Modulating F-actin organization induces organ growth by affecting the Hippo pathway

Homeostasis of the Drosophila adult retina by actin-capping protein and the Hippo pathway

The conserved Hippo signaling pathway regulates multiple cellular events, including tissue growth, cell fate decision and neuronal homeostasis. While the core Hippo kinase module appears to mediate all the effects of the pathway, various upstream inputs have been identified depending on tissue context. We have recently shown that, in the Drosophila wing imaginal disc, actin-Capping Protein and Hippo pathway activities inhibit F-actin accumulation. In turn, the reduction in F-actin sustains Hippo pathway activity, preventing Yorkie nuclear translocation and the upregulation of proliferation and survival genes. Here, we investigate the role of Capping Protein in growth-unrelated events controlled by the Hippo pathway. We provide evidence that loss of Capping Protein induces degeneration of the adult Drosophila retina through misregulation of the Hippo pathway. We propose a model by which F-actin dynamics might be involved in all processes that require the activity of the core Hippo kinase module.

Hippo signaling in Drosophila: recent advances and insights

The Hippo signaling pathway emerged from studies of Drosophila tumor suppressor genes, and is now appreciated as a major growth control pathway in vertebrates as well as arthropods. As a recently discovered pathway, key components of the pathway are continually being identified, and new insights into how the pathway is regulated and deployed are arising at a rapid pace. Over the past year and a half, significant advances have been made in our understanding of upstream regulatory inputs into Hippo signaling, key negative regulators of Hippo pathway activity have been identified, and important roles for the pathway in regeneration have been described. This review describes these and other advances, focusing on recent progress in our understanding of Hippo signaling that has come from continued studies in Drosophila.

Hippo pathway regulation by cell morphology and stress fibers

The Hippo signaling pathway plays an important role in regulation of cell proliferation. Cell density regulates the Hippo pathway in cultured cells; however, the mechanism by which cells detect density remains unclear. In this study, we demonstrated that changes in cell morphology are a key factor. Morphological manipulation of single cells without cell-cell contact resulted in flat spread or round compact cells with nuclear or cytoplasmic Yap, respectively. Stress fibers increased in response to expanded cell areas, and F-actin regulated Yap downstream of cell morphology. Cell morphology- and F-actin-regulated phosphorylation of Yap, and the effects of F-actin were suppressed by modulation of Lats. Our results suggest that cell morphology is an important factor in the regulation of the Hippo pathway, which is mediated by stress fibers consisting of F-actin acting upstream of, or on Lats, and that cells can detect density through their resulting morphology. This cell morphology (stress-fiber)-mediated mechanism probably cooperates with a cell-cell contact (adhesion)-mediated mechanism involving the Hippo pathway to achieve density-dependent control of cell proliferation.