Hippo signaling pathway and organ size control

Initially discovered in Drosophila, the Hippo (Hpo) pathway has been recognized as a conserved signaling pathway that controls organ size during development by restricting cell growth and proliferation and by promoting apoptosis. In addition, abnormal activities of several Hpo pathway components have been implicated in human cancer. Here, we review the current understanding of the molecular and cellular basis of Hpo signaling in development and tumorigenesis, and discuss how the Hpo pathway integrates spatial and temporal signals to control tissue growth and organ size.     

Mob as tumor suppressor is activated at the cell membrane to control tissue growth and organ size in Drosophila

Growth inhibition mediated by Hippo (Hpo) signaling is essential for tissue growth and organ size control in Drosophila. However, the cellular mechanism by which the core components like Mob as tumor suppressor (Mats) and Warts (Wts) protein kinase are activated is poorly understood. In this work, we found that the endogenous Mats is located at the plasma membrane in developing tissues. Membrane targeting constitutively activates Mats to promote apoptosis and reduce cell proliferation, which leads to reduced tissue growth and organ size. Moreover, the ability of membrane-targeted Mats to inhibit tissue growth required the wts gene activity and Wts kinase activity was increased by the activated Mats in developing tissues. Consistent with the idea that Mats is a key component of the Hpo pathway, Mats is required and sufficient to regulate Yki nuclear localization. These results support a model in which the plasma membrane is an important site of action for Mats tumor suppressor to control tissue growth and organ size.     

Mob as tumor suppressor is activated by Hippo kinase for growth inhibition in Drosophila

Tissue growth and organ size are determined by coordinated cell proliferation and apoptosis in development. Recent studies have demonstrated that Hippo (Hpo) signaling plays a crucial role in coordinating these processes by restricting cell proliferation and promoting apoptosis. Here we provide evidence that the Mob as tumor suppressor protein, Mats, functions as a key component of the Hpo signaling pathway. We found that Mats associates with Hpo in a protein complex and is a target of the Hpo serine/threonine protein kinase. Mats phosphorylation by Hpo increases its affinity with Warts (Wts)/large tumor suppressor (Lats) serine/threonine protein kinase and ability to upregulate Wts catalytic activity to target downstream molecules such as Yorkie (Yki). Consistently, our epistatic analysis suggests that mats acts downstream of hpo. Coexpression analysis indicated that Mats can indeed potentiate Hpo-mediated growth inhibition in vivo. Our results support a model in which Mats is activated by Hpo through phosphorylation for growth inhibition, and this regulatory mechanism is conserved from flies to mammals.     

The Salvador partner Hippo promotes apoptosis and cell-cycle exit in Drosophila

Tissue growth during animal development is tightly controlled so that the organism can develop harmoniously. The salvador (sav) gene, which encodes a scaffold protein, has been shown to restrict cell number by coordinating cell-cycle exit and apoptosis during Drosophila development. Here we identify Hippo (Hpo), the Drosophila orthologue of the mammalian MST1 and MST2 serine/threonine kinases, as a partner of Sav. Loss of hpo function leads to sav-like phenotypes, whereas gain of hpo function results in the opposite phenotype. Whereas Sav and Hpo normally restrict cellular quantities of the Drosophila inhibitor of apoptosis protein DIAP1, overexpression of Hpo destabilizes DIAP1 in cell culture. We show that DIAP1 is phosphorylated in a Hpo-dependent manner in S2 cells and that Hpo can phosphorylate DIAP1 in vitro. Thus, Hpo may promote apoptosis by reducing cellular amounts of DIAP1. In addition, we show that Sav is an unstable protein that is stabilized by Hpo. We propose that Hpo and Sav function together to restrict tissue growth in vivo.     

Hippo promotes proliferation arrest and apoptosis in the Salvador/Warts pathway

Proliferation and apoptosis must be precisely regulated to form organs with appropriate cell numbers and to avoid tumour growth. Here we show that Hippo (Hpo), the Drosophila homologue of the mammalian Ste20-like kinases, MST1/2, promotes proper termination of cell proliferation and stimulates apoptosis during development. hpo mutant tissues are larger than normal because mutant cells continue to proliferate beyond normal tissue size and are resistant to apoptotic stimuli that usually eliminate extra cells. Hpo negatively regulates expression of Cyclin E to restrict cell proliferation, downregulates the Drosophila inhibitor of apoptosis protein DIAP1, and induces the proapoptotic gene head involution defective (hid) to promote apoptosis. The mutant phenotypes of hpo are similar to those of warts (wts), which encodes a serine/threonine kinase of the myotonic dystrophy protein kinase family, and salvador (sav), which encodes a WW domain protein that binds to Wts. We find that Sav binds to a regulatory domain of Hpo that is essential for its function, indicating that Hpo acts together with Sav and Wts in a signalling module that coordinately regulates cell proliferation and apoptosis.     

The mob as tumor suppressor gene is essential for early development and regulates tissue growth in Drosophila

Studies in Drosophila have defined a new growth inhibitory pathway mediated by Fat (Ft), Merlin (Mer), Expanded (Ex), Hippo (Hpo), Salvador (Sav)/Shar-pei, Warts (Wts)/Large tumor suppressor (Lats), and Mob as tumor suppressor (Mats), which are all evolutionarily conserved in vertebrate animals. We previously found that the Mob family protein Mats functions as a coactivator of Wts kinase. Here we show that mats is essential for early development and is required for proper chromosomal segregation in developing embryos. Mats is expressed at low levels ubiquitously, which is consistent with the role of Mats as a general growth regulator. Like mammalian Mats, Drosophila Mats colocalizes with Wts/Lats kinase and cyclin E proteins at the centrosome. This raises the possibility that Mats may function together with Wts/Lats to regulate cyclin E activity in the centrosome for mitotic control. While Hpo/Wts signaling has been implicated in the control of cyclin E and diap1 expression, we found that it also modulates the expression of cyclin A and cyclin B. Although mats depletion leads to aberrant mitoses, this does not seem to be due to compromised mitotic spindle checkpoint function.     

Uif, a large transmembrane protein with EGF-like repeats, can antagonize Notch signaling in Drosophila

Background

Notch signaling is a highly conserved pathway in multi-cellular organisms ranging from flies to humans. It controls a variety of developmental processes by stimulating the expression of its target genes in a highly specific manner both spatially and temporally. The diversity, specificity and sensitivity of the Notch signaling output are regulated at distinct levels, particularly at the level of ligand-receptor interactions.

Methodology/Principal Findings

Here, we report that the Drosophila gene uninflatable (uif), which encodes a large transmembrane protein with eighteen EGF-like repeats in its extracellular domain, can antagonize the canonical Notch signaling pathway. Overexpression of Uif or ectopic expression of a neomorphic form of Uif, Uif*, causes Notch signaling defects in both the wing and the sensory organ precursors. Further experiments suggest that ectopic expression of Uif* inhibits Notch signaling in cis and acts at a step that is dependent on the extracellular domain of Notch. Our results suggest that Uif can alter the accessibility of the Notch extracellular domain to its ligands during Notch activation.

Conclusions/Significance

Our study shows that Uif can modulate Notch activity, illustrating the importance of a delicate regulation of this signaling pathway for normal patterning.