Epithelial sheets exhibit several defining characteristics, including mechanically strong cell-cell junctions and a coordinated cell polarity, which maintain epithelial integrity and impart correct cell shape and tissue organization. Key to the acquisition of these characteristics is the intimate interplay between cell-cell adhesion, polarity proteins and regulators of the actin cytoskeleton.
By combining genetic and cell biological analyses we show that epithelial cells within the fly dorsal thorax possess distinct classes of dynamic protrusion along their apical-basal axis. Cells possess apical microvilli, lateral sheet-like protrusions at an intermediate level, and filopodia and lamellipodia at the base of the cell. Using in vivo inducible constructs that can sense or modify Rac activity, we demonstrate an apicobasal gradient of Rac activity that is required to correctly form and position these distinct classes of protrusion along the apicobasal axis of the cell. Apicobasal polarity is required to form this gradient, and we show that we can modify the Rac activity gradient in genetic mutants for specific polarity proteins, with consequent changes in protrusion form and position. We additionally show, using photo-activatable Rac constructs, that it is the level of Rac activity that determines protrusion form, with high levels of Rac activity required to form filopodia and lower levels required to form lamellipodia. Following exposure to pulses of laser light, thereby increasing Rac activity in photo-activatable-Rac expressing cells, we are able to convert lamellipodial protrusions to filopodial protrusions in the living animal.
It has long been known that polarity proteins are essential in maintaining epithelial cell shape, with loss of apicobasal polarity being a prerequisite for epithelial-to-mesenchymal transitions, and abnormal cell invasiveness. Our data provide a mechanism by which polarity proteins can influence Rac activity and the actin cytoskeleton, thereby ensuring the maintenance of epithelial cell shape, and consequently the architecture of the epithelium as a whole.