The Drosophila tumor suppressors Expanded and Merlin differentially regulate cell cycle exit, apoptosis, and Wingless signaling

Redesigning the Drosophila histone gene cluster: an improved genetic platform for spatiotemporal manipulation of histone function

Mutating replication-dependent (RD) histone genes is an important tool for understanding chromatin-based epigenetic regulation. Deploying this tool in metazoans is particularly challenging because RD histones in these organisms are typically encoded by many genes, often located at multiple loci. Such gene arrangements make the ability to generate homogenous histone mutant genotypes by site-specific gene editing quite difficult. Drosophila melanogaster provides a solution to this problem because the RD histone genes are organized into a single large tandem array that can be deleted and replaced with transgenes containing mutant histone genes. In the last ∼15 years several different RD histone gene replacement platforms were developed using this simple strategy. However, each platform contains weaknesses that preclude full use of the powerful developmental genetic capabilities available to Drosophila researchers. Here we describe the development of a newly engineered platform that rectifies many of these weaknesses. We used CRISPR to precisely delete the RD histone gene array (HisC), replacing it with a multifunctional cassette that permits site-specific insertion of either one or two synthetic gene arrays using selectable markers. We designed this cassette with the ability to selectively delete each of the integrated gene arrays in specific tissues using site-specific recombinases. We also present a method for rapidly synthesizing histone gene arrays of any genotype using Golden Gate cloning technologies. These improvements facilitate the generation of histone mutant cells in various tissues at different stages of Drosophila development and provide an opportunity to apply forward genetic strategies to interrogate chromatin structure and gene regulation.

Redesigning the Drosophila histone gene cluster: An improved genetic platform for spatiotemporal manipulation of histone function

Mutating replication-dependent (RD) histone genes is an important tool for understanding chromatin-based epigenetic regulation. Deploying this tool in metazoan models is particularly challenging because RD histones in these organisms are typically encoded by many genes, often located at multiple loci. Such RD histone gene arrangements make the ability to generate homogenous histone mutant genotypes by site-specific gene editing quite difficult. Drosophila melanogaster provides a solution to this problem because the RD histone genes are organized into a single large tandem array that can be deleted and replaced with transgenes containing mutant histone genes. In the last ∼15 years several different RD histone gene replacement platforms have been developed using this simple strategy. However, each platform contains weaknesses that preclude full use of the powerful developmental genetic capabilities available to Drosophila researchers. Here we describe the development of a newly engineered platform that rectifies many of these weaknesses. We used CRISPR to precisely delete the RD histone gene array ( HisC ), replacing it with a multifunctional cassette that permits site-specific insertion of either one or two synthetic gene arrays using selectable markers. We designed this cassette with the ability to selectively delete each of the integrated gene arrays in specific tissues using site-specific recombinases. We also present a method for rapidly synthesizing histone gene arrays of any genotype using Golden Gate cloning technologies. These improvements facilitate generation of histone mutant cells in various tissues at different stages of Drosophila development and provide an opportunity to apply forward genetic strategies to interrogate chromatin structure and gene regulation.

Hippo pathway and Bonus control developmental cell fate decisions in the Drosophila eye

The canonical function of the Hippo signaling pathway is the regulation of organ growth. How this pathway controls cell-fate determination is less well understood. Here, we identify a function of the Hippo pathway in cell-fate decisions in the developing Drosophila eye, exerted through the interaction of Yorkie (Yki) with the transcriptional regulator Bonus (Bon), an ortholog of mammalian transcriptional intermediary factor 1/tripartite motif (TIF1/TRIM) family proteins. Instead of controlling tissue growth, Yki and Bon promote epidermal and antennal fates at the expense of the eye fate. Proteomic, transcriptomic, and genetic analyses reveal that Yki and Bon control these cell-fate decisions by recruiting transcriptional and post-transcriptional co-regulators and by repressing Notch target genes and activating epidermal differentiation genes. Our work expands the range of functions and regulatory mechanisms under Hippo pathway control.

Patched and Costal-2 mutations lead to differences in tissue overgrowth autonomy

Genetic screens are used in Drosophila melanogaster to identify genes key in the regulation of organismal development and growth. These screens have defined signalling pathways necessary for tissue and organismal development, which are evolutionarily conserved across species, including Drosophila. Here, we have used an FLP/FRT mosaic system to screen for conditional regulators of cell growth and cell division in the Drosophila eye. The conditional nature of this screen utilizes a block in the apoptotic pathway to prohibit the mosaic mutant cells from dying via apoptosis. From this screen, we identified two different mutants that mapped to the Hedgehog signalling pathway. Previously, we described a novel Ptc mutation and here we add to the understanding of disrupting the Hh pathway with a novel allele of Cos2. Both of these Hh components are negative regulators of the pathway, yet they depict mutant differences in the type of overgrowth created. Ptc mutations lead to overgrowth consisting of almost entirely wild-type tissue (non-autonomous overgrowth), while the Cos2 mutation results in tissue that is overgrown in both the mutant and wild-type clones (both autonomous and non-autonomous). These differences in tissue overgrowth are consistent in the Drosophila eye and wing. The observed difference is correlated with different deregulation patterns of pMad, the downstream effector of DPP signalling. This finding provides insight into pathway-specific differences that help to better understand intricacies of developmental processes and human diseases that result from deregulated Hedgehog signalling, such as basal cell carcinoma.

Phosphorylation of Merlin by Aurora A kinase appears necessary for mitotic progression

Although Merlin's function as a tumor suppressor and regulator of mitogenic signaling networks such as the Ras/rac, Akt, and Hippo pathways is well-documented, in mammals as well as in insects, its role during cell cycle progression remains unclear. In this study, using a combination of approaches, including FACS analysis, time-lapse imaging, immunofluorescence microscopy, and co-immunoprecipitation, we show that Ser-518 of Merlin is a substrate of the Aurora protein kinase A during mitosis and that its phosphorylation facilitates the phosphorylation of a newly discovered site, Thr-581. We found that the expression in HeLa cells of a Merlin variant that is phosphorylation-defective on both sites leads to a defect in centrosomes and mitotic spindles positioning during metaphase and delays the transition from metaphase to anaphase. We also show that the dual mitotic phosphorylation not only reduces Merlin binding to microtubules but also timely modulates ezrin interaction with the cytoskeleton. Finally, we identify several point mutants of Merlin associated with neurofibromatosis type 2 that display an aberrant phosphorylation profile along with defective α-tubulin-binding properties. Altogether, our findings of an Aurora A-mediated interaction of Merlin with α-tubulin and ezrin suggest a potential role for Merlin in cell cycle progression.

The Power of Drosophila Genetics: The Discovery of the Hippo Pathway

The Hippo Pathway comprises a vast network of components that integrate diverse signals including mechanical cues and cell surface or cell-surface-associated molecules to define cellular outputs of growth, proliferation, cell fate, and cell survival on both the cellular and tissue level. Because of the importance of the regulators, core components, and targets of this pathway in human health and disease, individual components were often identified by efforts in mammalian models or for a role in a specific process such as stress response or cell death. However, multiple components were originally discovered in the Drosophila system, and the breakthrough of conceiving that these components worked together in a signaling pathway came from a series of Drosophila genetic screens and fundamental genetic and phenotypic characterization efforts. In this chapter, we will review the original discoveries leading to the conceptual framework of these components as a tumor suppressor network. We will review chronologically the early efforts that established our initial understanding of the core machinery that then launched the growing and vibrant field to be discussed throughout later chapters of this book.

Upstream paths for Hippo signaling in Drosophila organ development

Organ growth is fundamental to animal development. One of major mechanisms for growth control is mediated by the conserved Hippo signaling pathway initially identified in Drosophila. The core of this pathway in Drosophila consists of a cascade of protein kinases Hippo and Warts that negatively regulate transcriptional coactivator Yorkie (Yki). Activation of Yki promotes cell survival and proliferation to induce organ growth. A key issue in Hippo signaling is to understand how core kinase cascade is activated. Activation of Hippo kinase cascade is regulated in the upstream by at least two transmembrane proteins Crumbs and Fat that act in parallel. These membrane proteins interact with additional factors such as FERM-domain proteins Expanded and Merlin to modulate subcellular localization and function of the Hippo kinase cascade. Hippo signaling is also influenced by cytoskeletal networks and cell tension in epithelia of developing organs. These upstream events in the regulation of Hippo signaling are only partially understood. This review focuses on our current understanding of some upstream processes involved in Hippo signaling in developing Drosophila organs.

A Drosophila Tumor Suppressor Gene Prevents Tonic TNF Signaling through Receptor N-Glycosylation

Drosophila tumor suppressor genes have revealed molecular pathways that control tissue growth, but mechanisms that regulate mitogenic signaling are far from understood. Here we report that the Drosophila TSG tumorous imaginal discs (tid), whose phenotypes were previously attributed to mutations in a DnaJ-like chaperone, are in fact driven by the loss of the N-linked glycosylation pathway component ALG3. tid/alg3 imaginal discs display tissue growth and architecture defects that share characteristics of both neoplastic and hyperplastic mutants. Tumorous growth is driven by inhibited Hippo signaling, induced by excess Jun N-terminal kinase (JNK) activity. We show that ectopic JNK activation is caused by aberrant glycosylation of a single protein, the fly tumor necrosis factor (TNF) receptor homolog, which results in increased binding to the continually circulating TNF. Our results suggest that N-linked glycosylation sets the threshold of TNF receptor signaling by modifying ligand-receptor interactions and that cells may alter this modification to respond appropriately to physiological cues.

The Hippo Pathway: A Master Regulatory Network Important in Development and Dysregulated in Disease

The Hippo Pathway is a master regulatory network that regulates proliferation, cell growth, stemness, differentiation, and cell death. Coordination of these processes by the Hippo Pathway throughout development and in mature organisms in response to diverse external and internal cues plays a role in morphogenesis, in controlling organ size, and in maintaining organ homeostasis. Given the importance of these processes, the Hippo Pathway also plays an important role in organismal health and has been implicated in a variety of diseases including eye disease, cardiovascular disease, neurodegeneration, and cancer. This review will focus on Drosophila reports that identified the core components of the Hippo Pathway revealing specific downstream biological outputs of this complicated network. A brief description of mammalian reports will complement review of the Drosophila studies. This review will also survey upstream regulation of the core components with a focus on feedback mechanisms.

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.

Drosophila Schip1 Links Expanded and Tao-1 to Regulate Hippo Signaling

Regulation of organ size is essential in animal development, and Hippo (Hpo) signaling is a major conserved mechanism for controlling organ growth. In Drosophila, Hpo and Warts kinases are core components of this pathway and function as tumor suppressors by inhibiting Yorkie (Yki). Expanded (Ex) is a regulator of the Hpo activity, but how they are linked is unknown. Here, we show that Schip1, a Drosophila homolog of the mammalian Schwannomin interacting protein 1 (SCHIP1), provides a link between Ex and Hpo. Ex is required for apical localization of Schip1 in imaginal discs. Schip1 is necessary for promoting membrane localization and phosphorylation of Hpo by recruiting the Hpo kinase Tao-1. Taking these findings together, we conclude that Schip1 directly links Ex to Hpo signaling by recruiting Tao-1. This study provides insights into the mechanism of Tao-1 regulation and a potential growth control function for SCHIP1 in mammals.

Willing to Be Involved in Cancer

Genome sequencing is now a common procedure, but prior to this, screening experiments using protein baits was one of the routinely used methods that, occasionally, allowed the identification of new gene products. One such experiment uncovered the gene product called willin/human Expanded/FRMD6. Initial characterization studies found that willin bound phospholipids and was strongly co-localised with actin. However, subsequently, willin was found to be the closest human sequence homologue of the Drosophila protein Expanded (Ex), sharing 60% homology with the Ex FERM domain. This in turn suggested, and then was proven that willin could activate the Hippo signalling pathway. This review describes the increasing body of knowledge about the actions of willin in a number of cellular functions related to cancer. However, like many gene products involved in aspects of cell signalling, a convincing direct role for willin in cancer remains tantalisingly elusive, at present.

The T-box transcription factor Midline regulates wing development by repressing wingless and hedgehog in Drosophila

Wingless (Wg) and Hedgehog (Hh) signaling pathways are key players in animal development. However, regulation of the expression of wg and hh are not well understood. Here, we show that Midline (Mid), an evolutionarily conserved transcription factor, expresses in the wing disc of Drosophila and plays a vital role in wing development. Loss or knock down of mid in the wing disc induced hyper-expression of wingless (wg) and yielded cocked and non-flat wings. Over-expression of mid in the wing disc markedly repressed the expression of wg, DE-Cadherin (DE-Cad) and armadillo (arm), and resulted in a small and blistered wing. In addition, a reduction in the dose of mid enhanced phenotypes of a gain-of-function mutant of hedgehog (hh). We also observed repression of hh upon overexpression of mid in the wing disc. Taken together, we propose that Mid regulates wing development by repressing wg and hh in Drosophila.

Merlin/NF2-Lin28B-let-7 Is a Tumor-Suppressive Pathway that Is Cell-Density Dependent and Hippo Independent

Contact inhibition of proliferation is critical for tissue organization, and its dysregulation contributes to tumorigenesis. Merlin/NF2 is a tumor suppressor that governs contact inhibition. Although Merlin/NF2 inhibits YAP1 and TAZ, which are paralogous Hippo pathway transcriptional co-activators and oncoproteins, it is not fully understood how Merlin/NF2-mediated signal transduction triggered by cell-cell contact exerts tumor suppression. Here, we identify Lin28B, an inhibitor of let-7 microRNAs (miRNAs), as an important downstream target of Merlin/NF2. Functional studies revealed that, at low cell density, Merlin/NF2 is phosphorylated and does not bind to Lin28B, allowing Lin28B to enter the nucleus, bind to pri-let-7 miRNAs, and inhibit their maturation in a YAP1/TAZ-independent manner. This inhibition of pri-let-7 maturation then promotes cell growth. However, cell-cell contact triggers Merlin/NF2 dephosphorylation, which sequesters Lin28B in the cytoplasm and permits pri-let-7 maturation. Our results reveal that Merlin/NF2-mediated signaling drives a tumor-suppressive pathway that is cell-density dependent and Hippo independent.

Ack promotes tissue growth via phosphorylation and suppression of the Hippo pathway component Expanded

Non-receptor tyrosine kinase activated cdc42 kinase was reported to participate in several types of cancers in mammals. It is also believed to have an anti-apoptotic function in Drosophila. Here, we report the identification of Drosophila activated cdc42 kinase as a growth promoter and a novel Hippo signaling pathway regulator. We find that activated cdc42 kinase promotes tissue growth through modulating Yorkie activity. Furthermore, we demonstrate that activated cdc42 kinase interacts with Expanded and induces tyrosine phosphorylation of Expanded on multiple sites. We propose a model that activated cdc42 kinase negatively regulates Expanded by changing its phosphorylation status to promote tissue growth. Moreover, we show that ack genetically interacts with merlin and expanded. Thus, we identify Drosophila activated cdc42 kinase as a Hippo pathway regulator.

Overlapping functions of the MAP4K family kinases Hppy and Msn in Hippo signaling

The Hippo (Hpo) tumor suppressor pathway is an evolutionarily conserved signaling pathway that controls tissue growth and organ size in species ranging from Drosophila to human, and its malfunction has been implicated in many types of human cancer. In this study, we conducted a kinome screen and identified Happyhour (Hppy)/MAP4K3 as a novel player in the Hpo pathway. Our biochemical study showed that Hppy binds and phosphorylates Wts. Our genetic experiments suggest that Hppy acts in parallel and partial redundantly with Misshapen (Msn)/MAP4K4 to regulate Yki nuclear localization and Hpo target gene expression in Drosophila wing imaginal discs. Furthermore, we showed that cytoskeleton stress restricts Yki nuclear localization through Hppy and Msn when Hpo activity is compromised, thus providing an explanation for the Wts-dependent but Hpo-independent regulation of Yki in certain contexts. Our study has unraveled an additional layer of complexity in the Hpo signaling pathway and laid down a foundation for exploring how different upstream regulators feed into the core Hpo pathway.

Scaling the Drosophila Wing: TOR-Dependent Target Gene Access by the Hippo Pathway Transducer Yorkie

Organ growth is controlled by patterning signals that operate locally (e.g., Wingless/Ints [Wnts], Bone Morphogenetic Proteins [BMPs], and Hedgehogs [Hhs]) and scaled by nutrient-dependent signals that act systemically (e.g., Insulin-like peptides [ILPs] transduced by the Target of Rapamycin [TOR] pathway). How cells integrate these distinct inputs to generate organs of the appropriate size and shape is largely unknown. The transcriptional coactivator Yorkie (Yki, a YES-Associated Protein, or YAP) acts downstream of patterning morphogens and other tissue-intrinsic signals to promote organ growth. Yki activity is regulated primarily by the Warts/Hippo (Wts/Hpo) tumour suppressor pathway, which impedes nuclear access of Yki by a cytoplasmic tethering mechanism. Here, we show that the TOR pathway regulates Yki by a separate and novel mechanism in the Drosophila wing. Instead of controlling Yki nuclear access, TOR signaling governs Yki action after it reaches the nucleus by allowing it to gain access to its target genes. When TOR activity is inhibited, Yki accumulates in the nucleus but is sequestered from its normal growth-promoting target genes—a phenomenon we term “nuclear seclusion.” Hence, we posit that in addition to its well-known role in stimulating cellular metabolism in response to nutrients, TOR also promotes wing growth by liberating Yki from nuclear seclusion, a parallel pathway that we propose contributes to the scaling of wing size with nutrient availability.

From dwarves to giants, scaling is a universal property of animal organs, but its mechanistic basis is poorly understood. Here, the authors identify a molecular circuit underlying scaling of the Drosophila wing.

What mechanisms control the sizes of animal organs? It is known that organ growth is the product of two systems: an intrinsic system that coordinates cell proliferation with the specification of cell fate (patterning), and an extrinsic system that synchronizes growth with nutrient levels. Developing organs integrate these two inputs to ensure that properly proportioned structures develop which are of the right scale to match overall body size. However, the mechanisms used to integrate these distinct growth control systems have remained largely mysterious. In this study, we have addressed how intrinsic and extrinsic systems combine to drive growth of the Drosophila wing. Focusing on the Target of Rapamycin (TOR) pathway—a major, nutrient-dependent regulator of organ growth—and Yorkie—the transcriptional activator downstream of the Hippo pathway and a key, organ-intrinsic growth regulator—we have identified a circuit in which TOR activity limits Yorkie’s capacity to promote wing growth, in part through a novel mode of transcription factor regulation that we term “nuclear seclusion.” We find that inhibiting TOR leads to the retention of Yorkie in the nucleus but diminishes its transcriptional activity by diverting it away from target genes. We posit that subjugating Yorkie in this way contributes to how fluctuations in TOR activity scale wing size according to nutrient levels.

Spectrin regulates Hippo signaling by modulating cortical actomyosin activity

The Hippo pathway controls tissue growth through a core kinase cascade that impinges on the transcription of growth-regulatory genes. Understanding how this pathway is regulated in development remains a major challenge. Recent studies suggested that Hippo signaling can be modulated by cytoskeletal tension through a Rok-myosin II pathway. How cytoskeletal tension is regulated or its relationship to the other known upstream regulators of the Hippo pathway remains poorly defined. In this study, we identify spectrin, a contractile protein at the cytoskeleton-membrane interface, as an upstream regulator of the Hippo signaling pathway. We show that, in contrast to canonical upstream regulators such as Crumbs, Kibra, Expanded, and Merlin, spectrin regulates Hippo signaling in a distinct way by modulating cortical actomyosin activity through non-muscle myosin II. These results uncover an essential mediator of Hippo signaling by cytoskeleton tension, providing a new entry point to dissecting how mechanical signals regulate Hippo signaling in living tissues.

DOI:http://dx.doi.org/10.7554/eLife.06567.001

Organs including the liver, eyes, and lungs are made up of millions of cells, and how these organs stop growing once they reach their final size has fascinated scientists for decades. The cells in developing organs must communicate with each other and respond appropriately to the signals that they receive from other cells. This requires so-called “signaling pathways”. One such pathway that involves a protein called Hippo is known to control when cells should grow and divide and when they should stop. If this pathway does not work correctly, it can cause too many cells to be formed, which may result in cancer.

The Hippo signaling pathway can also be regulated by an extensive network of protein filaments found within cells, called the cytoskeleton. This network can exert forces on the cells, which can have a major impact on cell growth. However, the mechanism behind the interaction between the cytoskeleton and the Hippo signaling pathway is poorly understood.

Now, Deng et al. have engineered fruit flies in which the expression of individual genes had been artificially reduced, and looked for flies that had enlarged wings. Three genes identified in these experiments encode different subunits of a large spring-like protein, called spectrin, which is part of the cytoskeleton. This suggests that normally spectrin limits wing size. Furthermore, spectrin was also found to control the size of other organs in the fruit flies, such as the eyes and ovaries. In all of these organs, the Hippo signaling pathway failed to work properly in the absence of spectrin. Deng et al. then further explored the relationship between spectrin and Hippo signaling and found that cells without spectrin show abnormally high levels of tension in their cytoskeleton. When flies that lacked spectrin were engineered to reduce this tension, these flies developed normal sized organs. These findings reveal the importance of cytoskeleton tension in controlling tissue growth, and provide a new entry point to understand how normal tissues grow to their characteristic size and how such process goes awry in cancer.

DOI:http://dx.doi.org/10.7554/eLife.06567.002

Zyxin antagonizes the FERM protein expanded to couple F-actin and Yorkie-dependent organ growth

BACKGROUND

Coordinated multicellular growth during development is achieved by the sensing of spatial and nutritional boundaries. The conserved Hippo (Hpo) signaling pathway has been proposed to restrict tissue growth by perceiving mechanical constraints through actin cytoskeleton networks. The actin-associated LIM proteins Zyxin (Zyx) and Ajuba (Jub) have been linked to the control of tissue growth via regulation of Hpo signaling, but the study of Zyx has been hampered by a lack of genetic tools.

RESULTS

We generated a zyx mutant in Drosophila using TALEN endonucleases and used this to show that Zyx antagonizes the FERM-domain protein Expanded (Ex) to control tissue growth, eye differentiation, and F-actin accumulation. Zyx membrane targeting promotes the interaction between the transcriptional co-activator Yorkie (Yki) and the transcription factor Scalloped (Sd), leading to activation of Yki target gene expression and promoting tissue growth. Finally, we show that Zyx's growth-promoting function is dependent on its interaction with the actin-associated protein Enabled (Ena) via a conserved LPPPP motif and is antagonized by Capping Protein (CP).

CONCLUSIONS

Our results show that Zyx is a functional antagonist of Ex in growth control and establish a link between actin filament polymerization and Yki activity.

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.

The two faces of Hippo: targeting the Hippo pathway for regenerative medicine and cancer treatment

The Hippo signalling pathway is an emerging growth control and tumour suppressor pathway that regulates cell proliferation and stem cell functions. Defects in Hippo signalling and hyperactivation of its downstream effectors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) contribute to the development of cancer, which suggests that pharmacological inhibition of YAP and TAZ activity may be an effective anticancer strategy. Conversely, YAP and TAZ can also have beneficial roles in stimulating tissue repair and regeneration following injury, so their activation may be therapeutically useful in these contexts. A complex network of intracellular and extracellular signalling pathways that modulate YAP and TAZ activities have recently been identified. Here, we review the regulation of the Hippo signalling pathway, its functions in normal homeostasis and disease, and recent progress in the identification of small-molecule pathway modulators.

Drosophila as a model for context-dependent tumorigenesis

Drosophila can exhibit classic hallmarks of cancer, such as evasion of apoptosis, sustained proliferation, metastasis, prolonged survival, genome instability, and metabolic reprogramming, when cancer-related genes are perturbed. In the last two decades, studies in flies have identified several tumor suppressor and oncogenes. However, the greatest strength of the fly lies in its ability to model cancer hallmarks in a variety of tissue types, which enables the study of context-dependent tumorigenesis. We review the organs and tissues that have been used to model tumor formation, and propose new strategies to maximize the potential of Drosophila in cancer research.

Alcohol interacts with genetic alteration of the Hippo tumor suppressor pathway to modulate tissue growth in Drosophila

Alcohol-mediated cancers represent more than 3.5% of cancer-related deaths, yet how alcohol promotes cancer is a major open question. Using Drosophila, we identified novel interactions between dietary ethanol and loss of tumor suppressor components of the Hippo Pathway. The Hippo Pathway suppresses tumors in flies and mammals by inactivating transcriptional co-activator Yorkie, and the spectrum of cancers associated with impaired Hippo signaling overlaps strikingly with those associated with alcohol. Therefore, our findings may implicate loss of Hippo Pathway tumor suppression in alcohol-mediated cancers. Ethanol enhanced overgrowth from loss of the expanded, hippo, or warts tumor suppressors but, surprisingly, not from over-expressing the yorkie oncogene. We propose that in parallel to Yorkie-dependent overgrowth, impairing Hippo signaling in the presence of alcohol may promote overgrowth via additional alcohol-relevant targets. We also identified interactions between alcohol and Hippo Pathway over-activation. We propose that exceeding certain thresholds of alcohol exposure activates Hippo signaling to maintain proper growth control and prevent alcohol-mediated mis-patterning and tissue overgrowth.

Defining the protein-protein interaction network of the human hippo pathway

The Hippo pathway, which is conserved from Drosophila to mammals, has been recognized as a tumor suppressor signaling pathway governing cell proliferation and apoptosis, two key events involved in organ size control and tumorigenesis. Although several upstream regulators, the conserved kinase cascade and key downstream effectors including nuclear transcriptional factors have been defined, the global organization of this signaling pathway is not been fully understood. Thus, we conducted a proteomic analysis of human Hippo pathway, which revealed the involvement of an extensive protein-protein interaction network in this pathway. The mass spectrometry data were deposited to ProteomeXchange with identifier PXD000415. Our data suggest that 550 interactions within 343 unique protein components constitute the central protein-protein interaction landscape of human Hippo pathway. Our study provides a glimpse into the global organization of Hippo pathway, reveals previously unknown interactions within this pathway, and uncovers new potential components involved in the regulation of this pathway. Understanding these interactions will help us further dissect the Hippo signaling-pathway and extend our knowledge of organ size control.

Hippo gains weight: added insights and complexity to pathway control

The Hippo pathway is a kinase cascade, formed by Hippo, Salvador, Warts, and Mats, that regulates the subcellular distribution and transcriptional activity of Yorkie. Yorkie is a transcriptional coactivator that promotes the expression of genes that inhibit apoptosis and drive cell proliferation. We review recent studies indicating that activity of the Hippo pathway is controlled by cell-cell junctions, cell adhesion molecules, scaffolding proteins, and cytoskeletal proteins, as well as by regulators of apical-basal polarity and extracellular tension.

The expanding family of FERM proteins

Our understanding of the FERM (4.1/ezrin/radixin/moesin) protein family has been rapidly expanding in the last few years, with the result that many new physiological functions have been ascribed to these biochemically unique proteins. In the present review, we will discuss a number of new FRMD (FERM domain)-containing proteins that were initially discovered from genome sequencing but are now being established through biochemical and genetic studies to be involved both in normal cellular processes, but are also associated with a variety of human diseases.

An evolutionary shift in the regulation of the Hippo pathway between mice and flies

The Hippo pathway plays a key role in controlling organ growth in many animal species and its deregulation is associated with different types of cancer. Understanding the regulation of the Hippo pathway and discovering upstream regulators is thus a major quest. Interestingly, while the core of the Hippo pathway contains a highly conserved kinase cascade, different components have been identified as upstream regulators in Drosophila and vertebrates. However, whether the regulation of the Hippo pathway is indeed different between Drosophila and vertebrates or whether these differences are due to our limited analysis of these components in different organisms is not known. Here we show that the mouse Fat4 cadherin, the ortholog of the Hippo pathway regulator Fat in Drosophila, does not apparently regulate the Hippo pathway in the murine liver. In fact, we uncovered an evolutionary shift in many of the known upstream regulators at the base of the arthropod lineage. In this evolutionary transition, Fat and the adaptor protein Expanded gained novel domains that connected them to the Hippo pathway, whereas the cell-adhesion receptor Echinoid evolved as a new protein. Subsequently, the junctional adaptor protein Angiomotin (Amot) was lost and the downstream effector Yap lost its PDZ-binding motif that interacts with cell junction proteins. We conclude that fundamental differences exist in the upstream regulatory mechanisms of Hippo signaling between Drosophila and vertebrates.

Regulation of cytoskeletal organization and junctional remodeling by the atypical cadherin Fat

The atypical cadherin Fat is a conserved regulator of planar cell polarity, but the mechanisms by which Fat controls cell shape and tissue structure are not well understood. Here, we show that Fat is required for the planar polarized organization of actin denticle precursors, adherens junction proteins and microtubules in the epidermis of the late Drosophila embryo. In wild-type embryos, spatially regulated cell-shape changes and rearrangements organize cells into highly aligned columns. Junctional remodeling is suppressed at dorsal and ventral cell boundaries, where adherens junction proteins accumulate. By contrast, adherens junction proteins fail to accumulate to the wild-type extent and all cell boundaries are equally engaged in junctional remodeling in fat mutants. The effects of loss of Fat on cell shape and junctional localization, but not its role in denticle organization, are recapitulated by mutations in Expanded, an upstream regulator of the conserved Hippo pathway, and mutations in Hippo and Warts, two kinases in the Hippo kinase cascade. However, the cell shape and planar polarity defects in fat mutants are not suppressed by removing the transcriptional co-activator Yorkie, suggesting that these roles of Fat are independent of Yorkie-mediated transcription. The effects of Fat on cell shape, junctional remodeling and microtubule localization are recapitulated by expression of activated Notch. These results demonstrate that cell shape, junctional localization and cytoskeletal planar polarity in the Drosophila embryo are regulated by a common signal provided by the atypical cadherin Fat and suggest that Fat influences tissue organization through its role in polarized junctional remodeling.

Regulation of Yorkie activity in Drosophila imaginal discs by the Hedgehog receptor gene patched

The Hedgehog (Hh) pathway was first defined by its role in segment polarity in the Drosophila melanogaster embryonic epidermis and has since been linked to many aspects of vertebrate development and disease. In humans, mutation of the Patched1 (PTCH1) gene, which encodes an inhibitor of Hh signaling, leads to tumors of the skin and pediatric brain. Despite the high level of conservation between the vertebrate and invertebrate Hh pathways, studies in Drosophila have yet to find direct evidence that ptc limits organ size. Here we report identification of Drosophila ptc in a screen for mutations that require a synergistic apoptotic block in order to drive overgrowth. Developing imaginal discs containing clones of ptc mutant cells immortalized by the concurrent loss of the Apaf-1-related killer (Ark) gene are overgrown due, in large part, to the overgrowth of wild type portions of these discs. This phenotype correlates with overexpression of the morphogen Dpp in ptc,Ark double-mutant cells, leading to elevated phosphorylation of the Dpp pathway effector Mad (p-Mad) in cells surrounding ptc,Ark mutant clones. p-Mad functions with the Hippo pathway oncoprotein Yorkie (Yki) to induce expression of the pro-growth/anti-apoptotic microRNA bantam. Accordingly, Yki activity is elevated among wild type cells surrounding ptc,Ark clones and alleles of bantam and yki dominantly suppress the enlarged-disc phenotype produced by loss of ptc. These data suggest that ptc can regulate Yki in a non-cell autonomous manner and reveal an intercellular link between the Hh and Hippo pathways that may contribute to growth-regulatory properties of the Hh pathway in development and disease.

KIBRA exhibits MST-independent functional regulation of the Hippo signaling pathway in mammals

The Salvador/Warts/Hippo (Hippo) signaling pathway defines a novel signaling cascade regulating cell contact inhibition, organ size control, cell growth, proliferation, apoptosis and cancer development in mammals. The upstream regulation of this pathway has been less well defined than the core kinase cassette. KIBRA has been shown to function as an upstream member of the Hippo pathway by influencing the phosphorylation of LATS and YAP, but functional consequences of these biochemical changes have not been previously addressed. We show that in MCF10A cells, loss of KIBRA expression displays epithelial-to-mesenchymal transition (EMT) features, which are concomitant with decreased LATS and YAP phosphorylation, but not MST1/2. In addition, ectopic KIBRA expression antagonizes YAP via the serine 127 phosphorylation site and we show that KIBRA, Willin and Merlin differentially regulate genes controlled by YAP. Finally, reduced KIBRA expression in primary breast cancer specimens correlates with the recently described claudin-low subtype, an aggressive sub-group with EMT features and a poor prognosis.

Organ size control by Hippo and TOR pathways

The determination of final organ size is a highly coordinated and complex process that relies on the precise regulation of cell number and/or cell size. Perturbation of organ size control contributes to many human diseases, including hypertrophy, degenerative diseases, and cancer. Hippo and TOR are among the key signaling pathways involved in the regulation of organ size through their respective functions in the regulation of cell number and cell size. Here, we review the general mechanisms that regulate organ growth, describe how Hippo and TOR control key aspects of growth, and discuss recent findings that highlight a possible coordination between Hippo and TOR in organ size regulation.

Drosophila Pez acts in Hippo signaling to restrict intestinal stem cell proliferation

The conserved Hippo signaling pathway acts in growth control and is fundamental to animal development and oncogenesis. Hippo signaling has also been implicated in adult midgut homeostasis in Drosophila. Regulated divisions of intestinal stem cells (ISCs), giving rise to an ISC and an enteroblast (EB) that differentiates into an enterocyte (EC) or an enteroendocrine (EE) cell, enable rapid tissue turnover in response to intestinal stress. The damage-related increase in ISC proliferation requires deactivation of the Hippo pathway and consequential activation of the transcriptional coactivator Yorkie (Yki) in both ECs and ISCs. Here, we identify Pez, an evolutionarily conserved FERM domain protein containing a protein tyrosine phosphatase (PTP) domain, as a novel binding partner of the upstream Hippo signaling component Kibra. Pez function--but not its PTP domain--is essential for Hippo pathway activity specifically in the fly midgut epithelium. Thus, Pez displays a tissue-specific requirement and functions as a negative upstream regulator of Yki in the regulation of ISC proliferation.

The cell adhesion molecule echinoid functions as a tumor suppressor and upstream regulator of the Hippo signaling pathway

The Hippo (Hpo) signaling pathway controls tissue growth and organ size in species ranging from Drosophila to mammals and is deregulated in a wide range of human cancers. The core pathway consists of the Hpo/Warts (Wts) kinase cassette that phosphorylates and inactivates the transcriptional coactivator Yorkie (Yki). Here, we report that Echinoid (Ed), an immunoglobulin domain-containing cell adhesion molecule, acts as an upstream regulator of the Hpo pathway. Loss of Ed compromises Yki phosphorylation, resulting in elevated Yki activity that increases Hpo target gene expression and drives tissue overgrowth. Ed physically interacts with and stabilizes the Hpo-binding partner Salvador (Sav) at adherens junctions. Ed/Sav interaction is promoted by cell-cell contact and requires dimerization of Ed cytoplasmic domain. Overexpression of Sav or dimerized Ed cytoplasmic domain suppressed loss-of-Ed phenotypes. We propose that Ed may link cell-cell contact to Hpo signaling through binding and stabilizing Sav, thus modulating the Hpo kinase activity.

Regulation of Drosophila glial cell proliferation by Merlin-Hippo signaling

Glia perform diverse and essential roles in the nervous system, but the mechanisms that regulate glial cell numbers are not well understood. Here, we identify and characterize a requirement for the Hippo pathway and its transcriptional co-activator Yorkie in controlling Drosophila glial proliferation. We find that Yorkie is both necessary for normal glial cell numbers and, when activated, sufficient to drive glial over-proliferation. Yorkie activity in glial cells is controlled by a Merlin-Hippo signaling pathway, whereas the upstream Hippo pathway regulators Fat, Expanded, Crumbs and Lethal giant larvae have no detectable role. We extend functional characterization of Merlin-Hippo signaling by showing that Merlin and Hippo can be physically linked by the Salvador tumor suppressor. Yorkie promotes expression of the microRNA gene bantam in glia, and bantam promotes expression of Myc, which is required for Yorkie and bantam-induced glial proliferation. Our results provide new insights into the control of glial growth, and establish glia as a model for Merlin-specific Hippo signaling. Moreover, as several of the genes we studied have been linked to human gliomas, our results suggest that this linkage could reflect their organization into a conserved pathway for the control of glial cell proliferation.

Dimerization and cytoplasmic localization regulate Hippo kinase signaling activity in organ size control

The Hippo (Hpo) signaling pathway controls organ size by regulating the balance between cell proliferation and apoptosis. Although the Hpo function is conserved, little is known about the mechanism of how its kinase activity is regulated. Based on structural information, we performed mutation-function analysis and provided in vitro and in vivo evidence that Hpo activation requires proper dimerization of its N-terminal kinase domain as well as the C-terminal SARAH domain. Hpo carrying point mutation M242E can still dimerize, yet the dimers formed between intermolecular kinase domains were altered in conformation. As a result, autophosphorylation of Hpo at Thr-195 was blocked, and its kinase activity was abolished. In contrast, Hpo carrying I634D, a single mutation introduced in the Hpo C-terminal SARAH domain, disrupted the dimerization of the SARAH domain, leading to reduced Hippo activity. We also find that the Hpo C-terminal half contains two nuclear export signals that promote cytoplasmic localization and activity of Hpo. Taken together, our results suggest that dimerization and nucleocytoplasmic translocation of Hpo are crucial for its biological function and indicate that a proper dimer conformation of the kinase domain is essential for Hpo autophosphorylation and kinase activity.

Binary regulation of Hippo pathway by Merlin/NF2, Kibra, Lgl, and Melted specifies and maintains postmitotic neuronal fate

Patterning the Drosophila retina for color vision relies on postmitotic specification of photoreceptor subtypes. R8 photoreceptors express one of two light-sensing Rhodopsins, Rh5 or Rh6. This fate decision involves a bistable feedback loop between Melted, a PH-domain protein, and Warts, a kinase in the Hippo growth pathway. Here, we show that a subset of the Hippo pathway-Merlin, Kibra, and Lethal(2)giant larvae (Lgl), but not Expanded or Fat-is required for Warts expression and activity in R8 to specify Rh6 fate. Melted represses warts transcription to disrupt Hippo pathway activity and specify Rh5 fate. Therefore, R8 Hippo signaling exhibits ON-or-OFF regulation, promoting mutually exclusive fates. Furthermore, Merlin and Lgl are continuously required to maintain R8 neuronal subtypes. These results reveal roles for Merlin, Kibra, and Lgl in neuronal specification and maintenance and show that the Hippo pathway is reimplemented for sensory neuron fate by combining canonical and noncanonical regulatory steps.

Loss of the Drosophila cell polarity regulator Scribbled promotes epithelial tissue overgrowth and cooperation with oncogenic Ras-Raf through impaired Hippo pathway signaling

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.

A screen for conditional growth suppressor genes identifies the Drosophila homolog of HD-PTP as a regulator of the oncoprotein Yorkie

Mammalian cancers depend on "multiple hits," some of which promote growth and some of which block apoptosis. We screened for mutations that require a synergistic block in apoptosis to promote tissue overgrowth and identified myopic (mop), the Drosophila homolog of the candidate tumor-suppressor and endosomal regulator His-domain protein tyrosine phosphatase (HD-PTP). We find that Myopic regulates the Salvador/Warts/Hippo (SWH) tumor suppressor pathway: Myopic PPxY motifs bind conserved residues in the WW domains of the transcriptional coactivator Yorkie, and Myopic colocalizes with Yorkie at endosomes. Myopic controls Yorkie endosomal association and protein levels, ultimately influencing expression of some Yorkie target genes. However, the antiapoptotic gene diap1 is not affected, which may explain the conditional nature of the myopic growth phenotype. These data establish Myopic as a Yorkie regulator and implicate Myopic-dependent association of Yorkie with endosomal compartments as a regulatory step in nuclear outputs of the SWH pathway.

Zyxin links fat signaling to the hippo pathway

Using genetic and molecular analyses, the authors identify Zyx as a positive regulator of Hippo signaling and characterize its role within the pathway.

The Hippo signaling pathway has a conserved role in growth control and is of fundamental importance during both normal development and oncogenesis. Despite rapid progress in recent years, key steps in the pathway remain poorly understood, in part due to the incomplete identification of components. Through a genetic screen, we identified the Drosophila Zyxin family gene, Zyx102 (Zyx), as a component of the Hippo pathway. Zyx positively regulates the Hippo pathway transcriptional co-activator Yorkie, as its loss reduces Yorkie activity and organ growth. Through epistasis tests, we position the requirement for Zyx within the Fat branch of Hippo signaling, downstream of Fat and Dco, and upstream of the Yorkie kinase Warts, and we find that Zyx is required for the influence of Fat on Warts protein levels. Zyx localizes to the sub-apical membrane, with distinctive peaks of accumulation at intercellular vertices. This partially overlaps the membrane localization of the myosin Dachs, which has similar effects on Fat-Hippo signaling. Co-immunoprecipitation experiments show that Zyx can bind to Dachs and that Dachs stimulates binding of Zyx to Warts. We also extend characterization of the Ajuba LIM protein Jub and determine that although Jub and Zyx share C-terminal LIM domains, they regulate Hippo signaling in distinct ways. Our results identify a role for Zyx in the Hippo pathway and suggest a mechanism for the role of Dachs: because Fat regulates the localization of Dachs to the membrane, where it can overlap with Zyx, we propose that the regulated localization of Dachs influences downstream signaling by modulating Zyx-Warts binding. Mammalian Zyxin proteins have been implicated in linking effects of mechanical strain to cell behavior. Our identification of Zyx as a regulator of Hippo signaling thus also raises the possibility that mechanical strain could be linked to the regulation of gene expression and growth through Hippo signaling.

Processes that control cell numbers are essential during normal development, when they are required to generate organs of the correct size, and during cancinogenesis, when they influence tumor growth. The Hippo pathway is an intercellular signaling pathway that relays information about cell-cell contact and cell polarity to a signal transduction pathway that regulates the transcription of genes controlling cell numbers. The role of Hippo signaling in controlling growth is conserved from fruit flies to humans, but many aspects of the Hippo signal transduction pathway remain poorly understood. In this article, we identify Zyx as a previously unknown component of the Hippo pathway in Drosophila, and characterize its role within the pathway. We show that Zyx plays an essential role in a branch of Hippo signaling that involves the transmembrane receptor protein Fat and its target Dachs, which is a myosin family protein. Our results suggest a model in which Fat regulates the localization of Dachs, Dachs subsequently binds Zyx, stimulating its binding with the kinase Warts/Lats, and thereby regulates downstream signaling events. Zyx is conserved in vertebrates and we suggest that vertebrate Zyx proteins might also be involved in the regulation of Hippo signaling and, thereby, organ growth.

The Hippo pathway and apico–basal cell polarity

The establishment and maintenance of apico–basal cell polarity is a pre-requisite for the formation of a functioning epithelial tissue. Many lines of evidence suggest that cell polarity perturbations favour cancer formation, even though the mechanistic basis for this link remains unclear. Studies in Drosophila have uncovered complex interactions between the conserved Hpo (Hippo) tumour suppressor pathway and apico–basal polarity determinants. The Hpo pathway is a crucial growth regulatory network whose inactivation in Drosophila epithelial tissues induces massive overproliferation. Its core consists of a phosphorylation cascade (comprising the kinases Hpo and Warts) that mediates the inactivation of the pro-growth transcriptional co-activator Yki [Yorkie; YAP (Yes-associated protein) in mammals]. Several apically located proteins, such as Merlin, Expanded or Kibra, have been identified as upstream regulators of the Hpo pathway, leading to the notion that an apical multi-molecular complex modulates core kinase activity and promotes Yki/YAP inactivation. In the present review, we explore the links between apico–basal polarity and Hpo signalling. We focus on the regulation of Yki/YAP by apical proteins, but also on how the Hpo pathway might in turn influence apical domain size as part of a regulatory feedback loop.

Hippo signaling: growth control and beyond

The Hippo pathway has emerged as a conserved signaling pathway that is essential for the proper regulation of organ growth in Drosophila and vertebrates. Although the mechanisms of signal transduction of the core kinases Hippo/Mst and Warts/Lats are relatively well understood, less is known about the upstream inputs of the pathway and about the downstream cellular and developmental outputs. Here, we review recently discovered mechanisms that contribute to the dynamic regulation of Hippo signaling during Drosophila and vertebrate development. We also discuss the expanding diversity of Hippo signaling functions during development, discoveries that shed light on a complex regulatory system and provide exciting new insights into the elusive mechanisms that regulate organ growth and regeneration.

The Salvador/Warts/Hippo pathway controls regenerative tissue growth in Drosophila melanogaster

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.

The hippo signaling pathway in development and cancer

First discovered in Drosophila, the Hippo signaling pathway is a conserved regulator of organ size. Central to this pathway is a kinase cascade leading from the tumor suppressor Hippo (Mst1 and Mst2 in mammals) to the oncoprotein Yki (YAP and TAZ in mammals), a transcriptional coactivator of target genes involved in cell proliferation and survival. Here, I review recent progress in elucidating the molecular mechanism and physiological function of Hippo signaling in Drosophila and mammals. These studies suggest that the core Hippo kinase cascade integrates multiple upstream inputs, enabling dynamic regulation of tissue homeostasis in animal development and physiology.