Planar cell polarity (PCP) regulates basal body (BB) docking and positioning during cilia formation, but the underlying mechanisms remain elusive. In this study, we investigate the uncharacterized gene Flattop (Fltp) that is transcriptionally activated during PCP acquisition in ciliated tissues. Fltp knock-out mice show BB docking and ciliogenesis defects in multiciliated lung cells. Furthermore, Fltp is necessary for kinocilium positioning in monociliated inner ear hair cells. In these cells, the core PCP molecule Dishevelled 2, the BB/spindle positioning protein Dlg3, and Fltp localize directly adjacent to the apical plasma membrane, physically interact and surround the BB at the interface of the microtubule and actin cytoskeleton. Dlg3 and Fltp knock-outs suggest that both cooperatively translate PCP cues for BB positioning in the inner ear. Taken together, the identification of novel BB/spindle positioning components as potential mediators of PCP signaling might have broader implications for other cell types, ciliary disease, and asymmetric cell division.
Epithelial tissues are sheets of cells that line the surface of many parts of the body, including the airways and the inner ear. Small hair-like structures called cilia can be found on the top surface of many epithelial cells and are arranged in a precise, ordered pattern. Such patterning ensures that cilia can work in a co-ordinated manner, for example by beating together to help clearing mucus from airways.
Cilia grow out from ‘basal bodies’ and, like many other important structures in a cell, these basal bodies must be oriented along the correct side of an epithelial tissue. This is achieved by ‘planar cell polarity signaling’, which makes sure that the structures inside a cell are correctly aligned, and ensures that polarized cells themselves are correctly oriented across the epithelial tissue. Disruption of this signaling can result in developmental defects.
Some proteins help to establish polarity in a cell by altering the cell's cytoskeleton—the structural support and transport network of the cell. A ‘core’ complex of proteins then coordinates how the cells are arranged throughout the epithelial tissue. Although many of the proteins involved in each of these roles are known, how they interact with each other to establish planar cell polarity remains poorly understood.
Now, Gegg et al. report that, in mice, a protein called Flattop functions to position basal bodies—and thus cilia—by working together with another protein called Dlg3. In mice that cannot produce Flattop, cilia formation is defective in the lung, and the cilia in the inner ear are positioned incorrectly. Gegg et al. found that in the inner ear, Flattop and Dlg3 physically interact with each other and two other proteins—including one of the core proteins involved in planar cell polarity. This protein complex then surrounds the basal bodies at the point where they connect to the cell's cytoskeleton.
Future challenges will be to clarify how the protein complex anchors to the cytoskeleton and how it interacts with other core planar cell polarity proteins in the cells of the inner ear. It will also be important to see whether this protein complex fulfills a similar role in other ciliated epithelial tissues.