Jaffe et al. show how a polarizing protein ensures that the cells face the right direction.
A newborn epithelial cell in, say, the intestinal lining will polarize so that its apical surface is oriented toward the gut lumen. The protein Cdc42 helps set up polarity in a range of organisms and cell types, and previous studies suggested that it does the same in epithelial cells. The mechanism remained obscure, however.
To determine how Cdc42 differentiates top from bottom, Jaffe et al. grew human intestinal cells in a three-dimensional culture. The cells divided repeatedly to form a sphere, with their apical surfaces directed toward a central lumen.
When the researchers used RNAi to slash Cdc42 levels, they expected the cells to become symmetric. Instead, the cells polarized, but they clustered around multiple lumens. That result indicates that rather than instigating polarization, Cdc42 determines where it occurs.
Cdc42 orients cells by helping position the mitotic spindle so that it is perpendicular to the middle of the sphere. Thus, division splits each cell radially, and the apical surface of each daughter remains in the center of the cluster. If Cdc42 is missing, the spindle orients randomly, and the apical surface of a particular daughter cell can end up outside the center—hence, the extra lumens.
The researchers also found that the apical surface forms at the last point of contact between dividing cells. They showed that the midbody, the tether between daughter cells, is located asymmetrically so that all apical surfaces form at the center of the cluster. Determining whether Cdc42 controls midbody position will require further research.