A novel role of the hippo signalling pathway during asymmetric cell division

Resumen

Asymmetric cell division is a universal mechanism to generate cellular diversity during development and a crucial process in cancer and stem cell biology. Drosophila neuroblasts, the neural stem cells of the central nervous system, divide asymmetrically to give rise to another neuroblast that keeps on proliferating and a ganglion mother cell that is committed to initiate a process of differentiation. Some years ago, we isolated Warts (Lats1/2 in mammals) in a yeast two-hybrid screening as a potential partner of the PDZ protein Canoe, which we showed is a key player in modulating asymmetric cell division. Warts is a serine/threonine kinase that function at the core kinase cascade of the Hippo tumor suppressor signalling pathway involved in regulating cell growth, apoptosis and organ size. Hence, we decided to analyze whether the Hippo pathway functions in the process of asymmetric cell division. In this work, we have found specific genetic interactions between Canoe and all components of the Hippo pathway. Additionally, warts genetically interacts with multiple components specifically involved in modulating asymmetric cell division, such as aPKC, Inscuteable and Numb. Moreover, warts loss-of-function mutant embryos display clear defects in the localization of the apical proteins aPKC, Bazooka/Par3 and Canoe, as well as in the basal cell-fate determinants Numb and Prospero. Failures in the mitotic spindle orientation in metaphase neuroblasts and in the generation of unequeal-sized daughter cells are also detected. Finally, co-immunoprecipitation experiments have revealed that Warts is forming a complex in vivo with Inscuteable and Bazooka, and in vitro kinase assay experiments have shown that Warts is phosphorylating both Canoe and Bazooka. Not only neuroblasts but also muscle/heart progenitors require Warts and other components of the Hippo pathway for their asymmetric division. Altogether, these data unveil a novel function for the Hippo signalling pathway in asymmetric cell division.