The Salvador-Warts-Hippo (Hippo) pathway is an evolutionarily conserved regulator of organ growth and cell fate. It performs these functions in epithelial and neural tissues of both insects and mammals, as well as in mammalian organs such as the liver and heart. Despite rapid advances in Hippo pathway research, a definitive role for this pathway in hematopoiesis has remained enigmatic. The hematopoietic compartments of Drosophila melanogaster and mammals possess several conserved features [1, 2]. D. melanogaster possess three types of hematopoietic cells that most closely resemble mammalian myeloid cells: plasmatocytes (macrophage-like cells), crystal cells (involved in wound healing), and lamellocytes (which encapsulate parasites). The proteins that control differentiation of these cells also control important blood lineage decisions in mammals [3-10]. Here, we define the Hippo pathway as a key mediator of hematopoiesis by showing that it controls differentiation and proliferation of the two major types of D. melanogaster blood cells, plasmatocytes and crystal cells. In animals lacking the downstream Hippo pathway kinase Warts, lymph gland cells overproliferated, differentiated prematurely, and often adopted a mixed lineage fate. The Hippo pathway regulated crystal cell numbers by both cell-autonomous and non-cell-autonomous mechanisms. Yorkie and its partner transcription factor Scalloped were found to regulate transcription of the Runx family transcription factor Lozenge, which is a key regulator of crystal cell fate. Further, Yorkie or Scalloped hyperactivation induced ectopic crystal cells in a non-cell-autonomous and Notch-pathway-dependent fashion.

The Salvador-Warts-Hippo (SWH) pathway is an important regulator of tissue growth that is frequently subverted in human cancer. The key oncoprotein of the SWH pathway is the transcriptional co-activator, Yes-associated protein (YAP). YAP promotes tissue growth and transformation of cultured cells by interacting with transcriptional regulatory proteins via its WW domains, or, in the case of the TEAD1-4 transcription factors, an N-terminal binding domain. YAP possesses a putative transactivation domain in its C-terminus that is necessary to stimulate transcription factors in vitro, but its requirement for YAP function has not been investigated in detail. Interestingly, whilst the WW domains and TEAD-binding domain are highly conserved in the Drosophila melanogaster YAP orthologue, Yorkie, the majority of the C-terminal region of YAP is not present in Yorkie. To investigate this apparent conundrum, we assessed the functional roles of the YAP and Yorkie C-termini. We found that these regions were not required for Yorkie's ability to drive tissue growth in vivo, or YAP's ability to promote anchorage-independent growth or resistance to contact inhibition. However, the YAP transactivation domain was required for YAP's ability to induce cell migration and invasion. Moreover, a role for the YAP transactivation domain in cell transformation was uncovered when the YAP WW domains were mutated together with the transactivation domain. This shows that YAP can promote cell transformation in a flexible manner, presumably by contacting transcriptional regulatory proteins either via its WW domains or its transactivation domain.

During tissue regeneration, cell proliferation replaces missing structures to restore organ function. Regenerative potential differs greatly between organs and organisms; for example some amphibians can regrow entire limbs whereas mammals cannot. The process of regeneration relies on several signaling pathways that control developmental tissue growth, and implies the existence of organ size-control checkpoints that regulate both developmental, and regenerative, growth. Here we explore the role of one such checkpoint, the Salvador–Warts–Hippo pathway, in tissue regeneration. The Salvador–Warts–Hippo pathway limits tissue growth by repressing the Yorkie transcriptional co-activator. Several proteins serve as upstream modulators of this pathway including the atypical cadherins, Dachsous and Fat, whilst the atypical myosin, Dachs, functions downstream of Fat to activate Yorkie. Using Drosophila melanogaster imaginal discs we show that Salvador–Warts–Hippo pathway activity is repressed in regenerating tissue and that Yorkie is rate-limiting for regeneration of the developing wing. We show that regeneration is compromised in dachs mutant wing discs, but that proteins in addition to Fat and Dachs are likely to modulate Yorkie activity in regenerating cells. In conclusion our data reveal the importance of Yorkie hyperactivation for tissue regeneration and suggest that multiple upstream inputs, including Fat–Dachsous signaling, sense tissue damage and regulate Yorkie activity during regeneration of epithelial tissues.

BACKGROUND:Epithelial neoplasias are associated with alterations in cell polarity and excessive cell proliferation, yet how these neoplastic properties are related to one another is still poorly understood. The study of Drosophila genes that function as neoplastic tumor suppressors by regulating both of these properties has significant potential to clarify this relationship.RESULTS:Here we show in Drosophila that loss of Scribbled (Scrib), a cell polarity regulator and neoplastic tumor suppressor, results in impaired Hippo pathway signaling in the epithelial tissues of both the eye and wing imaginal disc. scrib mutant tissue overgrowth, but not the loss of cell polarity, is dependent upon defective Hippo signaling and can be rescued by knockdown of either the TEAD/TEF family transcription factor Scalloped or the transcriptional coactivator Yorkie in the eye disc, or reducing levels of Yorkie in the wing disc. Furthermore, loss of Scrib sensitizes tissue to transformation by oncogenic Ras-Raf signaling, and Yorkie-Scalloped activity is required to promote this cooperative tumor overgrowth. The inhibition of Hippo signaling in scrib mutant eye disc clones is not dependent upon JNK activity, but can be significantly rescued by reducing aPKC kinase activity, and ectopic aPKC activity is sufficient to impair Hippo signaling in the eye disc, even when JNK signaling is blocked. In contrast, warts mutant overgrowth does not require aPKC activity. Moreover, reducing endogenous levels of aPKC or increasing Scrib or Lethal giant larvae levels does not promote increased Hippo signaling, suggesting that aPKC activity is not normally rate limiting for Hippo pathway activity. Epistasis experiments suggest that Hippo pathway inhibition in scrib mutants occurs, at least in part, downstream or in parallel to both the Expanded and Fat arms of Hippo pathway regulation.CONCLUSIONS:Loss of Scrib promotes Yorkie/Scalloped-dependent epithelial tissue overgrowth, and this is also important for driving cooperative tumor overgrowth with oncogenic Ras-Raf signaling. Whether this is also the case in human cancers now warrants investigation since the cell polarity function of Scrib and its capacity to restrain oncogene-mediated transformation, as well as the tissue growth control function of the Hippo pathway, are conserved in mammals.

The newly discovered Salvador-Warts-Hippo (SWH) pathway is a key regulator of tissue growth during both development and disease. The first identified components of this pathway represent core downstream effector proteins: the kinases Warts and Hippo; the adaptor proteins Salvador and Mats; and the transcriptional coactivator Yorkie. More recently, a surprising number of proteins have been implicated as upstream regulators of the SWH pathway, including: the planar cell polarity cadherins Fat and Dachsous; the FERM-domain proteins Expanded and Merlin; the WW-domain protein Kibra; the Ras-association family protein dRASSF; and the apicobasal polarity proteins lethal giant larvae, atypical protein kinase C and Crumbs. The identification of a large cohort of upstream regulatory proteins suggests that core SWH pathway proteins are poised to respond to diverse stimuli that must be integrated in a coordinated fashion. Here, we review the existing knowledge of upstream SWH pathway proteins and discuss possible mechanisms of action and signal integration.

Background How epithelial cells adopt their particular polarised forms is poorly understood. In a screen for genes regulating epithelial morphology in Drosophila , we identified sds22 , a conserved gene previously characterised in yeast. Results In the columnar epithelia of imaginal discs or follicle cells, mutation of sds22 causes contraction of cells along their apical-basal axis, resulting in a more cuboidal morphology. In addition, the mutant cells can also display altered cell polarity, forming multiple layers in follicle cells and leaving the epithelium in imaginal discs. In yeast, sds22 encodes a PP1 phosphatase regulatory subunit. Consistent with this, we show that Drosophila Sds22 binds to all four Drosophila PP1s and shares an overlapping phenotype with PP1beta9c . We also show that two previously postulated PP1 targets, Spaghetti Squash and Moesin are hyper-phosphorylated in sds22 mutants. This function is shared by the human homologue of Sds22, PPP1R7. Conclusion Sds22 is a conserved PP1 phosphatase regulatory subunit that controls cell shape and polarity.