasting apical-basal polarity in contacting cells.

Author: Kim Baumann - Copyright © 2007 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission