The Hippo (Hpo; called Mst in vertebrates) signaling pathway limits cell growth by preventing the nuclear translocation of the transcriptional coactivator Yorkie (Yki; Yap in vertebrates), which promotes the expression of genes that stimulate cell proliferation. When activated, the kinase Hpo phosphorylates and activates the kinase Warts (Wts; Lats in vertebrates), which then phosphorylates Yki. Mats is a cofactor for Wts, and the scaffold protein Sav promotes the association between Hpo and Wts. Two groups report that Hippo signaling also affects spindle orientation in two different cellular contexts in Drosophila melanogaster. Neuroblasts are neural stem cells that divide asymmetrically to produce one self-renewing neuroblast and one daughter cell that divides once more to generate two neurons or glial cells. Asymmetric inheritance of determinants of cell fate requires the apically localized protein Partner of Inscuteable (Pins), which interacts with the Par complex through the adaptor protein Inscuteable (Insc). Pins also interacts with Discs large (Dlg) and Mushroom body defect (Mud), both of which also interact with the spindle pole. Thus, Pins orients cell divisions by ensuring that one spindle pole localizes to the apical side of the neuroblast.
Keder et al. found that Hpo, Wts, Mats, and Sav were required for proper asymmetric division of neuroblasts in the Drosophila embryo. In wts mutants, asymmetric localization of several proteins important for asymmetric division or daughter cell fate was reduced or absent, and neuroblasts had improperly oriented mitotic spindles. Wts also interacted genetically with Canoe (Cno; Afadin in vertebrates), which cooperates with Pins to control the localization of Mud. In vitro, Wts phosphorylated the Par3 homolog Bazooka, and the authors identified three sites at which Wts phosphorylated Cno. Mutating the Wts phosphorylation sites in Cno prevented the proper localization of Cno and Dlg at the apical cortex. Genetic experiments also implicated Wts and Mats in the asymmetric divisions of embryonic muscle and heart progenitor cells. In a related paper, Dewey et al. found that Hpo, Wts, and Sav, but not Yki, were required for asymmetric division when cultured Drosophila S2 cells were engineered to divide asymmetrically in a Pins-dependent manner. Wts promoted the colocalization of Mud and Pins during mitosis in these asymmetrically dividing S2 cells. In vitro, Wts phosphorylated the coiled-coil domain of Mud, a domain that may be involved in regulating the interaction between Mud and Pins. Experiments in Drosophila wing imaginal discs indicated that Wts-mediated phosphorylation of Mud was also required for the recruitment of Mud to sites of Pin localization and for properly oriented cell divisions in this epithelial tissue.
Thus, Hippo signaling influences spindle orientation through the Wts-mediated phosphorylation of the Pins-interacting proteins Cno and Mud. Reduced Hippo signaling promotes tumorigenesis by stimulating cell proliferation in a Yki-dependent manner, and these findings suggest that reduced Hippo signaling could also contribute to tumor formation by disrupting tissue organization or interfering with the asymmetric divisions that are important for controlling the balance between differentiation and self-renewal in stem cells.
A. Keder, N. Rives-Quinto, B. L. Aerne, M. Franco, N. Tapon, A. Carmena, The Hippo pathway core cassette regulates asymmetric cell division. Curr. Biol. 25, 2739–2750 (2015). [Online Journal]
E. B. Dewey, D. Sanchez, C. A. Johnston, Warts phosphorylates Mud to promote Pins-mediated mitotic spindle orientation in Drosophila, independent of Yorkie. Curr. Biol. 25, 2751–2762 (2015). [Online Journal]