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

The Hippo pathway in the heart: pivotal roles in development, disease, and regeneration

The Hippo-YAP (Yes-associated protein) pathway is an evolutionarily and functionally conserved regulator of organ size and growth with crucial roles in cell proliferation, apoptosis, and differentiation. This pathway has great potential for therapeutic manipulation in different disease states and to promote organ regeneration. In this Review, we summarize findings from the past decade revealing the function and regulation of the Hippo-YAP pathway in cardiac development, growth, homeostasis, disease, and regeneration. In particular, we highlight the roles of the Hippo-YAP pathway in endogenous heart muscle renewal, including the pivotal role of the Hippo-YAP pathway in regulating cardiomyocyte proliferation and differentiation, stress response, and mechanical signalling. The human heart lacks the capacity to self-repair; therefore, the loss of cardiomyocytes after injury such as myocardial infarction can result in heart failure and death. Despite substantial advances in the treatment of heart failure, an enormous unmet clinical need exists for alternative treatment options. Targeting the Hippo-YAP pathway has tremendous potential for developing therapeutic strategies for cardiac repair and regeneration for currently intractable cardiovascular diseases such as heart failure. The lessons learned from cardiac repair and regeneration studies will also bring new insights into the regeneration of other tissues with limited regenerative capacity.

Nuclear envelope deformation controls cell cycle progression in response to mechanical force

The shape of the cell nucleus can vary considerably during developmental and pathological processes; however, the impact of nuclear morphology on cell behavior is not known. Here, we observed that the nuclear envelope flattens as cells transit from G1 to S phase and inhibition of myosin II prevents nuclear flattening and impedes progression to S phase. Strikingly, we show that applying compressive force on the nucleus in the absence of myosin II-mediated tension is sufficient to restore G1 to S transition. Using a combination of tools to manipulate nuclear morphology, we observed that nuclear flattening activates a subset of transcription factors, including TEAD and AP1, leading to transcriptional induction of target genes that promote G1 to S transition. In addition, we found that nuclear flattening mediates TEAD and AP1 activation in response to ROCK-generated contractility or cell spreading. Our results reveal that the nuclear envelope can operate as a mechanical sensor whose deformation controls cell growth in response to tension.

A new, easily generated mouse model of diabetic kidney fibrosis

Our understanding of diabetic kidney disease pathogenesis has been hampered by the lack of easily generated pre-clinical animal models that faithfully recapitulate critical features of human disease. While most standard animal models develop manifestations of early stage diabetic injury such as hyperfiltration and mesangial matrix expansion, only a select few develop key late stage features such as interstitial fibrosis and reduced glomerular filtration rate. An underlying theme in these late stage disease models has been the addition of renin-angiotensin system hyperactivation, an important contributor to human disease pathogenesis. Widespread use of these models has been limited, however, as they are either labour intensive to generate, or have been developed in the rat, preventing the use of the many powerful genetic tools developed for mice. Here we describe the Akita^+/− Ren^+/− mouse, a new, easily generated murine model of diabetic kidney disease that develops many features of late stage human injury, including not only hyperglycemia, hypertension, and albuminuria, but also reduced glomerular filtration rate, glomerulosclerosis, and interstitial fibrosis.

CCN1-Yes-Associated Protein Feedback Loop Regulates Physiological and Pathological Angiogenesis

Cellular communication network factor 1 (CCN1) is a dynamically expressed, matricellular protein required for vascular development and tissue repair. The CCN1 gene is a presumed target of Yes-associated protein (YAP), a transcriptional coactivator that regulates cell growth and organ size. Herein, we demonstrate that the CCN1 promoter is indeed a direct genomic target of YAP in endothelial cells (ECs) of new blood vessel sprouts and that YAP deficiency in mice downregulates CCN1 and alters cytoskeletal and mitogenic gene expression. Interestingly, CCN1 overexpression in cultured ECs inactivates YAP in a negative feedback and causes its nuclear exclusion. Accordingly, EC-specific deletion of the CCN1 gene in mice mimics a YAP gain-of-function phenotype, characterized by EC hyperproliferation and blood vessel enlargement. CCN1 brings about its effect by providing cells with a soft compliant matrix that creates YAP-repressive cytoskeletal states. Concordantly, pharmacological inhibition of cell stiffness recapitulates the CCN1 deletion vascular phenotype. Furthermore, adeno-associated virus-mediated expression of CCN1 reversed the pathology of YAP hyperactivation and the subsequent aberrant growth of blood vessels in mice with ischemic retinopathy. Our studies unravel a new paradigm of functional interaction between CCN1 and YAP and underscore the significance of their interplay in the pathogenesis of neovascular diseases.

The Hippo Signaling Pathway in Development and Disease

The Hippo signaling pathway regulates diverse physiological processes, and its dysfunction has been implicated in an increasing number of human diseases, including cancer. Here, we provide an updated review of the Hippo pathway; discuss its roles in development, homeostasis, regeneration, and diseases; and highlight outstanding questions for future investigation and opportunities for Hippo-targeted therapies.

Yorkie controls tube length and apical barrier integrity during airway development

Skouloudaki et al. identify an alternative role of the transcriptional coactivator Yorkie (Yki) in controlling water impermeability and tube size of developing Drosophila airways. Tracheal impermeability is triggered by Yki-mediated transcriptional regulation of δ-aminolevulinate synthase (Alas), whereas tube elongation is controlled by binding of Yki to the actin-severing factor Twinstar.

Epithelial organ size and shape depend on cell shape changes, cell–matrix communication, and apical membrane growth. The Drosophila melanogaster embryonic tracheal network is an excellent model to study these processes. Here, we show that the transcriptional coactivator of the Hippo pathway, Yorkie (YAP/TAZ in vertebrates), plays distinct roles in the developing Drosophila airways. Yorkie exerts a cytoplasmic function by binding Drosophila Twinstar, the orthologue of the vertebrate actin-severing protein Cofilin, to regulate F-actin levels and apical cell membrane size, which are required for proper tracheal tube elongation. Second, Yorkie controls water tightness of tracheal tubes by transcriptional regulation of the δ-aminolevulinate synthase gene (Alas). We conclude that Yorkie has a dual role in tracheal development to ensure proper tracheal growth and functionality.

F-actin dynamics regulates mammalian organ growth and cell fate maintenance

BACKGROUND & AIMS

In vitro, cell function can be potently regulated by the mechanical properties of cells and of their microenvironment. Cells measure these features by developing forces via their actomyosin cytoskeleton, and respond accordingly by regulating intracellular pathways, including the transcriptional coactivators YAP/TAZ. Whether mechanical cues are relevant for in vivo regulation of adult organ homeostasis, and whether this occurs through YAP/TAZ, remains largely unaddressed.

METHODS

We developed Capzb conditional knockout mice and obtained primary fibroblasts to characterize the role of CAPZ in vitro. In vivo functional analyses were carried out by inducing Capzb inactivation in adult hepatocytes, manipulating YAP/Hippo activity by hydrodynamic tail vein injections, and treating mice with the ROCK inhibitor, fasudil.

RESULTS

We found that the F-actin capping protein CAPZ restrains actomyosin contractility: Capzb inactivation alters stress fiber and focal adhesion dynamics leading to enhanced myosin activity, increased traction forces, and increased liver stiffness. In vitro, this rescues YAP from inhibition by a small cellular geometry; in vivo, it induces YAP activation in parallel to the Hippo pathway, causing extensive hepatocyte proliferation and leading to striking organ overgrowth. Moreover, Capzb is required for the maintenance of the differentiated hepatocyte state, for metabolic zonation, and for gluconeogenesis. In keeping with changes in tissue mechanics, inhibition of the contractility regulator ROCK, or deletion of the Yap1 mechanotransducer, reverse the phenotypes emerging in Capzb-null livers.

CONCLUSIONS

These results indicate a previously unsuspected role for CAPZ in tuning the mechanical properties of cells and tissues, which is required in hepatocytes for the maintenance of the differentiated state and to regulate organ size. More generally, it indicates for the first time that mechanotransduction has a physiological role in maintaining liver homeostasis in mammals.

LAY SUMMARY

The mechanical properties of cells and tissues (i.e. whether they are soft or stiff) are thought to be important regulators of cell behavior. Herein, we found that inactivation of the protein CAPZ alters the mechanical properties of cells and liver tissues, leading to YAP hyperactivation. In turn, this profoundly alters liver physiology, causing organ overgrowth, defects in liver cell differentiation and metabolism. These results reveal a previously uncharacterized role for mechanical signals in the maintenance of adult liver homeostasis.

Switch-like enhancement of epithelial-mesenchymal transition by YAP through feedback regulation of WT1 and Rho-family GTPases

Collective cell migration occurs in many patho-physiological states, including wound healing and invasive cancer growth. The integrity of the expanding epithelial sheets depends on extracellular cues, including cell-cell and cell-matrix interactions. We show that the nano-scale topography of the extracellular matrix underlying epithelial cell layers can strongly affect the speed and morphology of the fronts of the expanding sheet, triggering partial and complete epithelial-mesenchymal transitions (EMTs). We further demonstrate that this behavior depends on the mechano-sensitivity of the transcription regulator YAP and two new YAP-mediated cross-regulating feedback mechanisms: Wilms Tumor-1-YAP-mediated downregulation of E-cadherin, loosening cell-cell contacts, and YAP-TRIO-Merlin mediated regulation of Rho GTPase family proteins, enhancing cell migration. These YAP-dependent feedback loops result in a switch-like change in the signaling and the expression of EMT-related markers, leading to a robust enhancement in invasive cell spread, which may lead to a worsened clinical outcome in renal and other cancers.

Reorganisation of the extracellular matrix (ECM) controls processes involving epithelial-mesenchymal transition (EMT). Here, the authors show that EMT occurring in epithelial cells on a fabricated nano-engineered cell adhesion surface is triggered by mechanical cues from the ECM.

Rap1 Negatively Regulates the Hippo Pathway to Polarize Directional Protrusions in Collective Cell Migration

In collective cell migration, directional protrusions orient cells in response to external cues, which requires coordinated polarity among the migrating cohort. However, the molecular mechanism has not been well defined. Drosophila border cells (BCs) migrate collectively and invade via the confined space between nurse cells, offering an in vivo model to examine how group polarity is organized. Here, we show that the front/back polarity of BCs requires Rap1, hyperactivation of which disrupts cluster polarity and induces misoriented protrusions and loss of asymmetry in the actin network. Conversely, hypoactive Rap1 causes fewer protrusions and cluster spinning during migration. A forward genetic screen revealed that downregulation of the Hippo (Hpo) pathway core components hpo or mats enhances the Rap1^V12-induced migration defect and misdirected protrusions. Mechanistically, association of Rap1^V12 with the kinase domain of Hpo suppresses its activity, which releases Hpo signaling-mediated suppression of F-actin elongation, promoting cellular protrusions in collective cell migration.

Role of the Hippo Pathway in Fibrosis and Cancer

The Hippo pathway is the key player in various signaling processes, including organ development and maintenance of tissue homeostasis. This pathway comprises a core kinases module and transcriptional activation module, representing a highly conserved mechanism from Drosophila to vertebrates. The central MST1/2-LATS1/2 kinase cascade in this pathway negatively regulates YAP/TAZ transcription co-activators in a phosphorylation-dependent manner. Nuclear YAP/TAZ bind to transcription factors to stimulate gene expression, contributing to the regenerative potential and regulation of cell growth and death. Recent studies have also highlighted the potential role of Hippo pathway dysfunctions in the pathology of several diseases. Here, we review the functional characteristics of the Hippo pathway in organ fibrosis and tumorigenesis, and discuss its potential as new therapeutic targets.

The complex entanglement of Hippo-Yap/Taz signaling in tumor immunity

The Hippo-Yap/Taz pathway, originally identified as a central developmental regulator of organ size, has been found perturbed in many types of human tumors, and linked to tumor growth, survival, evasion, metastasis, stemness, and drug resistance. Beside these tumor-cell-intrinsic functions, Hippo signaling also plays important immune-regulatory roles. In this review, we will summarize and discuss recent breakthroughs in our understanding of how various components of the Hippo-Yap/Taz pathway influence the tumor immune microenvironment, including their effects on the tumor secretome and immune infiltrates, their roles in regulating crosstalk between tumor cells and T cells, and finally their intrinsic functions in various types of innate and adaptive immune cells. While further research is needed to integrate and reconcile existing findings and to discern the overall effects of Hippo signaling on tumor immunity, it is clear that Hippo signaling functions as a key bridge connecting tumor cells with both the adaptive and innate immune systems. Thus, all future therapeutic development against the Hippo-Yap/Taz pathway should take into account their multi-faceted roles in regulating tumor immunity in addition to their growth-regulatory functions. Given that immune therapies have become the mainstay of cancer treatment, it is also important to pursue how to manipulate Hippo signaling to boost response or overcome resistance to existing immune therapies.

Deciphering Nuclear Mechanobiology in Laminopathy

Extracellular mechanical stimuli are translated into biochemical signals inside the cell via mechanotransduction. The nucleus plays a critical role in mechanoregulation, which encompasses mechanosensing and mechanotransduction. The nuclear lamina underlying the inner nuclear membrane not only maintains the structural integrity, but also connects the cytoskeleton to the nuclear envelope. Lamin mutations, therefore, dysregulate the nuclear response, resulting in abnormal mechanoregulations, and ultimately, disease progression. Impaired mechanoregulations even induce malfunction in nuclear positioning, cell migration, mechanosensation, as well as differentiation. To know how to overcome laminopathies, we need to understand the mechanisms of laminopathies in a mechanobiological way. Recently, emerging studies have demonstrated the varying defects from lamin mutation in cellular homeostasis within mechanical surroundings. Therefore, this review summarizes recent findings highlighting the role of lamins, the architecture of nuclear lamina, and their disease relevance in the context of nuclear mechanobiology. We will also provide an overview of the differentiation of cellular mechanics in laminopathy.

Thy-1 in Integrin Mediated Mechanotransduction

The glycosylphosphatidylinositol (GPI) anchored glycoprotein Thy-1 has been prevalently expressed on the surface of various cell types. The biological function of Thy-1 ranges from T cell activation, cell adhesion, neurite growth, differentiation, metastasis and fibrogenesis and has been extensively reviewed elsewhere. However, current discoveries implicate Thy-1 also functions as a key mechanotransduction mediator. In this review, we will be focusing on the role of Thy-1 in translating extracellular mechanic cues into intracellular biological cascades. The mechanotransduction capability of Thy-1 relies on trans and cis interaction between Thy-1 and RGD-binding integrins; and will be discussed in depth in the review.

Regulation of YAP/TAZ Activity by Mechanical Cues: An Experimental Overview

YAP/TAZ activity is regulated by a complex network of signals that include the Hippo pathway, cell polarity complexes, and signaling receptors of the RTK, GPCR, and WNT pathways and by a seamlessly expanding number of intracellular cues including energy and mevalonate metabolism. Among these inputs, we here concentrate on mechanical cues embedded in the extracellular matrix (ECM) microenvironment, which are key regulators of YAP/TAZ activity. We review the techniques that have been used to study mechano-regulation of YAP/TAZ, including conceptual and practical considerations on how these experiments should be designed and controlled. Finally, we briefly review the most appropriate techniques to monitor YAP/TAZ activity in these experiments and their significance to study the mechanisms linking YAP/TAZ to mechanical cues.

Luciferase Reporter Assays to Determine YAP/TAZ Activity in Mammalian Cells

This chapter describes the luciferase assays that are available to monitor YAP/TAZ activity in cell lines and to study their regulation, including the choice for the normalizer, a description of the main YAP-/TAZ-responsive luciferase reporters used so far by the community, and technical notes and experimental considerations on the most appropriate positive controls. Some specific examples are provided to use luciferase assays as the basis to distinguish between Hippo-mediated and phosphorylation-mediated regulatory events and regulatory events that regulate YAP/TAZ independent of these inputs. Finally, typical experimental protocols are outlined briefly for an easier setup of YAP/TAZ luciferase assays.

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.

Shavenbaby and Yorkie mediate Hippo signaling to protect adult stem cells from apoptosis

To compensate for accumulating damages and cell death, adult homeostasis (e.g., body fluids and secretion) requires organ regeneration, operated by long-lived stem cells. How stem cells can survive throughout the animal life remains poorly understood. Here we show that the transcription factor Shavenbaby (Svb, OvoL in vertebrates) is expressed in renal/nephric stem cells (RNSCs) of Drosophila and required for their maintenance during adulthood. As recently shown in embryos, Svb function in adult RNSCs further needs a post-translational processing mediated by the Polished rice (Pri) smORF peptides and impairing Svb function leads to RNSC apoptosis. We show that Svb interacts both genetically and physically with Yorkie (YAP/TAZ in vertebrates), a nuclear effector of the Hippo pathway, to activate the expression of the inhibitor of apoptosis DIAP1. These data therefore identify Svb as a nuclear effector in the Hippo pathway, critical for the survival of adult somatic stem cells.

The Hippo Signaling Network and Its Biological Functions

Hippo signaling is an evolutionarily conserved network that has a central role in regulating cell proliferation and cell fate to control organ growth and regeneration. It promotes activation of the LATS kinases, which control gene expression by inhibiting the activity of the transcriptional coactivator proteins YAP and TAZ in mammals and Yorkie in Drosophila. Diverse upstream inputs, including both biochemical cues and biomechanical cues, regulate Hippo signaling and enable it to have a key role as a sensor of cells' physical environment and an integrator of growth control signals. Several components of this pathway localize to cell-cell junctions and contribute to regulation of Hippo signaling by cell polarity, cell contacts, and the cytoskeleton. Downregulation of Hippo signaling promotes uncontrolled cell proliferation, impairs differentiation, and is associated with cancer. We review the current understanding of Hippo signaling and highlight progress in the elucidation of its regulatory mechanisms and biological functions.

YAP/TAZ Signaling as a Molecular Link between Fibrosis and Cancer

Tissue fibrosis is a pathological condition that is associated with impaired epithelial repair and excessive deposition of extracellular matrix (ECM). Fibrotic lesions increase the risk of cancer in various tissues, but the mechanism linking fibrosis and cancer is unclear. Yes-associated protein (YAP) and the transcriptional coactivator with PDZ-binding motif (TAZ) are core components of the Hippo pathway, which have multiple biological functions in the development, homeostasis, and regeneration of tissues and organs. YAP/TAZ act as sensors of the structural and mechanical features of the cell microenvironment. Recent studies have shown aberrant YAP/TAZ activation in both fibrosis and cancer in animal models and human tissues. In fibroblasts, ECM stiffness mechanoactivates YAP/TAZ, which promote the production of profibrotic mediators and ECM proteins. This results in tissue stiffness, thus establishing a feed-forward loop of fibroblast activation and tissue fibrosis. In contrast, in epithelial cells, YAP/TAZ are activated by the disruption of cell polarity and increased ECM stiffness in fibrotic tissues, which promotes the proliferation and survival of epithelial cells. YAP/TAZ are also involved in the epithelial–mesenchymal transition (EMT), which contributes to tumor progression and cancer stemness. Importantly, the crosstalk with transforming growth factor (TGF)-β signaling and Wnt signaling is essential for the profibrotic and tumorigenic roles of YAP/TAZ. In this article, we review the latest advances in the pathobiological roles of YAP/TAZ signaling and their function as a molecular link between fibrosis and cancer.

NUAK2 is a critical YAP target in liver cancer

The Hippo-YAP signaling pathway is a critical regulator of proliferation, apoptosis, and cell fate. The main downstream effector of this pathway, YAP, has been shown to be misregulated in human cancer and has emerged as an attractive target for therapeutics. A significant insufficiency in our understanding of the pathway is the identity of transcriptional targets of YAP that drive its potent growth phenotypes. Here, using liver cancer as a model, we identify NUAK2 as an essential mediator of YAP-driven hepatomegaly and tumorigenesis in vivo. By evaluating several human cancer cell lines we determine that NUAK2 is selectively required for YAP-driven growth. Mechanistically, we found that NUAK2 participates in a feedback loop to maximize YAP activity via promotion of actin polymerization and myosin activity. Additionally, pharmacological inactivation of NUAK2 suppresses YAP-dependent cancer cell proliferation and liver overgrowth. Importantly, our work here identifies a specific, potent, and actionable target for YAP-driven malignancies.

Hippo-YAP pathway plays an important role in cancers; however the in vivo relevance of YAP/TAZ target genes is unclear. Here, the authors show that NUAK2 is a target of YAP and participates in a feedback loop to maximize YAP activity. Inhibition of NUAK2 suppresses YAP-driven hepatomegaly and liver cancer growth, offering a new target for cancer therapy.

Programming Niche Accessibility and In Vitro Stemness with Intercellular DNA Reactions

Stem cells generally exist in low abundance and tend to lose stemness in the absence of self-renewal signals. While extracellular-matrix-mimicking techniques have been developed to support stem cell proliferation, the lack of niche cells in these synthetic systems often hampers continuous stem cell expansion and maintenance of pluripotency, which are indispensable for regenerative medicine. Here, an intercellular DNA-reaction-programmed ESPN (expansion of stem cells with pairing niches) strategy is developed for 3D culture of mammary stem cells (MaSCs). Boolean logic operations are implemented to confer DNA-programmed mechanical signaling and genetically engineered morphogen signaling by niche cells, resulting in sustained expansion of MaSCs in vitro. The creation of stem cell niches improves the proliferation of pluripotent cells by four times during one-week culture. This method thus provides a novel approach for logical regulation of stemness and proliferation of stem cells for biomedicine.

Angiomotins stimulate LATS kinase autophosphorylation and act as scaffolds that promote Hippo signaling

The Hippo pathway controls cell proliferation, differentiation, and survival by regulating the Yes-associated protein (YAP) transcriptional coactivator in response to various stimuli, including the mechanical environment. The major YAP regulators are the LATS1/2 kinases, which phosphorylate and inhibit YAP. LATS1/2 are activated by phosphorylation on a hydrophobic motif (HM) outside of the kinase domain by MST1/2 and other kinases. Phosphorylation of the HM motif then triggers autophosphorylation of the kinase in the activation loop to fully activate the kinase, a process facilitated by MOB1. The angiomotin family of proteins (AMOT, AMOTL1, and AMOTL2) bind LATS1/2 and promote its kinase activity and YAP phosphorylation through an unknown mechanism. Here we show that angiomotins increase Hippo signaling through multiple mechanisms. We found that, by binding LATS1/2, SAV1, and YAP, angiomotins function as a scaffold that connects LATS1/2 to both its activator SAV1-MST1 and its target YAP. Deletion of all three angiomotins reduced the association of LATS1 with SAV1-MST1 and decreased MST1/2-mediated LATS1/2-HM phosphorylation. Angiomotin deletion also reduced LATS1/2's ability to associate with and phosphorylate YAP. In addition, we found that angiomotins have an unexpected function along with MOB1 to promote autophosphorylation of LATS1/2 on the activation loop motif independent of HM phosphorylation. These results indicate that angiomotins enhance Hippo signaling by stimulating LATS1/2 autophosphorylation and by connecting LATS1/2 with both its activator SAV1-MST1/2 and its substrate YAP.

Modulation of the Hippo pathway and organ growth by RNA processing proteins

The Hippo tumor-suppressor pathway regulates organ growth, cell proliferation, and stem cell biology. Defects in Hippo signaling and hyperactivation of its downstream effectors-Yorkie (Yki) in Drosophila and YAP/TAZ in mammals-result in progenitor cell expansion and overgrowth of multiple organs and contribute to cancer development. Deciphering the mechanisms that regulate the activity of the Hippo pathway is key to understanding its function and for therapeutic targeting. However, although the Hippo kinase cascade and several other upstream inputs have been identified, the mechanisms that regulate Yki/YAP/TAZ activity are still incompletely understood. To identify new regulators of Yki activity, we screened in Drosophila for suppressors of tissue overgrowth and Yki activation caused by overexpression of atypical protein kinase C (aPKC), a member of the apical cell polarity complex. In this screen, we identified mutations in the heterogeneous nuclear ribonucleoprotein Hrb27C that strongly suppressed the tissue defects induced by ectopic expression of aPKC. Hrb27C was required for aPKC-induced tissue growth and Yki target gene expression but did not affect general gene expression. Genetic and biochemical experiments showed that Hrb27C affects Yki phosphorylation. Other RNA-binding proteins known to interact with Hrb27C for mRNA transport in oocytes were also required for normal Yki activity, although they suppressed Yki output. Based on the known functions of Hrb27C, we conclude that Hrb27C-mediated control of mRNA splicing, localization, or translation is essential for coordinated activity of the Hippo pathway.

Regulation of Hippo signaling by actin remodeling

The Hippo signaling pathway controls nuclear accumulation and stability of the transcriptional coregulator YAP and its paralog TAZ. The activity of Hippo-YAP signaling is influenced not only by biochemical signals, but also by cell shape and mechanical tension transmitted through cell-cell junctions and cell-matrix adhesions. Data accumulated thus far indicates that the actin cytoskeleton is a key mediator of the regulation of Hippo-YAP signaling by means of a variety of biochemical and mechanical cues. In this review, we have outlined the role of actin dynamics and actin-associated proteins in the regulation of Hippo-YAP signaling. In addition, we discuss actinmediated regulation of YAP/TAZ activity independent of the core Hippo kinases MST and LATS. Although our understanding of the link between Hippo-YAP signaling and the actin cytoskeleton is progressing rapidly, many open questions remain. [BMB Reports 2018; 51(3): 151-156].

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.

The regulation and function of the Hippo pathway in heart regeneration

Heart failure caused by cardiomyocyte loss and fibrosis is a leading cause of death worldwide. Although current treatments for heart failure such as heart transplantation and left ventricular assist device implantation have obvious value, new approaches are needed. Endogenous adult cardiomyocyte renewal is measurable but inefficient and inadequate in response to extensive acute heart damage. Stimulating self-renewal of endogenous cardiomyocytes holds great promise for heart repair. Uncovering the genetic mechanisms underlying cardiomyocyte renewal is a critical step in developing new approaches to repairing the heart. Recent studies have revealed that the inhibition of the Hippo pathway is sufficient to promote the proliferation of endogenous cardiomyocytes, indicating that the manipulation of the Hippo pathway in the heart may be a promising treatment for heart failure in the future. We summarize recent findings that have shed light on the function of the Hippo pathway in heart regeneration. We also discuss the mechanisms by which Hippo pathway inhibition promotes heart regeneration and how the Hippo pathway responds to different types of injury or stress during the regenerative process. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration.

AMOT130 linking F-actin to YAP is involved in intervertebral disc degeneration

OBJECTIVES

Dysregulation of YAP by the Hippo signalling is associated with intervertebral disc degeneration (IDD). However, the relationship between the F-actin and Hippo pathway in IDD, and their effects on YAP remain poorly understood.

METHODS

The characteristics of Hippo pathway and F-actin the in the NP (nucleus pulposus) and annulus fibrosus of immature, mature, ageing and disc degeneration model rats were observed by immunofluorescence, western blot and qPCR. Nucleus pulposus cells (NPCs) were transfected with lentivirus Sh-LATS A, Sh-LATS B and harvested for SA-β-gal staining, qPCR, western blotting and immunofluorescence staining to investigate the mechanism of Hippo pathway and F-actin interact in NPCs.

RESULTS

We observed moderate decreases in F-actin and YAP expression with age in healthy intervertebral discs (IVDs). F-actin stress fibres distributed throughout the cytoplasm disappeared following treatment with latrunculin B (Lat B), resulting in a punctate distribution. Depletion of large tumour suppressor homologues 1/2 (LATS1/2) did not decrease the rate of cellular senescence, and YAP remained in the cytoplasm following Lat B treatment. Furthermore, angiomotin 130 (AMOT130) was associated with F-actin through a conserved actin-binding domain to retain YAP in the cytoplasm.

CONCLUSIONS

This study showed that a mechanism by which Hippo pathway and F-actin synergize to modulate YAP activation and localization in the context of IDD and help to control NPCs proliferation.

Zyxin promotes colon cancer tumorigenesis in a mitotic phosphorylation-dependent manner and through CDK8-mediated YAP activation

Zyxin is a member of the focal adhesion complex and plays a critical role in actin filament polymerization and cell motility. Several recent studies showed that Zyxin is a positive regulator of Yki/YAP (Yes-associated protein) signaling. However, little is known about the mechanisms by which Zyxin itself is regulated and how Zyxin affects Hippo-YAP activity. We first showed that Zyxin is phosphorylated by CDK1 during mitosis. Depletion of Zyxin resulted in significantly impaired colon cancer cell proliferation, migration, anchorage-independent growth, and tumor formation in xenograft animal models. Mitotic phosphorylation is required for Zyxin activity in promoting growth. Zyxin regulates YAP activity through the colon cancer oncogene CDK8. CDK8 knockout phenocopied Zyxin knockdown in colon cancer cells, while ectopic expression of CDK8 substantially restored the tumorigenic defects of Zyxin-depletion cells. Mechanistically, we showed that CDK8 directly phosphorylated YAP and promoted its activation. Fully activated YAP is required to support the growth in CDK8-knockout colon cancer cells in vitro and in vivo. Together, these observations suggest that Zyxin promotes colon cancer tumorigenesis in a mitotic-phosphorylation-dependent manner and through CDK8-mediated YAP activation.

Regulation of the Hippo pathway in cancer biology

The Hippo tumor suppressor pathway, which is well conserved from Drosophila to humans, has emerged as the master regulator of organ size, as well as major cellular properties, such as cell proliferation, survival, stemness, and tissue homeostasis. The biological significance and deregulation of the Hippo pathway in tumorigenesis have received a surge of interest in the past decade. In the current review, we present the major discoveries that made substantial contributions to our understanding of the Hippo pathway and discuss how Hippo pathway components contribute to cellular signaling, physiology, and their potential implications in anticancer therapeutics.

Multifaceted regulation and functions of YAP/TAZ in tumors (Review)

The Hippo pathway, initially identified through screenings for mutant tumor suppressors in Drosophila, is an evolutionarily conserved signaling pathway that controls organ size by regulating cell proliferation and apoptosis. Abnormal regulation of the Hippo pathway may lead to cancer in mammals. As the major downstream effectors of the Hippo pathway, unphosphorylated Yes-associated protein (YAP) and its homolog transcriptional co-activator TAZ (also called WWTR1) (hereafter called YAP/TAZ) are translocated into the nucleus. In the nucleus, in order to induce target gene expression, YAP/TAZ bind to the TEA domain (TEAD) proteins, and this binding subsequently promotes cell proliferation and inhibits apoptosis. In contrast, as key regulators of tumorigenesis and development, YAP/TAZ are phosphorylated and regulated by multiple molecules and pathways including Lats1/2 of Hippo, Wnt and G-protein-coupled receptor (GPCR) signaling, with a regulatory role in cell physiology, tumor cell development and pathological abnormalities simultaneously. In particular, the crucial role of YAP/TAZ in tumors ensures their potential as targets in designing anticancer drugs. To date, mounting research has elucidated the suppression of YAP/TAZ via effective inhibitors, which significantly highlights their application in cancer treatment. In the present review, we focus on the functions of YAP/TAZ in cancer, discuss their potential as new therapeutic target for tumor treatment, and provide valuable suggestions for further study in this field.

Quantitative Analysis Reveals that Actin and Src-Family Kinases Regulate Nuclear YAP1 and Its Export

The transcriptional regulator YAP1 is critical for the pathological activation of fibroblasts. In normal fibroblasts, YAP1 is located in the cytoplasm, while in activated cancer-associated fibroblasts, it is nuclear and promotes the expression of genes required for pro-tumorigenic functions. Here, we investigate the dynamics of YAP1 shuttling in normal and activated fibroblasts, using EYFP-YAP1, quantitative photobleaching methods, and mathematical modeling. Imaging of migrating fibroblasts reveals the tight temporal coupling of cell shape change and altered YAP1 localization. Both 14-3-3 and TEAD binding modulate YAP1 shuttling, but neither affects nuclear import. Instead, we find that YAP1 nuclear accumulation in activated fibroblasts results from Src and actomyosin-dependent suppression of phosphorylated YAP1 export. Finally, we show that nuclear-constrained YAP1, upon XPO1 depletion, remains sensitive to blockade of actomyosin function. Together, these data place nuclear export at the center of YAP1 regulation and indicate that the cytoskeleton can regulate YAP1 within the nucleus.

Growth and size control during development

The size and shape of organs are characteristic for each species. Even when organisms develop to different sizes due to varying environmental conditions, such as nutrition, organ size follows species-specific rules of proportionality to the rest of the body, a phenomenon referred to as allometry. Therefore, for a given environment, organs stop growth at a predictable size set by the species's genotype. How do organs stop growth? How can related species give rise to organs of strikingly different size? No definitive answer has been given to date. One of the major models for the studies of growth termination is the vinegar fly Drosophila melanogaster. Therefore, this review will focus mostly on work carried out in Drosophila to try to tease apart potential mechanisms and identify routes for further investigation. One general rule, found across the animal kingdom, is that the rate of growth declines with developmental time. Therefore, answers to the problem of growth termination should explain this seemingly universal fact. In addition, growth termination is intimately related to the problems of robustness (i.e. precision) and plasticity in organ size, symmetric and asymmetric organ development, and of how the ‘target’ size depends on extrinsic, environmental factors.

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.

STEF/TIAM2-mediated Rac1 activity at the nuclear envelope regulates the perinuclear actin cap

The perinuclear actin cap is an important cytoskeletal structure that regulates nuclear morphology and re-orientation during front-rear polarisation. The mechanisms regulating the actin cap are currently poorly understood. Here, we demonstrate that STEF/TIAM2, a Rac1 selective guanine nucleotide exchange factor, localises at the nuclear envelope, co-localising with the key perinuclear proteins Nesprin-2G and Non-muscle myosin IIB (NMMIIB), where it regulates perinuclear Rac1 activity. We show that STEF depletion reduces apical perinuclear actin cables (a phenotype rescued by targeting active Rac1 to the nuclear envelope), increases nuclear height and impairs nuclear re-orientation. STEF down-regulation also reduces perinuclear pMLC and decreases myosin-generated tension at the nuclear envelope, suggesting that STEF-mediated Rac1 activity regulates NMMIIB activity to promote stabilisation of the perinuclear actin cap. Finally, STEF depletion decreases nuclear stiffness and reduces expression of TAZ-regulated genes, indicating an alteration in mechanosensing pathways as a consequence of disruption of the actin cap.

Src Cooperates with Oncogenic Ras in Tumourigenesis via the JNK and PI3K Pathways in Drosophila epithelial Tissue

The Ras oncogene (Rat Sarcoma oncogene, a small GTPase) is a key driver of human cancer, however alone it is insufficient to produce malignancy, due to the induction of cell cycle arrest or senescence. In a Drosophila melanogaster genetic screen for genes that cooperate with oncogenic Ras (bearing the Ras^V12 mutation, or Ras^ACT), we identified the Drosophila Src (Sarcoma virus oncogene) family non-receptor tyrosine protein kinase genes, Src42A and Src64B, as promoting increased hyperplasia in a whole epithelial tissue context in the Drosophila eye. Moreover, overexpression of Src cooperated with Ras^ACT in epithelial cell clones to drive neoplastic tumourigenesis. We found that Src overexpression alone activated the Jun N-terminal Kinase (JNK) signalling pathway to promote actin cytoskeletal and cell polarity defects and drive apoptosis, whereas, in cooperation with Ras^ACT, JNK led to a loss of differentiation and an invasive phenotype. Src + Ras^ACT cooperative tumourigenesis was dependent on JNK as well as Phosphoinositide 3-Kinase (PI3K) signalling, suggesting that targeting these pathways might provide novel therapeutic opportunities in cancers dependent on Src and Ras signalling.

Ingestion of Food Particles Regulates the Mechanosensing Misshapen-Yorkie Pathway in Drosophila Intestinal Growth

The intestinal epithelium has a high cell turnover rate and is an excellent system to study stem cell-mediated adaptive growth. In the Drosophila midgut, the Ste20 kinase Misshapen, which is distally related to Hippo, has a niche function to restrict intestinal stem cell activity. We show here that, under low growth conditions, Misshapen is localized near the cytoplasmic membrane, is phosphorylated at the threonine 194 by the upstream kinase Tao, and is more active toward Warts, which in turn inhibits Yorkie. Ingestion of yeast particles causes a midgut distention and a reduction of Misshapen membrane association and activity. Moreover, Misshapen phosphorylation is regulated by the stiffness of cell culture substrate, changing of actin cytoskeleton, and ingestion of inert particles. These results together suggest that dynamic membrane association and Tao phosphorylation of Misshapen are steps that link the mechanosensing of intestinal stretching after food particle ingestion to control adaptive growth.

E-cadherin in contact inhibition and cancer

E-cadherin is a key component of the adherens junctions that are integral in cell adhesion and maintaining epithelial phenotype of cells. Homophilic E-cadherin binding between cells is important in mediating contact inhibition of proliferation when cells reach confluence. Loss of E-cadherin expression results in loss of contact inhibition and is associated with increased cell motility and advanced stages of cancer. In this review we discuss the role of E-cadherin and its downstream signaling in regulation of contact inhibition and the development and progression of cancer.

Cell Junctions in Hippo Signaling

The Hippo signal transduction pathway is an important regulator of organ growth and cell differentiation, and its deregulation contributes to the development of cancer. The activity of the Hippo pathway is strongly dependent on cell junctions, cellular architecture, and the mechanical properties of the microenvironment. In this review, we discuss recent advances in our understanding of how cell junctions transduce signals from the microenvironment and control the activity of the Hippo pathway. We also discuss how these mechanisms may control organ growth during development and regeneration, and how defects in them deregulate Hippo signaling in cancer cells.

Role of Hippo Pathway Effector Tafazzin Protein in Maintaining Stemness of Umbilical Cord-Derived Mesenchymal Stem Cells (UC-MSC)

Tafazzin (TAZ) protein has been upregulated in various types of human cancers, although the basis for elevation is uncertain, it has been made definite that the effect of mutation in the hippo pathway, particularly when it is switched off, considerably activates tafazzin transcriptionally and thus this results in tissue or tumor overgrowth. Recent perceptions into the activity of tafazzin, have ascribed to it, a role as stem cell factor in mouse mesenchymal and as well as in neural stem cells. Being a downstream molecule in Hippo signalling, phosphorylation or dephosphorylation of tafazzin gene regulates its transcriptional activity and the stemness of mesenchymal stem cells. Commonly, extracellular matrix controls the stem cell fate commitment and perhaps tafazzin controls stemness through altering the extra cellular matrix. Extracellular matrix is generally made up of prime proteoglycans and the fate stabilization of the resulting lineages is surveilled by engineering these glycans. Tafazzin degradation and addition of proteoglycans affect physical attributes of the extracellular matrix that drives cell differentiation into various lineages. Thus, tafazzin along with major glycans present in the extracellular matrix is involved in imparting stemness. However, there are incoherent molecular events, wherein both tafazzin and the extracellular matrix components, together either activate or inhibit differentiation of stem cells. This review discusses about the role of tafazzin oncoprotein as a stemness factor.

Upstairs, downstairs: spatial regulation of Hippo signalling

Cellular signalling lies at the heart of every decision involved in the development and homeostasis of multicellular organisms. The Hippo pathway was discovered nearly two decades ago through seminal work in Drosophila and rapidly emerged as a crucial signalling network implicated in developmental and oncogenic growth, tissue regeneration and stem cell biology. Here, we review recent advances in the field relating to the upstream regulation of Hippo signalling and the intracellular tug-of-war that tightly controls its main target, the transcriptional co-activator Yorkie/YAP.

Alternative Splicing in the Hippo Pathway-Implications for Disease and Potential Therapeutic Targets

Alternative splicing is a well-studied gene regulatory mechanism that produces biological diversity by allowing the production of multiple protein isoforms from a single gene. An involvement of alternative splicing in the key biological signalling Hippo pathway is emerging and offers new therapeutic avenues. This review discusses examples of alternative splicing in the Hippo pathway, how deregulation of these processes may contribute to disease and whether these processes offer new potential therapeutic targets.

Modelling Cooperative Tumorigenesis in Drosophila

The development of human metastatic cancer is a multistep process, involving the acquisition of several genetic mutations, tumour heterogeneity, and interactions with the surrounding microenvironment. Due to the complexity of cancer development in mammals, simpler model organisms, such as the vinegar fly, Drosophila melanogaster, are being utilized to provide novel insights into the molecular mechanisms involved. In this review, we highlight recent advances in modelling tumorigenesis using the Drosophila model, focusing on the cooperation of oncogenes or tumour suppressors, and the interaction of mutant cells with the surrounding tissue in epithelial tumour initiation and progression.

Tension-dependent regulation of mammalian Hippo signaling through LIMD1

Hippo signaling is regulated by biochemical and biomechanical cues that influence the cytoskeleton, but the mechanisms that mediate this have remained unclear. We show that all three mammalian Ajuba family proteins - AJUBA, LIMD1 and WTIP - exhibit tension-dependent localization to adherens junctions, and that both LATS family proteins, LATS1 and LATS2, exhibit an overlapping tension-dependent junctional localization. This localization of Ajuba and LATS family proteins is also influenced by cell density, and by Rho activation. We establish that junctional localization of LATS kinases requires LIMD1, and that LIMD1 is also specifically required for the regulation of LATS kinases and YAP1 by Rho. Our results identify a biomechanical pathway that contributes to regulation of mammalian Hippo signaling, establish that this occurs through tension-dependent LIMD1-mediated recruitment and inhibition of LATS kinases in junctional complexes, and identify roles for this pathway in both Rho-mediated and density-dependent regulation of Hippo signaling.

Linking Extracellular Matrix Agrin to the Hippo Pathway in Liver Cancer and Beyond

In addition to the structural and scaffolding role, the extracellular matrix (ECM) is emerging as a hub for biomechanical signal transduction that is frequently relayed to intracellular sensors to regulate diverse cellular processes. At a macroscopic scale, matrix rigidity confers long-ranging effects contributing towards tissue fibrosis and cancer. The transcriptional co-activators YAP/TAZ, better known as the converging effectors of the Hippo pathway, are widely recognized for their new role as nuclear mechanosensors during organ homeostasis and cancer. Still, how YAP/TAZ senses these “stiffness cues” from the ECM remains enigmatic. Here, we highlight the recent perspectives on the role of agrin in mechanosignaling from the ECM via antagonizing the Hippo pathway to activate YAP/TAZ in the contexts of cancer, neuromuscular junctions, and cardiac regeneration.

The Hippo pathway as a drug target in gastric cancer

The Hippo tumor suppressor pathway is critical for balancing cellular differentiation and proliferation in response to cell-cell contact, mechanical signals and diffusible signals such as lysophosphatidic acid. Hippo pathway signaling is frequently dysregulated in gastric cancer (GC), as well as many other kinds of solid tumors, contributing to multiple aspects of malignant progression including unchecked cell division and metastasis. Considering the importance of this Hippo pathway in cancer, its pharmacological disruption may be of huge benefit in the fight against this disease. In this review, we summarize the components of the Hippo pathway, its crosstalk with other major oncogenic signaling pathways, common mechanisms of its dysregulation, as well as potential therapeutic approaches of targeting this pathway for cancer treatment, specifically in a GC context.

An evolutionarily conserved negative feedback mechanism in the Hippo pathway reflects functional difference between LATS1 and LATS2

The Hippo pathway represses YAP oncoprotein activity through phosphorylation by LATS kinases. Although variety of upstream components has been found to participate in the Hippo pathway, the existence and function of negative feedback has remained uncertain. We found that activated YAP, together with TEAD transcription factors, directly induces transcription of LATS2, but not LATS1, to form a negative feedback loop. We also observed increased mRNA levels of Hippo upstream components upon YAP activation. To reveal the physiological role of this negative feedback regulation, we deleted Lats2 or Lats1 in the liver-specific Sav1-knockout mouse model which develops a YAP-induced tumor. Additional deletion of Lats2 severely enhanced YAP-induced tumorigenic phenotypes in a liver specific Sav1 knock-out mouse model while additional deletion of Lats1 mildly affected the phenotype. Only Sav1 and Lats2 double knock-down cells formed larger colonies in soft agar assay, thereby recapitulating accelerated tumorigenesis seen in vivo. Importantly, this negative feedback is evolutionarily conserved, as Drosophila Yorkie (YAP ortholog) induces transcription of Warts (LATS2 ortholog) with Scalloped (TEAD ortholog). Collectively, we demonstrated the existence and function of an evolutionarily conserved negative feedback mechanism in the Hippo pathway, as well as the functional difference between LATS1 and LATS2 in regulation of YAP.

The apical scaffold big bang binds to spectrins and regulates the growth of Drosophila melanogaster wing discs

During development, cell proliferation is regulated, ensuring that tissues reach their correct size and shape. Forest et al. show that the Drosophila melanogaster scaffold protein big bang (Bbg) controls epithelial tissue growth without affecting epithelial polarity and architecture. Bbg interacts with spectrins at the apical cortex and promotes Yki signaling and actomyosin contractility.

During development, cell numbers are tightly regulated, ensuring that tissues and organs reach their correct size and shape. Recent evidence has highlighted the intricate connections between the cytoskeleton and the regulation of the key growth control Hippo pathway. Looking for apical scaffolds regulating tissue growth, we describe that Drosophila melanogaster big bang (Bbg), a poorly characterized multi-PDZ scaffold, controls epithelial tissue growth without affecting epithelial polarity and architecture. bbg-mutant tissues are smaller, with fewer cells that are less apically constricted than normal. We show that Bbg binds to and colocalizes tightly with the β-heavy–Spectrin/Kst subunit at the apical cortex and promotes Yki activity, F-actin enrichment, and the phosphorylation of the myosin II regulatory light chain Spaghetti squash. We propose a model in which the spectrin cytoskeleton recruits Bbg to the cortex, where Bbg promotes actomyosin contractility to regulate epithelial tissue growth.

Drosophila Big bang regulates the apical cytocortex and wing growth through junctional tension

Growth of epithelial tissues is regulated by a plethora of components, including signaling and scaffolding proteins, but also by junctional tension, mediated by the actomyosin cytoskeleton. However, how these players are spatially organized and functionally coordinated is not well understood. Here, we identify the Drosophila melanogaster scaffolding protein Big bang as a novel regulator of growth in epithelial cells of the wing disc by ensuring proper junctional tension. Loss of big bang results in the reduction of the regulatory light chain of nonmuscle myosin, Spaghetti squash. This is associated with an increased apical cell surface, decreased junctional tension, and smaller wings. Strikingly, these phenotypic traits of big bang mutant discs can be rescued by expressing constitutively active Spaghetti squash. Big bang colocalizes with Spaghetti squash in the apical cytocortex and is found in the same protein complex. These results suggest that in epithelial cells of developing wings, the scaffolding protein Big bang controls apical cytocortex organization, which is important for regulating cell shape and tissue growth.

Two faces of Hippo: activate or suppress the Hippo pathway in cancer

The Hippo pathway has generated considerable interest in recent years because of its involvement in several key hallmarks of cancer progression and metastasis. Research on the Hippo signaling pathway in cancer has been used to determine the activity of yes-associated protein (YAP) in tumorigenesis and disease progression. Previous studies have shown that the Hippo pathway can be used as a target to inhibit YAP activity and is a viable treatment for cancer. However, more studies are required to further advance our understanding of the Hippo signaling pathway in cancer. It has been shown that knockout of serine/threonine-kinases LATS1/2 in the Hippo pathway suppresses cancer immunity in mice. In addition, suppression of the oncogene YAP could contribute toward cancer immune therapy. Therefore, regulation of Hippo signaling can be an attractive alternative strategy for cancer treatment. This review will provide a summary of currently known compounds that activate or suppress the Hippo pathway.