Cell biology of planar polarity transmission in the Drosophila wing.

The coordination of epithelial planar polarization is a critical step in the formation of well-ordered tissues. The process has been extensively studied in Drosophila, where genetic analysis has identified a set of "tissue polarity" genes that serve to coordinate planar polarity of cells in the developing wings, bristles and eyes. In the last several years, it has emerged that six of these genes encode junctional proteins. In the wing epithelium, these proteins undergo a polarized redistribution, forming separate proximal and distal cortical domains within each cell. The mechanisms that mediate cortical polarization and cue its direction have been the subject of intense investigation. Cuing the orientation of cortical polarization appears to depend on the atypical Cadherins Fat and Dachsous, although these proteins do not become polarized themselves, nor do they colocalize with components of polarized cortical domains. Interestingly, these Cadherins also act at earlier developmental stages to polarize tissue growth along the proximal-distal axis and it will be interesting to see whether these processes are mechanistically related. Once the axis of polarization is determined, cortical polarity seems to be propagated, at least locally, by a cascade of direct cell-cell interactions mediated by the proximal and distal domains. The cell biological mechanisms leading to polarization are still unclear, but the process depends on the control of Protein Phosphatase 2A activity by its regulatory subunit, Widerborst. Interestingly, Widerborst is found on a planar web of microtubules with connections to apical junctions, suggesting that these microtubules may have an important function in polarizing the cortex.