Hippo signaling in Drosophila: recent advances and insights

The Strip-Hippo Pathway Regulates Synaptic Terminal Formation by Modulating Actin Organization at the Drosophila Neuromuscular Synapses

Synapse formation requires the precise coordination of axon elongation, cytoskeletal stability, and diverse modes of cell signaling. The underlying mechanisms of this interplay, however, remain unclear. Here, we demonstrate that Strip, a component of the striatin-interacting phosphatase and kinase (STRIPAK) complex that regulates these processes, is required to ensure the proper development of synaptic boutons at the Drosophila neuromuscular junction. In doing so, Strip negatively regulates the activity of the Hippo (Hpo) pathway, an evolutionarily conserved regulator of organ size whose role in synapse formation is currently unappreciated. Strip functions genetically with Enabled, an actin assembly/elongation factor and the presumptive downstream target of Hpo signaling, to modulate local actin organization at synaptic termini. This regulation occurs independently of the transcriptional co-activator Yorkie, the canonical downstream target of the Hpo pathway. Our study identifies a previously unanticipated role of the Strip-Hippo pathway in synaptic development, linking cell signaling to actin organization.

The Hippo Pathway Targets Rae1 to Regulate Mitosis and Organ Size and to Feed Back to Regulate Upstream Components Merlin, Hippo, and Warts

Hippo signaling acts as a master regulatory pathway controlling growth, proliferation, and apoptosis and also ensures that variations in proliferation do not alter organ size. How the pathway coordinates restricting proliferation with organ size control remains a major unanswered question. Here we identify Rae1 as a highly-conserved target of the Hippo Pathway integrating proliferation and organ size. Genetic and biochemical studies in Drosophila cells and tissues and in mammalian cells indicate that Hippo signaling promotes Rae1 degradation downstream of Warts/Lats. In proliferating cells, Rae1 loss restricts cyclin B levels and organ size while Rae1 over-expression increases cyclin B levels and organ size, similar to Hippo Pathway over-activation or loss-of-function, respectively. Importantly, Rae1 regulation by the Hippo Pathway is crucial for its regulation of cyclin B and organ size; reducing Rae1 blocks cyclin B accumulation and suppresses overgrowth caused by Hippo Pathway loss. Surprisingly, in addition to suppressing overgrowth, reducing Rae1 also compromises survival of epithelial tissue overgrowing due to loss of Hippo signaling leading to a tissue “synthetic lethality” phenotype. Excitingly, Rae1 plays a highly conserved role to reduce the levels and activity of the Yki/YAP oncogene. Rae1 increases activation of the core kinases Hippo and Warts and plays a post-transcriptional role to increase the protein levels of the Merlin, Hippo, and Warts components of the pathway; therefore, in addition to Rae1 coordinating organ size regulation with proliferative control, we propose that Rae1 also acts in a feedback circuit to regulate pathway homeostasis.

Exquisite control of organ size is critical during animal development and its loss results in pathological conditions. The Hippo Tumor Suppressor Pathway coordinates regulation of proliferation, growth, apoptosis, and autophagy to determine and maintain precise control of organ size. However, the genes responsible for Hippo-mediated regulation of mitosis or coordination of proliferation within organ size control have evaded characterization. Here, we describe Rae1, an essential WD-repeat containing protein, as a new organ size regulator. By genetic analysis, we show that Rae1 acts downstream of the Hippo Pathway to regulate mitotic cyclins and organ size. In contexts where organ size control is lost by compromised Hippo signaling, we show that there is a requirement for Rae1 that is distinct from the requriement for Yki: reducing Yki levels causes suppression of overgrowth, while reducing Rae1 levels dramatically compromises the survival of Hippo-deficient tissue. Lastly, our studies of Rae1 uncovered a potential post-transcriptional feedback loop that reinforces Yorkie-mediated transcriptional feedback for the Hippo Pathway.

A critical role for NF2 and the Hippo pathway in branching morphogenesis

Branching morphogenesis is a complex biological process common to the development of most epithelial organs. Here we demonstrate that NF2, LATS1/2 and YAP play a critical role in branching morphogenesis in the mouse kidney. Removal of Nf2 or Lats1/2 from the ureteric bud (UB) lineage causes loss of branching morphogenesis that is rescued by loss of one copy of Yap and Taz, and phenocopied by YAP overexpression. Mosaic analysis demonstrates that cells with high YAP expression have reduced contribution to UB tips, similar to Ret^−/− cells, and that YAP suppresses RET signalling and tip identity. Conversely, Yap/Taz UB-deletion leads to cyst-like branching and expansion of UB tip markers, suggesting a shift towards tip cell identity. Based on these data we propose that NF2 and the Hippo pathway locally repress YAP/TAZ activity in the UB to promote subsequent splitting of the tip to allow branching morphogenesis.

Branching morphogenesis is essential for the formation of most epithelial organs. Here, the authors show that Neurofibromatosis 2 (NF2), the Hippo pathway kinases LATS1 and LATS2, and the transcriptional co-activators YAP and TAZ control tip identity, RET signalling and branching morphogenesis in the mouse kidney.

Regulation of Hippo signalling by p38 signalling

The Hippo signalling pathway has a crucial role in growth control during development, and its dysregulation contributes to tumorigenesis. Recent studies uncover multiple upstream regulatory inputs into Hippo signalling, which affects phosphorylation of the transcriptional coactivator Yki/YAP/TAZ by Wts/Lats. Here we identify the p38 mitogen-activated protein kinase (MAPK) pathway as a new upstream branch of the Hippo pathway. In Drosophila, overexpression of MAPKK gene licorne (lic), or MAPKKK gene Mekk1, promotes Yki activity and induces Hippo target gene expression. Loss-of-function studies show that lic regulates Hippo signalling in ovary follicle cells and in the wing disc. Epistasis analysis indicates that Mekk1 and lic affect Hippo signalling via p38b and wts. We further demonstrate that the Mekk1-Lic-p38b cascade inhibits Hippo signalling by promoting F-actin accumulation and Jub phosphorylation. In addition, p38 signalling modulates actin filaments and Hippo signalling in parallel to small GTPases Ras, Rac1, and Rho1. Lastly, we show that p38 signalling regulates Hippo signalling in mammalian cell lines. The Lic homologue MKK3 promotes nuclear localization of YAP via the actin cytoskeleton. Upregulation or downregulation of the p38 pathway regulates YAP-mediated transcription. Our work thus reveals a conserved crosstalk between the p38 MAPK pathway and the Hippo pathway in growth regulation.

The MST/Hippo Pathway and Cell Death: A Non-Canonical Affair

The MST/Hippo signalling pathway was first described over a decade ago in Drosophila melanogaster and the core of the pathway is evolutionary conserved in mammals. The mammalian MST/Hippo pathway regulates organ size, cell proliferation and cell death. In addition, it has been shown to play a central role in the regulation of cellular homeostasis and it is commonly deregulated in human tumours. The delineation of the canonical pathway resembles the behaviour of the Hippo pathway in the fly where the activation of the core kinases of the pathway prevents the proliferative signal mediated by the key effector of the pathway YAP. Nevertheless, several lines of evidence support the idea that the mammalian MST/Hippo pathway has acquired new features during evolution, including different regulators and effectors, crosstalk with other essential signalling pathways involved in cellular homeostasis and the ability to actively trigger cell death. Here we describe the current knowledge of the mechanisms that mediate MST/Hippo dependent cell death, especially apoptosis. We include evidence for the existence of complex signalling networks where the core proteins of the pathway play a central role in controlling the balance between survival and cell death. Finally, we discuss the possible involvement of these signalling networks in several human diseases such as cancer, diabetes and neurodegenerative disorders.

Yorkie Facilitates Organ Growth and Metamorphosis in Bombyx

The Hippo pathway, which was identified from genetic screens in the fruit fly, Drosophila melanogaster, has a major size-control function in animals. All key components of the Hippo pathway, including the transcriptional coactivator Yorkie that is the most critical substrate and downstream effector of the Hippo kinase cassette, are found in the silkworm, Bombyx mori. As revealed by microarray and quantitative real-time PCR, expression of Hippo pathway genes is particularly enriched in several mitotic tissues, including the ovary, testis, and wing disc. Developmental profiles of Hippo pathway genes are generally similar (with the exception of Yorkie) within each organ, but vary greatly in different tissues showing nearly opposing expression patterns in the wing disc and the posterior silk gland (PSG) on day 2 of the prepupal stage. Importantly, the reduction of Yorkie expression by RNAi downregulated Yorkie target genes in the ovary, decreased egg number, and delayed larval-pupal-adult metamorphosis. In contrast, baculovirus-mediated Yorkie(CA) overexpression upregulated Yorkie target genes in the PSG, increased PSG size, and accelerated larval-pupal metamorphosis. Together the results show that Yorkie potentially facilitates organ growth and metamorphosis, and suggest that the evolutionarily conserved Hippo pathway is critical for size control, particularly for PSG growth, in the silkworm.

Drosophila Imaginal Discs as a Model of Epithelial Wound Repair and Regeneration

Significance: The Drosophila larval imaginal discs, which form the adult fly during metamorphosis, are an established model system for the study of epithelial tissue damage. The disc proper is a simple columnar epithelium, but it contains complex patterning and cell-fate specification, and is genetically tractable. These features enable unbiased genetic screens to identify genes involved in all aspects of the wound response, from sensing damage to wound closure, initiation of regeneration, and re-establishment of proper cell fates. Identification of the genes that facilitate epithelial wound closure and regeneration will enable development of more sophisticated wound treatments for clinical use. Recent Advances: Imaginal disc epithelia can be damaged in many different ways, including fragmentation, induction of cell death, and irradiation. Recent work has demonstrated that the tissue's response to damage varies depending on how the wound was induced. Here, we summarize the different responses activated in these epithelial tissues after the different types of damage. Critical Issues: These studies highlight that not all wounds elicit the same response from the surrounding tissue. A complete understanding of the various wound-healing mechanisms in Drosophila will be a first step in understanding how to manage damaged human tissues and optimize healing in different clinical contexts. Future Directions: Further work is necessary to understand the similarities and differences among an epithelial tissue's responses to different insults. Ongoing studies will identify the genes and pathways employed by injured imaginal discs. Thus, work in this genetically tractable system complements work in more conventional wound-healing models.

Fat/Dachsous Signaling Promotes Drosophila Wing Growth by Regulating the Conformational State of the NDR Kinase Warts

Nuclear Dbf2-related (NDR) kinases play a central role in limiting growth in most animals. Signals that promote growth do so in part by suppressing the activation of NDR kinases by STE20/Hippo kinases. Here, we identify another mechanism for downregulating NDR kinase activity. Specifically, we show that activity of the Drosophila NDR kinase Warts in the developing wing depends on its transition from an inactive, "closed" conformation to a potentially active, "open" conformation mediated by Mats, a conserved Mps1-binder (Mob) protein. Further, we show that signaling interactions between the protocadherins Fat and Dachsous, organized by the morphogens Wingless and Decapentaplegic, suppress Warts by acting via the atypical myosin Dachs to inhibit or reverse this transition. The regulation of Warts conformation by Mats, Fat/Dachsous signaling, and Dachs appears independent of Warts phosphorylation by Hippo kinase, establishing a precedent for the control of NDR kinases, and hence growth, by distinct allosteric and phosphorylation mechanisms.

Dchs1-Fat4 regulation of polarized cell behaviours during skeletal morphogenesis

Skeletal shape varies widely across species as adaptation to specialized modes of feeding and locomotion, but how skeletal shape is established is unknown. An example of extreme diversity in the shape of a skeletal structure can be seen in the sternum, which varies considerably across species. Here we show that the Dchs1–Fat4 planar cell polarity pathway controls cell orientation in the early skeletal condensation to define the shape and relative dimensions of the mouse sternum. These changes fit a model of cell intercalation along differential Dchs1–Fat4 activity that drives a simultaneous narrowing, thickening and elongation of the sternum. Our results identify the regulation of cellular polarity within the early pre-chondrogenic mesenchyme, when skeletal shape is established, and provide the first demonstration that Fat4 and Dchs1 establish polarized cell behaviour intrinsically within the mesenchyme. Our data also reveal the first indication that cell intercalation processes occur during ventral body wall elongation and closure.

How the shape of the sternum is regulated is unclear. Here, the authors identify the Dchs1-Fat4-planar cell polarity pathway as controlling cell orientation and cell intercalation of mesenchymal cells that form skeletal condensations for the mouse sternum, which defines the relative dimensions of the sternum.

Yap1 plays a protective role in suppressing free fatty acid-induced apoptosis and promoting beta-cell survival

Mammalian pancreatic β-cells play a pivotal role in development and glucose homeostasis through the production and secretion of insulin. Functional failure or decrease in β-cell number leads to type 2 diabetes (T2D). Despite the physiological importance of β-cells, the viability of β-cells is often challenged mainly due to its poor ability to adapt to their changing microenvironment. One of the factors that negatively affect β-cell viability is high concentration of free fatty acids (FFAs) such as palmitate. In this work, we demonstrated that Yes-associated protein (Yap1) is activated when β-cells are treated with palmitate. Our loss- and gain-of-function analyses using rodent insulinoma cell lines revealed that Yap1 suppresses palmitate-induced apoptosis in β-cells without regulating their proliferation. We also found that upon palmitate treatment, re-arrangement of F-actin mediates Yap1 activation. Palmitate treatment increases expression of one of the Yap1 target genes, connective tissue growth factor (CTGF). Our gain-of-function analysis with CTGF suggests CTGF may be the downstream factor of Yap1 in the protective mechanism against FFA-induced apoptosis.

Targeting the Hippo pathway: Clinical implications and therapeutics

The Hippo pathway plays a critical role in tissue and organ size regulation by restraining cell proliferation and apoptosis under homeostatic conditions. Deregulation of this pathway can promote tumorigenesis in multiple malignant human tumor types, including sarcoma, breast, lung and liver cancers. In this review, we summarize the current understanding of Hippo pathway function, it's role in human cancer, and address the potential of Hippo pathway member proteins as therapeutic targets for a variety of tumors.

Transcriptional analysis of the dachsous gene uncovers novel isoforms expressed during development in Drosophila

The Drosophila cadherin-related protein Dachsous (Ds) plays a prominent role in planar cell polarity (PCP) and growth. The regulation of these two processes is based on the interaction between Ds and Fat proteins, generating an intracellular response required for tissue polarization and modulation of Hippo pathway activity. Here we have performed a comprehensive molecular study of the ds gene during larval development that has shown an unexpected complexity in its transcriptional regulation and revealed the expression of hitherto unsuspected transcripts. Also, knockdown of several isoforms provides new evidence on the importance of the cytoplasmic domain in the mechanism of action of Ds during development.

The hippo pathway core cassette regulates asymmetric cell division

Asymmetric cell division (ACD) is a crucial process during development, homeostasis, and cancer. Stem and progenitor cells divide asymmetrically, giving rise to two daughter cells, one of which retains the parent cell self-renewal capacity, while the other is committed to differentiation. Any imbalance in this process can induce overgrowth or even a cancer-like state. Here, we show that core components of the Hippo signaling pathway, an evolutionarily conserved organ growth regulator, modulate ACD in Drosophila. Hippo pathway inactivation disrupts the asymmetric localization of ACD regulators, leading to aberrant mitotic spindle orientation and defects in the generation of unequal-sized daughter cells. The Hippo pathway downstream kinase Warts, LATS1-2 in mammals, associates with the ACD modulators Inscuteable and Bazooka in vivo and phosphorylates Canoe, the ortholog of Afadin/AF-6, in vitro. Moreover, phosphosite mutant Canoe protein fails to form apical crescents in dividing neuroblasts in vivo, and the lack of Canoe phosphorylation by Warts leads to failures of Discs Large apical localization in metaphase neuroblasts. Given the relevance of ACD in stem cells during tissue homeostasis, and the well-documented role of the Hippo pathway as a tumor suppressor, these results represent a potential route for perturbations in the Hippo signaling to induce tumorigenesis via aberrant stem cell divisions.

Yap and Taz are required for Ret-dependent urinary tract morphogenesis

Despite the high occurrence of congenital abnormalities of the lower urinary tract in humans, the molecular, cellular and morphological aspects of their development are still poorly understood. Here, we use a conditional knockout approach to inactivate within the nephric duct (ND) lineage the two effectors of the Hippo pathway, Yap and Taz. Deletion of Yap leads to hydronephrotic kidneys with blind-ending megaureters at birth. In Yap mutants, the ND successfully migrates towards, and contacts, the cloaca. However, close analysis reveals that the tip of the Yap(-/-) ND forms an aberrant connection with the cloaca and does not properly insert into the cloaca, leading to later detachment of the ND from the cloaca. Taz deletion from the ND does not cause any defect, but analysis of Yap(-/-);Taz(-/-) NDs indicates that both genes play partially redundant roles in ureterovesical junction formation. Aspects of the Yap(-/-) phenotype resemble hypersensitivity to RET signaling, including excess budding of the ND, increased phospho-ERK and increased expression of Crlf1, Sprouty1, Etv4 and Etv5. Importantly, the Yap(ND) (-/-) ND phenotype can be largely rescued by reducing Ret gene dosage. Taken together, these results suggest that disrupting Yap/Taz activities enhances Ret pathway activity and contributes to pathogenesis of lower urinary tract defects in human infants.

Localization of Hippo signalling complexes and Warts activation in vivo

Hippo signalling controls organ growth and cell fate by regulating the activity of the kinase Warts. Multiple Hippo pathway components localize to apical junctions in epithelial cells, but the spatial and functional relationships among components have not been clarified, nor is it known where Warts activation occurs. We report here that Hippo pathway components in Drosophila wing imaginal discs are organized into distinct junctional complexes, including separate distributions for Salvador, Expanded, Warts and Hippo. These complexes are reorganized on Hippo pathway activation, when Warts shifts from associating with its inhibitor Jub to its activator Expanded, and Hippo concentrates at Salvador sites. We identify mechanisms promoting Warts relocalization, and using a phospho-specific antisera and genetic manipulations, identify where Warts activation occurs: at apical junctions where Expanded, Salvador, Hippo and Warts overlap. Our observations define spatial relationships among Hippo signalling components and establish the functional importance of their localization to Warts activation.

Components of the Hippo signalling pathway localize to apical junctions in epithelial cells, where they regulate growth in response to mechanical and biochemical cues. Sun et al. show that these proteins are organized into distinct junctional complexes, which reorganize up on Hippo pathway activation.

Ci antagonizes Hippo signaling in the somatic cells of the ovary to drive germline stem cell differentiation

Many stem cell populations are tightly regulated by their local microenvironment (niche), which comprises distinct types of stromal cells. However, little is known about mechanisms by which niche subgroups coordinately determine the stem cell fate. Here we identify that Yki, the key Hippo pathway component, is essential for escort cell (EC) function in promoting germline differentiation in Drosophila ovary. We found that Hedgehog (Hh) signals emanating primarily from cap cells support the function of ECs, where Cubitus interruptus (Ci), the Hh signaling effector, acts to inhibit Hippo kinase cascade activity. Mechanistically, we found that Ci competitively interacts with Hpo and impairs the Hpo-Wts signaling complex formation, thereby promoting Yki nuclear localization. The actions of Ci ensure effective Yki signaling to antagonize Sd/Tgi/Vg-mediated default repression in ECs. This study uncovers a mechanism explaining how subgroups of niche cells coordinate to determine the stem cell fate via Hh-Hippo signaling crosstalk, and enhances our understanding of mechanistic regulations of the oncogenic Yki/YAP signaling.

Ligand-independent requirements of steroid receptors EcR and USP for cell survival

The active form of the Drosophila steroid hormone ecdysone, 20-hydroxyecdysone (20E), binds the heterodimer EcR/USP nuclear receptor to regulate target genes that elicit proliferation, cell death and differentiation during insect development. Although the 20E effects are relatively well known, the physiological relevance of its receptors remains poorly understood. We show here that the prothoracic gland (PG), the major steroid-producing organ of insect larvae, requires EcR and USP to survive in a critical period previous to metamorphosis, and that this requirement is 20E-independent. The cell death induced by the downregulation of these receptors involves the activation of the JNK-encoding basket gene and it can be rescued by upregulating EcR isoforms which are unable to respond to 20E. Also, while PG cell death prevents ecdysone production, blocking hormone synthesis or secretion in normal PG does not lead to cell death, demonstrating further the ecdysone-independent nature of the receptor-deprivation cell death. In contrast to PG cells, wing disc or salivary glands cells do not require these receptors for survival, revealing their cell and developmental time specificity. Exploring the potential use of this feature of steroid receptors in cancer, we assayed tumor overgrowth induced by altered yorkie signaling. This overgrowth is suppressed by EcR downregulation in PG, but not in wing disc, cells. The mechanism of all these cell death features is based on the transcriptional regulation of reaper. These novel and context-dependent functional properties for EcR and USP receptors may help to understand the heterogeneous responses to steroid-based therapies in human pathologies.

Fat1 interacts with Fat4 to regulate neural tube closure, neural progenitor proliferation and apical constriction during mouse brain development

Mammalian brain development requires coordination between neural precursor proliferation, differentiation and cellular organization to create the intricate neuronal networks of the adult brain. Here, we examined the role of the atypical cadherins Fat1 and Fat4 in this process. We show that mutation of Fat1 in mouse embryos causes defects in cranial neural tube closure, accompanied by an increase in the proliferation of cortical precursors and altered apical junctions, with perturbations in apical constriction and actin accumulation. Similarly, knockdown of Fat1 in cortical precursors by in utero electroporation leads to overproliferation of radial glial precursors. Fat1 interacts genetically with the related cadherin Fat4 to regulate these processes. Proteomic analysis reveals that Fat1 and Fat4 bind different sets of actin-regulating and junctional proteins. In vitro data suggest that Fat1 and Fat4 form cis-heterodimers, providing a mechanism for bringing together their diverse interactors. We propose a model in which Fat1 and Fat4 binding coordinates distinct pathways at apical junctions to regulate neural progenitor proliferation, neural tube closure and apical constriction.

The ecdysone receptor coactivator Taiman links Yorkie to transcriptional control of germline stem cell factors in somatic tissue

The Hippo pathway is a conserved signaling cascade that modulates tissue growth. Although its core elements are well defined, factors modulating Hippo transcriptional outputs remain elusive. Here we show that components of the steroid-responsive ecdysone (Ec) pathway modulate Hippo transcriptional effects in imaginal disc cells. The Ec receptor coactivator Taiman (Tai) interacts with the Hippo transcriptional coactivator Yorkie (Yki) and promotes expression of canonical Yki-responsive genes. Tai enhances Yki-driven growth, while Tai loss, or a form of Tai unable to bind Yki, suppresses Yki-driven tissue growth. This growth suppression is not correlated with impaired induction of canonical Hippo-responsive genes but with suppression of a distinct pro-growth program of Yki-induced/Tai-dependent genes, including the germline stem cell factors nanos and piwi. These data reveal Hippo/Ec pathway crosstalk in the form a Yki-Tai complex that collaboratively induces germline genes as part of a transcriptional program that is normally repressed in developing somatic epithelia.

Fat4/Dchs1 signaling between stromal and cap mesenchyme cells influences nephrogenesis and ureteric bud branching

Formation of the kidney requires reciprocal signaling among the ureteric tubules, cap mesenchyme and surrounding stromal mesenchyme to orchestrate complex morphogenetic events. The protocadherin Fat4 influences signaling from stromal to cap mesenchyme cells to regulate their differentiation into nephrons. Here, we characterize the role of a putative binding partner of Fat4, the protocadherin Dchs1. Mutation of Dchs1 in mice leads to increased numbers of cap mesenchyme cells, which are abnormally arranged around the ureteric bud tips, and impairment of nephron morphogenesis. Mutation of Dchs1 also reduces branching of the ureteric bud and impairs differentiation of ureteric bud tip cells into trunk cells. Genetically, Dchs1 is required specifically within cap mesenchyme cells. The similarity of Dchs1 phenotypes to stromal-less kidneys and to those of Fat4 mutants implicates Dchs1 in Fat4-dependent stroma-to-cap mesenchyme signaling. Antibody staining of genetic mosaics reveals that Dchs1 protein localization is polarized within cap mesenchyme cells, where it accumulates at the interface with stromal cells, implying that it interacts directly with a stromal protein. Our observations identify a role for Fat4 and Dchs1 in signaling between cell layers, implicate Dchs1 as a Fat4 receptor for stromal signaling that is essential for kidney development, and establish that vertebrate Dchs1 can be molecularly polarized in vivo.

Structural basis for Mob1-dependent activation of the core Mst-Lats kinase cascade in Hippo signaling

The Mst-Lats kinase cascade is central to the Hippo tumor-suppressive pathway that controls organ size and tissue homeostasis. The adaptor protein Mob1 promotes Lats activation by Mst, but the mechanism remains unknown. Here, we show that human Mob1 binds to autophosphorylated docking motifs in active Mst2. This binding enables Mob1 phosphorylation by Mst2. Phosphorylated Mob1 undergoes conformational activation and binds to Lats1. We determine the crystal structures of phospho-Mst2-Mob1 and phospho-Mob1-Lats1 complexes, revealing the structural basis of both phosphorylation-dependent binding events. Further biochemical and functional analyses demonstrate that Mob1 mediates Lats1 activation through dynamic scaffolding and allosteric mechanisms. Thus, Mob1 acts as a phosphorylation-regulated coupler of kinase activation by virtue of its ability to engage multiple ligands. We propose that stepwise, phosphorylation-triggered docking interactions of nonkinase elements enhance the specificity and robustness of kinase signaling cascades.

Systemic organ wasting induced by localized expression of the secreted insulin/IGF antagonist ImpL2

Organ wasting, related to changes in nutrition and metabolic activity of cells and tissues, is observed under conditions of starvation and in the context of diseases, including cancers. We have developed a model for organ wasting in adult Drosophila, whereby overproliferation induced by activation of Yorkie, the Yap1 oncogene ortholog, in intestinal stem cells leads to wasting of the ovary, fat body, and muscle. These organ-wasting phenotypes are associated with a reduction in systemic insulin/IGF signaling due to increased expression of the secreted insulin/IGF antagonist ImpL2 from the overproliferating gut. Strikingly, expression of rate-limiting glycolytic enzymes and central components of the insulin/IGF pathway is upregulated with activation of Yorkie in the gut, which may provide a mechanism for this overproliferating tissue to evade the effect of ImpL2. Altogether, our study provides insights into the mechanisms underlying organ-wasting phenotypes in Drosophila and how overproliferating tissues adapt to global changes in metabolism.

Control of organ growth by patterning and hippo signaling in Drosophila

Control of organ size is of fundamental importance and is controlled by genetic, environmental, and mechanical factors. Studies in many species have pointed to the existence of both organ-extrinsic and -intrinsic size-control mechanisms, which ultimately must coordinate to regulate organ size. Here, we discuss organ size control by organ patterning and the Hippo pathway, which both act in an organ-intrinsic fashion. The influence of morphogens and other patterning molecules couples growth and patterning, whereas emerging evidence suggests that the Hippo pathway controls growth in response to mechanical stimuli and signals emanating from cell-cell interactions. Several points of cross talk have been reported between signaling pathways that control organ patterning and the Hippo pathway, both at the level of membrane receptors and transcriptional regulators. However, despite substantial progress in the past decade, key questions in the growth-control field remain, including precisely how and when organ patterning and the Hippo pathway communicate to control size, and whether these communication mechanisms are organ specific or general. In addition, elucidating mechanisms by which organ-intrinsic cues, such as patterning factors and the Hippo pathway, interface with extrinsic cues, such as hormones to control organ size, remain unresolved.

The p53 control of apoptosis and proliferation: lessons from Drosophila

The canonical role of p53 in preserving genome integrity and limiting carcinogenesis has been well established. In the presence of acute DNA-damage, oncogene deregulation and other forms of cellular stress, p53 orchestrates a myriad of pleiotropic processes to repair cellular damages and maintain homeostasis. Beside these well-studied functions of p53, recent studies in Drosophila have unraveled intriguing roles of Dmp53 in promoting cell division in apoptosis-induced proliferation, enhancing fitness and proliferation of the winner cell in cell competition and coordinating growth at the organ and organismal level in the presence of stress. In this review, we describe these new functions of Dmp53 and discuss their relevance in the context of carcinogenesis.

The Hippo pathway effector Yki downregulates Wg signaling to promote retinal differentiation in the Drosophila eye

The evolutionarily conserved Hippo signaling pathway is known to regulate cell proliferation and maintain tissue homeostasis during development. We found that activation of Yorkie (Yki), the effector of the Hippo signaling pathway, causes separable effects on growth and differentiation of the Drosophila eye. We present evidence supporting a role for Yki in suppressing eye fate by downregulation of the core retinal determination genes. Other upstream regulators of the Hippo pathway mediate this effect of Yki on retinal differentiation. Here, we show that, in the developing eye, Yki can prevent retinal differentiation by blocking morphogenetic furrow (MF) progression and R8 specification. The inhibition of MF progression is due to ectopic induction of Wingless (Wg) signaling and Homothorax (Hth), the negative regulators of eye development. Modulating Wg signaling can modify Yki-mediated suppression of eye fate. Furthermore, ectopic Hth induction due to Yki activation in the eye is dependent on Wg. Last, using Cut (Ct), a marker for the antennal fate, we show that suppression of eye fate by hyperactivation of yki does not change the cell fate (from eye to antenna-specific fate). In summary, we provide the genetic mechanism by which yki plays a role in cell fate specification and differentiation - a novel aspect of Yki function that is emerging from multiple model organisms.

A conserved MST1/2-YAP axis mediates Hippo signaling during lung growth

Hippo signaling is a critical player in controlling the growth of several tissues and organs in diverse species. The current model of Hippo signaling postulates a cascade of kinase activity initiated by the MST1/2 kinases in response to external stimuli. This leads to inactivation of the transcriptional coactivators, YAP/TAZ, due to their cytoplasmic retention and degradation that is correlated with YAP/TAZ phosphorylation. In most tissues examined, YAP plays a more dominant role than TAZ. Whether a conserved Hippo pathway is utilized during lung growth and development is unclear. In particular, the regulatory relationship between MST1/2 and YAP/TAZ in the lung remains controversial. By employing the Shh-Cre mouse line to efficiently inactivate genes in the lung epithelium, we show that loss of MST1/2 kinases in the epithelium can lead to neonatal lethality caused by lung defects. This is manifested by perturbation of lung epithelial cell proliferation and differentiation. These phenotypes are more severe than those produced by Nkx2.1-Cre, highlighting the effects of differential Cre activity on phenotypic outcomes. Importantly, expression of YAP targets is upregulated and the ratio of phospho-YAP to total YAP protein levels is reduced in Mst1/2-deficient lungs, all of which are consistent with a negative role of MST1/2 in controlling YAP function. This model gains further support from both in vivo and in vitro studies. Genetic removal of one allele of Yap or one copy of both Yap and Taz rescues neonatal lethality and lung phenotypes due to loss of Mst1/2. Moreover, knockdown of Yap in lung epithelial cell lines restores diminished alveolar marker expression caused by Mst1/2 inactivation. These results demonstrate that MST1/2 inhibit YAP/TAZ activity and establish a conserved MST1/2-YAP axis in coordinating lung growth during development.

The Hippo Pathway and YAP/TAZ-TEAD Protein-Protein Interaction as Targets for Regenerative Medicine and Cancer Treatment

The Hippo pathway is an important organ size control signaling network and the major regulatory mechanism of cell-contact inhibition. Yes associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) are its targets and terminal effectors: inhibition of the pathway promotes YAP/TAZ translocation to the nucleus, where they interact with transcriptional enhancer associate domain (TEAD) transcription factors and coactivate the expression of target genes, promoting cell proliferation. Defects in the pathway can result in overgrowth phenotypes due to deregulation of stem-cell proliferation and apoptosis; members of the pathway are directly involved in cancer development. The pharmacological regulation of the pathway might be useful in cancer prevention, treatment, and regenerative medicine applications; currently, a few compounds can selectively modulate the pathway. In this review, we present an overview of the Hippo pathway, the sequence and structural analysis of YAP/TAZ, the known pharmacological modulators of the pathway, especially those targeting YAP/TAZ-TEAD interaction.

Inhibitory mechanism of FAT4 gene expression in response to actin dynamics during Src-induced carcinogenesis

Oncogenic transformation is characterized by morphological changes resulting from alterations in actin dynamics and adhesive activities. Emerging evidence suggests that the protocadherin FAT4 acts as a tumor suppressor in humans, and reduced FAT4 gene expression has been reported in breast and lung cancers and melanoma. However, the mechanism controlling FAT4 gene expression is poorly understood. In this study, we show that transient activation of the Src oncoprotein represses FAT4 mRNA expression through actin depolymerization in the immortalized normal human mammary epithelial cell line MCF-10A. Src activation causes actin depolymerization via the MEK/Erk/Cofilin cascade. The MEK inhibitor U0126 blocks the inhibitory effect of Src on FAT4 mRNA expression and Src-induced actin depolymerization. To determine whether actin dynamics act on the regulation of FAT4 mRNA expression, we treated MCF-10A cells with the ROCK inhibitor Y-27632. Y-27632 treatment decreased FAT4 mRNA expression. This suppressive effect was blocked by siRNA-mediated knockdown of Cofilin1. Furthermore, simultaneous administration of Latrunculin A (an actin depolymerizing agent), Y-27632, and Cofilin1 siRNA to the cells resulted in a marked reduction of FAT4 mRNA expression. Intriguingly, we also found that FAT4 mRNA expression was reduced under both low cell density and low stiffness conditions, which suggests that mechanotransduction affects FAT4 mRNA expression. Additionally, we show that siRNA-mediated FAT4 knockdown induced the activity of the Hippo effector YAP/TAZ in MCF-10A cells. Taken together, our results reveal a novel inhibitory mechanism of FAT4 gene expression through actin depolymerization during Src-induced carcinogenesis in human breast cells.

Regulation of DNA damage responses and cell cycle progression by hMOB2

Mps one binder proteins (MOBs) are conserved regulators of essential signalling pathways. Biochemically, human MOB2 (hMOB2) can inhibit NDR kinases by competing with hMOB1 for binding to NDRs. However, biological roles of hMOB2 have remained enigmatic. Here, we describe novel functions of hMOB2 in the DNA damage response (DDR) and cell cycle regulation. hMOB2 promotes DDR signalling, cell survival and cell cycle arrest after exogenously induced DNA damage. Under normal growth conditions in the absence of exogenously induced DNA damage hMOB2 plays a role in preventing the accumulation of endogenous DNA damage and a subsequent p53/p21-dependent G1/S cell cycle arrest. Unexpectedly, these molecular and cellular phenotypes are not observed upon NDR manipulations, indicating that hMOB2 performs these functions independent of NDR signalling. Thus, to gain mechanistic insight, we screened for novel binding partners of hMOB2, revealing that hMOB2 interacts with RAD50, facilitating the recruitment of the MRE11-RAD50-NBS1 (MRN) DNA damage sensor complex and activated ATM to DNA damaged chromatin. Taken together, we conclude that hMOB2 supports the DDR and cell cycle progression.

YAP-mediated induction of monoacylglycerol lipase restrains oncogenic transformation

The Hippo pathway is an evolutionarily conserved regulator of normal and oncogenic growth. Engagement of Hippo pathway signaling results in the inactivation of the transcriptional coactivator YAP by preventing its nuclear entry. The mechanisms underlying the oncogenic properties of YAP remain incompletely understood. Here we find that although the transactivation (TA) domain of YAP mediates YAP-dependent gene expression, it serves as an inhibitor of YAP-mediated anchorage-independent growth. We identify monoacylglycerol lipase (MAGL) as a YAP transcriptional target and an inhibitor of anchorage-dependent cell growth. Significantly, knockdown of MAGL expression leads to the augmentation of YAP-dependent cell transformation. Our results identify MAGL as a transcriptional target of YAP that restrains YAP-mediated cellular transformation.

Impaired Hippo signaling promotes Rho1-JNK-dependent growth

The Hippo and c-Jun N-terminal kinase (JNK) pathway both regulate growth and contribute to tumorigenesis when dysregulated. Whereas the Hippo pathway acts via the transcription coactivator Yki/YAP to regulate target gene expression, JNK signaling, triggered by various modulators including Rho GTPases, activates the transcription factors Jun and Fos. Here, we show that impaired Hippo signaling induces JNK activation through Rho1. Blocking Rho1-JNK signaling suppresses Yki-induced overgrowth in the wing disk, whereas ectopic Rho1 expression promotes tissue growth when apoptosis is prohibited. Furthermore, Yki directly regulates Rho1 transcription via the transcription factor Sd. Thus, our results have identified a novel molecular link between the Hippo and JNK pathways and implicated the essential role of the JNK pathway in Hippo signaling-related tumorigenesis.

Salvador-Warts-Hippo pathway in a developmental checkpoint monitoring helix-loop-helix proteins

The E proteins and Id proteins are, respectively, the positive and negative heterodimer partners for the basic-helix-loop-helix protein family and as such contribute to a remarkably large number of cell-fate decisions. E proteins and Id proteins also function to inhibit or promote cell proliferation and cancer. Using a genetic modifier screen in Drosophila, we show that the Id protein Extramacrochaetae enables growth by suppressing activation of the Salvador-Warts-Hippo pathway of tumor suppressors, activation that requires transcriptional activation of the expanded gene by the E protein Daughterless. Daughterless protein binds to an intronic enhancer in the expanded gene, both activating the SWH pathway independently of the transmembrane protein Crumbs and bypassing the negative feedback regulation that targets the same expanded enhancer. Thus, the Salvador-Warts-Hippo pathway has a cell-autonomous function to prevent inappropriate differentiation due to transcription factor imbalance and monitors the intrinsic developmental status of progenitor cells, distinct from any responses to cell-cell interactions.

SCF(Slmb) E3 ligase-mediated degradation of Expanded is inhibited by the Hippo pathway in Drosophila

Deregulation of the evolutionarily conserved Hippo pathway has been implicated in abnormal development of animals and in several types of cancer. One mechanism of Hippo pathway regulation is achieved by controlling the stability of its regulatory components. However, the executive E3 ligases that are involved in this process, and how the process is regulated, remain poorly defined. In this study, we identify, through a genetic candidate screen, the SCF(Slmb) E3 ligase as a novel negative regulator of the Hippo pathway in Drosophila imaginal tissues via mediation of the degradation of Expanded (Ex). Mechanistic study shows that Slmb-mediated degradation of Ex is inhibited by the Hippo signaling. Considering the fact that Hippo signaling suppresses the transcription of ex, we propose that the Hippo pathway employs a double security mechanism to ensure fine-tuned homeostasis during development.

The hippo signaling pathway: implications for heart regeneration and disease

Control of cell number and organ size is critical for appropriate development and tissue homeostasis. Studies in both Drosophila and mammals have established the Hippo signaling pathway as an important modulator of organ size and tumorigenesis. Upon activation, this kinase cascade modulates gene expression through the phosphorylation and inhibition of transcription co-activators that are involved in cell proliferation, differentiation, growth and apoptosis. Hippo signaling serves to limit organ size and suppress malignancies, and has been implicated in tissue regeneration following injury. These outcomes highlight the important role that Hippo signaling plays in regulating both physiologic and pathologic processes. In this review, an overview of the signaling pathway will be discussed as well as recent work that has investigated its role in cardiac development, regeneration and disease.

Unlike in Drosophila Meroistic Ovaries, hippo represses notch in Blattella germanica Panoistic ovaries, triggering the mitosis-endocycle switch in the follicular cells

During insect oogenesis, the follicular epithelium undergoes both cell proliferation and apoptosis, thus modulating ovarian follicle growth. The Hippo pathway is key in these processes, and has been thoroughly studied in the meroistic ovaries of Drosophila melanogaster. However, nothing is known about the role of the Hippo pathway in primitive panoistic ovaries. This work examines the mRNA expression levels of the main components of the Hippo pathway in the panoistic ovary of the basal insect species Blattella germanica, and demonstrates the function of Hippo through RNAi. In Hippo-depleted specimens, the follicular cells of the basal ovarian follicles proliferate without arresting cytokinesis; the epithelium therefore becomes bilayered, impairing ovarian follicle growth. This phenotype is accompanied by long stalks between the ovarian follicles. In D. melanogaster loss of function of Notch determines that the stalk is not developed. With this in mind, we tested whether Hippo and Notch pathways are related in B. germanica. In Notch (only)-depleted females, no stalks were formed between the ovarian follicles. Simultaneous depletion of Hippo and Notch rescued partially the stalk to wild-type. Unlike in the meroistic ovaries of D. melanogaster, in panoistic ovaries the Hippo pathway appears to regulate follicular cell proliferation by acting as a repressor of Notch, triggering the switch from mitosis to the endocycle in the follicular cells. The phylogenetically basal position of B. germanica suggests that this might be the ancestral function of Hippo in insect ovaries.

Protocadherin-9 involvement in retinal development in Xenopus laevis

Biological roles of most protocadherins (Pcdhs) are a largely unsolved problem. Therefore, we cloned cDNA for Xenopus laevis protocadherin-9 and characterized its properties to elucidate the role. The deduced amino acid sequence was highly homologous to those of mammalian protocadherin-9 s. X. laevis protocadherin-9 expressed from the cDNA in L cells showed basic properties similar to those of mammalian Pcdhs. Expression of X. laevis protocadherin-9 was first detected in stage-31 embryos and increased as the development proceeded. In the later stage embryos and the adults, the retina strongly expressed protocadherin-9, which was mainly localized at the plexiform layers. Injection of morpholino anti-sense oligonucleotide against protocadherin-9 into the fertilized eggs inhibited eye development; and eye growth and formation of the retinal laminar structure were hindered. Moreover, affected retina showed abnormal extension of neurites into the ganglion cell layer. Co-injection of protocadherin-9 mRNA with the morpholino anti-sense oligonucleotide rescued the embryos from the defects. These results suggest that X. laevis protocadherin-9 was involved in the development of retina structure possibly through survival of neurons, formation of the lamina structure and neurite localization.

Drosophila C-terminal Src kinase regulates growth via the Hippo signaling pathway

The Hippo signaling pathway is involved in regulating tissue size by inhibiting cell proliferation and promoting apoptosis. Aberrant Hippo pathway function is often detected in human cancers and correlates with poor prognosis. The Drosophila C-terminal Src kinase (d-Csk) is a genetic modifier of warts (wts), a tumor-suppressor gene in the Hippo pathway, and interacts with the Src oncogene. Reduction in d-Csk expression and the consequent activation of Src are frequently seen in several cancers including hepatocellular and colorectal tumors. Previous studies show that d-Csk regulates cell proliferation and tissue size during development. Given the similarity in the loss-of-function phenotypes of d-Csk and wts, we have investigated the interactions of d-Csk with the Hippo pathway. Here we present multiple lines of evidence suggesting that d-Csk regulates growth via the Hippo signaling pathway. We show that loss of dCsk caused increased Yki activity, and our genetic epistasis places dCsk downstream of Dachs. Furthermore, dCsk requires Yki for its growth regulatory functions, suggesting that dCsk is another upstream member of the network of genes that interact to regulate Wts and its effector Yki in the Hippo signaling pathway.

Cytoskeletal tension inhibits Hippo signaling through an Ajuba-Warts complex

Mechanical forces have been proposed to modulate organ growth, but a molecular mechanism that links them to growth regulation in vivo has been lacking. We report that increasing tension within the cytoskeleton increases Drosophila wing growth, whereas decreasing cytoskeletal tension decreases wing growth. These changes in growth can be accounted for by changes in the activity of Yorkie, a transcription factor regulated by the Hippo pathway. The influence of myosin activity on Yorkie depends genetically on the Ajuba LIM protein Jub, a negative regulator of Warts within the Hippo pathway. We further show that Jub associates with α-catenin and that its localization to adherens junctions and association with α-catenin are promoted by cytoskeletal tension. Jub recruits Warts to junctions in a tension-dependent manner. Our observations delineate a mechanism that links cytoskeletal tension to regulation of Hippo pathway activity, providing a molecular understanding of how mechanical forces can modulate organ growth.

Yorkie promotes transcription by recruiting a histone methyltransferase complex

Hippo signaling limits organ growth by inhibiting the transcriptional coactivator Yorkie. Despite the key role of Yorkie in both normal and oncogenic growth, the mechanism by which it activates transcription has not been defined. We report that Yorkie binding to chromatin correlates with histone H3K4 methylation and is sufficient to locally increase it. We show that Yorkie can recruit a histone methyltransferase complex through binding between WW domains of Yorkie and PPxY sequence motifs of NcoA6, a subunit of the Trithorax-related (Trr) methyltransferase complex. Cell culture and in vivo assays establish that this recruitment of NcoA6 contributes to Yorkie's ability to activate transcription. Mammalian NcoA6, a subunit of Trr-homologous methyltransferase complexes, can similarly interact with Yorkie's mammalian homolog YAP. Our results implicate direct recruitment of a histone methyltransferase complex as central to transcriptional activation by Yorkie, linking the control of cell proliferation by Hippo signaling to chromatin modification.

The Hippo pathway in disease and therapy: cancer and beyond

The Hippo tumour suppressor pathway co-ordinates cell proliferation, cell death and cell differentiation to regulate tissue growth control. In mammals, a conserved core Hippo signalling module receives signal inputs on different levels to ensure the proper regulation of YAP/TAZ activities as transcriptional co-activators. While the core module members MST1/2, Salvador, LATS1/2 and MOB1 have been attributed tumour suppressive functions, YAP/TAZ have been mainly described to have oncogenic roles, although some reports provided evidence supporting growth suppressive roles of YAP/TAZ in certain cancer settings. Intriguingly, mammalian Hippo signalling is also implicated in non-cancer diseases and plays a role in tissue regeneration following injury. Cumulatively, these findings indicate that the pharmacological inhibition or activation of the Hippo pathway could be desirable depending on the disease context. In this review, we first summarise the functions of the mammalian Hippo pathway in tumour formation, and then discuss non-cancer diseases involving Hippo signalling core components with a specific focus on our current understanding of the non-cancer roles of MST1/2 and YAP/TAZ. In addition, the pros and cons of possible pharmacological interventions with Hippo signalling will be reviewed, with particular emphasis on anti-cancer drug development and regenerative medicine.

Regulation of neuronal migration by Dchs1-Fat4 planar cell polarity

Planar cell polarity (PCP) describes the polarization of cell structures and behaviors within the plane of a tissue. PCP is essential for the generation of tissue architecture during embryogenesis and for postnatal growth and tissue repair, yet how it is oriented to coordinate cell polarity remains poorly understood [1]. In Drosophila, PCP is mediated via the Frizzled-Flamingo (Fz-PCP) and Dachsous-Fat (Fat-PCP) pathways [1-3]. Fz-PCP is conserved in vertebrates, but an understanding in vertebrates of whether and how Fat-PCP polarizes cells, and its relationship to Fz-PCP signaling, is lacking. Mutations in human FAT4 and DCHS1, key components of Fat-PCP signaling, cause Van Maldergem syndrome, characterized by severe neuronal abnormalities indicative of altered neuronal migration [4]. Here, we investigate the role and mechanisms of Fat-PCP during neuronal migration using the murine facial branchiomotor (FBM) neurons as a model. We find that Fat4 and Dchs1 are expressed in complementary gradients and are required for the collective tangential migration of FBM neurons and for their PCP. Fat4 and Dchs1 are required intrinsically within the FBM neurons and extrinsically within the neuroepithelium. Remarkably, Fat-PCP and Fz-PCP regulate FBM neuron migration along orthogonal axes. Disruption of the Dchs1 gradients by mosaic inactivation of Dchs1 alters FBM neuron polarity and migration. This study implies that PCP in vertebrates can be regulated via gradients of Fat4 and Dchs1 expression, which establish intracellular polarity across FBM cells during their migration. Our results also identify Fat-PCP as a novel neuronal guidance system and reveal that Fat-PCP and Fz-PCP can act along orthogonal axes.

The Hippo signaling pathway in stem cell biology and cancer

The Hippo signaling pathway, consisting of a highly conserved kinase cascade (MST and Lats) and downstream transcription coactivators (YAP and TAZ), plays a key role in tissue homeostasis and organ size control by regulating tissue-specific stem cells. Moreover, this pathway plays a prominent role in tissue repair and regeneration. Dysregulation of the Hippo pathway is associated with cancer development. Recent studies have revealed a complex network of upstream inputs, including cell density, mechanical sensation, and G-protein-coupled receptor (GPCR) signaling, that modulate Hippo pathway activity. This review focuses on the role of the Hippo pathway in stem cell biology and its potential implications in tissue homeostasis and cancer.

Coupling of Hedgehog and Hippo pathways promotes stem cell maintenance by stimulating proliferation

It is essential to define the mechanisms by which external signals regulate adult stem cell numbers, stem cell maintenance, and stem cell proliferation to guide regenerative stem cell therapies and to understand better how cancers originate in stem cells. In this paper, we show that Hedgehog (Hh) signaling in Drosophila melanogaster ovarian follicle stem cells (FSCs) induces the activity of Yorkie (Yki), the transcriptional coactivator of the Hippo pathway, by inducing yki transcription. Moreover, both Hh signaling and Yki positively regulate the rate of FSC proliferation, both are essential for FSC maintenance, and both promote increased FSC longevity and FSC duplication when in excess. We also found that responses to activated Yki depend on Cyclin E induction while responses to excess Hh signaling depend on Yki induction, and excess Yki can compensate for defective Hh signaling. These causal connections provide the most rigorous evidence to date that a niche signal can promote stem cell maintenance principally by stimulating stem cell proliferation.

Tissue communication in regenerative inflammatory signaling: lessons from the fly gut

The intestine, as a barrier epithelium, serves in the first line of defense against invading pathogens and damaging agents that enter the body via food ingestion. Maintenance of intestinal homeostasis is therefore key to organismal health. To maintain homeostasis, intestinal stem cells (ISCs) continuously replace lost or damaged intestinal epithelial cells in organisms ranging from Drosophila to humans. Interestingly, intestinal damage upon ingestion of chemicals or pathogenic bacteria leads to an inflammatory response in the Drosophila intestine, which promotes regeneration and predisposes to tumorigenesis. This regenerative inflammatory signaling culminates in proliferation and differentiation of ISCs that replenish the damaged intestinal cells and is regulated by the interplay of conserved cell-cell communication pathways, such as the JNK, JAK/STAT, Wnt/Wingless, Notch, InR, PVR, EGFR, and Hippo. These pathways are induced by signals emanating not only from the damaged intestinal epithelial cells, but also from neighboring tissues associated with the intestinal epithelium, such as the muscles and the trachea, or distant tissues, such as the wounded epidermis and the brain. Here we review tissue communication during homeostasis and regenerative inflammatory signaling in Drosophila focusing on the signals that emanate from non-intestinal epithelial tissues to ensure intestinal integrity.

Mammalian sterile 20-like kinase 1/2 inhibits the Wnt/β-catenin signalling pathway by directly binding casein kinase 1ε

The Hippo signalling pathway can suppress the Wnt/β-catenin signalling pathway through the last downstream effectors YAP (Yes-associated protein)/TAZ (tafazzin). MST (mammalian sterile 20-like kinase) 1 functions as the upstream kinase of the Hippo pathway, and CK1ε (casein kinase 1ε) plays roles in the up-stream signal transduction of the Wnt/β-catenin pathway. In the present study, using tandem affinity purification and MS analysis, CK1ε was identified as a novel partner of MST1. Further analysis showed that the interaction between MST1 and CK1ε was mediated by their kinase domains and enhanced by the activation of MST1. To exclude the interference of the phosphorylated YAP/TAZ, the transduction from MST1 to YAP/TAZ was blocked using anti-WW45 shRNA. In the sh-WW45 cells, MST1 still inhibited the Wnt3A-induced phosphorylation of DVL2 (dishevelled 2) and Wnt/β-catenin signalling by disturbing the interaction of DVL2 and CK1ε. The growth-suppressive effect of MST1 in the presence of Wnt3A was effectively relieved by the downstream activation of the Wnt/β-catenin pathway. Moreover, MST2, the close homologue of MST1, also displayed the similar function in suppressing the Wnt/β-catenin pathway. Therefore the results of the present study revealed that, in addition to the phosphorylated YAP/TAZ, the Hippo pathway can suppress the Wnt/β-catenin pathway directly through MST1/2.

Genetics and mechanisms of ovarian cancer: parallels between Drosophila and humans

Considering the degree of detail available at the genetic and cellular levels, the Drosophila ovary stands out as a powerful system to identify new players in the regulation of key aspects of cancer progression. In this review, we will comment on how the use of the Drosophila ovary has helped to elucidate some of the molecular bases of ovarian malignancies and to identify and characterize critical tumour suppressor genes and oncogenes with an impact in human pathologies.