The Hippo pathway regulates apical-domain size independently of its growth-control function.

The Hippo pathway, identified in Drosophila and conserved in vertebrates, regulates tissue growth by promoting cell cycle exit and apoptosis. In addition to their well-characterised overproliferation phenotype, adult Drosophila epithelial cells mutant for the kinases Hippo and Warts have hypertrophic apical domains. Here we examine the molecular basis of this apical hypertrophy and its impact on cell proliferation. In the wing imaginal disc epithelium, we observe increased staining for members of the apical polarity complexes aPKC and Crumbs as well as adherens junction components when Hippo activity is compromised, while basolateral markers are not affected. This increase in apical proteins is correlated with a hypertrophy of the apical domain and adherens junctions. The cell surface localisation of the Notch receptor is also increased in mutant clones, opening the possibility that aberrant receptor signalling may participate in overgrowth of hpo-deficient tissue. Interestingly, however, although the polarity determinant Crumbs is required for the accumulation of apical proteins, this does not appear to significantly contribute to the overproliferation defect elicited by loss of Hippo signalling. Therefore, Hippo signalling controls growth and apical domain size by distinct mechanisms.