The Hippo signalling pathway regulates organ growth: activation of the pathway inhibits proliferation and promotes apoptosis. The core pathway consists of two kinases, Hippo and Warts, along with their adaptor proteins. Warts phosphorylates and thus inactivates the transcriptional co-activator Yorkie, whose target genes include regulators of cell proliferation and survival. Upstream of Warts, various factors impact on kinase activity, including cytoskeletal and polarity regulators. While Hippo signalling is best known for regulating growth, on p. Madhuri Kango-Singh, Amit Singh and colleagues define a function for Yorkie in regulating differentiation in the Drosophila eye. Manipulation of the Hippo pathway is well known to regulate eye size, but here the authors find that mis-expression of Yorkie inhibits neuronal differentiation. Mechanistically, they find that Yorkie promotes the expression of Wingless, a known inhibitor of morphogenetic furrow progression and hence differentiation. Intriguingly, the two functions of the pathway – in regulating growth and in controlling differentiation – appear to be separable: while growth is regulated by the Hippo kinase, differentiation is controlled by the upstream regulators Fat and Dachs, which bypass Hippo. These data add to an emerging picture of Hippo signalling as a regulator not only of growth, but also of other developmental processes.
