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The evolutionarily conserved Hippo kinase signaling cascade governs cell proliferation, tissue differentiation and organ size, and can promote tumor growth and cancer metastasis when dysregulated. Unlike conventional signaling pathways driven by ligand-receptor binding to initiate downstream cascades, core Hippo kinases are activated not only by biochemical cues but also by mechanical ones generated from altered cell shape, cell polarity, cell-cell junctions or cell-extracellular matrix adhesion. In this review, we focus on recent advances showing how mechanical force acts through the actin cytoskeleton to regulate the Hippo pathway during cell movement and cancer invasion. We also discuss how this force affects YAP-dependent tissue growth and cell proliferation, and how disruption of that homeostatic relationship contributes to cancer metastasis.

Summary In collective cell migration, directional protrusions orient cells in response to external cues, which requires coordinated polarity among the migrating cohort. However, the molecular mechanism has not been well defined. Drosophila border cells (BCs) migrate collectively and invade via the confined space between nurse cells, offering an in vivo model to examine how group polarity is organized. Here, we show that the front/back polarity of BCs requires Rap1, hyperactivation of which disrupts cluster polarity and induces misoriented protrusions and loss of asymmetry in the actin network. Conversely, hypoactive Rap1 causes fewer protrusions and cluster spinning during migration. A forward genetic screen revealed that downregulation of the Hippo (Hpo) pathway core components hpo or mats enhances the Rap1 V12 -induced migration defect and misdirected protrusions. Mechanistically, association of Rap1 V12 with the kinase domain of Hpo suppresses its activity, which releases Hpo signaling-mediated suppression of F-actin elongation, promoting cellular protrusions in collective cell migration.

How temporal signals from the steroid hormone ecdysone are integrated with JAK/STAT-mediated spatial control of Drosophila border-cell migration has been unclear. JAK/STAT represses Abrupt, which in turn attenuates ecdysone signalling by interacting with the ecdysone receptor coactivator Taiman. During development, elaborate patterns of cell differentiation and movement must occur in the correct locations and at the proper times. Developmental timing has been studied less than spatial pattern formation, and the mechanisms integrating the two are poorly understood. Border-cell migration in the Drosophila ovary occurs specifically at stage 9. Timing of the migration is regulated by the steroid hormone ecdysone, whereas spatial patterning of the migratory population requires localized activity of the JAK–STAT pathway. Ecdysone signalling is patterned spatially as well as temporally, although the mechanisms are not well understood. In stage 9 egg chambers, ecdysone signalling is highest in anterior follicle cells including the border cells. We identify the gene abrupt as a repressor of ecdysone signalling and border-cell migration. Abrupt protein is normally lost from border-cell nuclei during stage 9, in response to JAK–STAT activity. This contributes to the spatial pattern of the ecdysone response. Abrupt attenuates ecdysone signalling by means of a direct interaction with the basic helix–loop–helix (bHLH) domain of the P160 ecdysone receptor coactivator Taiman (Tai). Taken together, these findings provide a molecular mechanism by which spatial and temporal cues are integrated.

Cell motility makes essential contributions to normal embryonic development and homeostasis. It is also thought to contribute in important ways to tumor metastasis. Because of this dual importance, cell migration has been extensively studied. The fruit fly Drosophila melanogaster has served as an important model organism for genetic analysis of many aspects of developmental biology, including cell migration. Here we describe the various types of cell movements that have been studied in detail, which represent models for epithelial-to-mesenchymal transition, transepithelial migration, inflammation, wound healing and invasion. We summarize what has been learned about the molecular control of cell migration from genetic studies in the fly. In addition, we describe recent efforts to model tumor metastasis directly in Drosophila by expressing oncogenes and/or mutating tumor suppressor genes. Together these studies suggest that Drosophila has much to offer as a model for varied aspects of tumor metastasis.

Asymmetric division of stem cells results in both self-renewal and differentiation of daughters. Understanding the molecules and mechanisms that govern differentiation of specific cell types from adult tissue stem cells is a major challenge in developmental biology and regenerative medicine. Drosophila follicle stem cells (FSCs) represent an excellent model system to study adult stem cell behavior; however, the earliest stages of follicle cell differentiation remain largely mysterious. Here we identify Castor (Cas) as a nuclear protein that is expressed in FSCs and early follicle cell precursors and then is restricted to differentiated polar and stalk cells once egg chambers form. Cas is required for FSC maintenance and polar and stalk cell fate specification. Eyes absent (Eya) is excluded from polar and stalk cells and represses their fate by inhibiting Cas expression. Hedgehog signaling is essential to repress Eya to allow Cas expression in polar and stalk cells. Finally, we show that the complementary patterns of Cas and Eya reveal the gradual differentiation of polar and stalk precursor cells at the earliest stages of their development. Our studies provide a marker for cell fates in this model and insight into the molecular and cellular mechanisms by which FSC progeny diverge into distinct fates.

This protocol describes a method for the dissection of egg chambers from intact Drosophila females and culture conditions that permit live imaging of them, with a particular emphasis on stage 9. This stage of development is characterized by oocyte growth and patterning, outer follicle cell rearrangement and migration of border cells. Although in vitro culture of egg chambers of later developmental stages has long been possible, until recently stage 9 egg chambers could only be kept alive for short periods, did not develop normally, and border cell migration failed entirely. We have established culture conditions that support overall egg chamber development including border cell migration in vitro. This protocol makes possible direct observation of molecular and cellular dynamics in both wild-type and mutant egg chambers, and opens the door to testing of pharmacological inhibitors and the use of biosensors. The entire protocol takes approximately 24 h while the preparation of egg chambers for live imaging requires only 15-20 min.