The TEAD/TEF Family Protein Scalloped Mediates Transcriptional Output of the Hippo Growth-Regulatory Pathway

The Prospect of Identifying Resistance Mechanisms for Castrate-Resistant Prostate Cancer Using Circulating Tumor Cells: Is Epithelial-to-Mesenchymal Transition a Key Player?

Prostate cancer (PCa) is initially driven by excessive androgen receptor (AR) signaling with androgen deprivation therapy (ADT) being a major therapeutic approach to its treatment. However, the development of drug resistance is a significant limitation on the effectiveness of both first-line and more recently developed second-line ADTs. There is a need then to study AR signaling within the context of other oncogenic signaling pathways that likely mediate this resistance. This review focuses on interactions between AR signaling, the well-known phosphatidylinositol-3-kinase/AKT pathway, and an emerging mediator of these pathways, the Hippo/YAP1 axis in metastatic castrate-resistant PCa, and their involvement in the regulation of epithelial-mesenchymal transition (EMT), a feature of disease progression and ADT resistance. Analysis of these pathways in circulating tumor cells (CTCs) may provide an opportunity to evaluate their utility as biomarkers and address their importance in the development of resistance to current ADT with potential to guide future therapies.

A Positive Feedback Loop of Hippo- and c-Jun-Amino-Terminal Kinase Signaling Pathways Regulates Amyloid-Beta-Mediated Neurodegeneration

Alzheimer's disease (AD, OMIM: 104300) is an age-related disorder that affects millions of people. One of the underlying causes of AD is generation of hydrophobic amyloid-beta 42 (Aβ42) peptides that accumulate to form amyloid plaques. These plaques induce oxidative stress and aberrant signaling, which result in the death of neurons and other pathologies linked to neurodegeneration. We have developed a Drosophila eye model of AD by targeted misexpression of human Aβ42 in the differentiating retinal neurons, where an accumulation of Aβ42 triggers a characteristic neurodegenerative phenotype. In a forward deficiency screen to look for genetic modifiers, we identified a molecularly defined deficiency, which suppresses Aβ42-mediated neurodegeneration. This deficiency uncovers hippo (hpo) gene, a member of evolutionarily conserved Hippo signaling pathway that regulates growth. Activation of Hippo signaling causes cell death, whereas downregulation of Hippo signaling triggers cell proliferation. We found that Hippo signaling is activated in Aβ42-mediated neurodegeneration. Downregulation of Hippo signaling rescues the Aβ42-mediated neurodegeneration, whereas upregulation of Hippo signaling enhances the Aβ42-mediated neurodegeneration phenotypes. It is known that c-Jun-amino-terminal kinase (JNK) signaling pathway is upregulated in AD. We found that activation of JNK signaling enhances the Aβ42-mediated neurodegeneration, whereas downregulation of JNK signaling rescues the Aβ42-mediated neurodegeneration. We tested the nature of interactions between Hippo signaling and JNK signaling in Aβ42-mediated neurodegeneration using genetic epistasis approach. Our data suggest that Hippo signaling and JNK signaling, two independent signaling pathways, act synergistically upon accumulation of Aβ42 plaques to trigger cell death. Our studies demonstrate a novel role of Hippo signaling pathway in Aβ42-mediated neurodegeneration.

Hippo/YAP Signaling Pathway: A Promising Therapeutic Target in Bone Paediatric Cancers?

Osteosarcoma and Ewing sarcoma are the most prevalent bone pediatric tumors. Despite intensive basic and medical research studies to discover new therapeutics and to improve current treatments, almost 40% of osteosarcoma and Ewing sarcoma patients succumb to the disease. Patients with poor prognosis are related to either the presence of metastases at diagnosis or resistance to chemotherapy. Over the past ten years, considerable interest for the Hippo/YAP signaling pathway has taken place within the cancer research community. This signaling pathway operates at different steps of tumor progression: Primary tumor growth, angiogenesis, epithelial to mesenchymal transition, and metastatic dissemination. This review discusses the current knowledge about the involvement of the Hippo signaling pathway in cancer and specifically in paediatric bone sarcoma progression.

Yes-Associated Protein and PDZ Binding Motif: A Critical Signaling Pathway in the Control of Human Pluripotent Stem Cells Self-Renewal and Differentiation

Human pluripotent stem cells (hPSCs) can self-renew indefinitely to generate cells like themselves with a normal karyotype and differentiate into other types of cells when stimulated with a proper set of internal and external signals. hPSCs including human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) are an alternative approach toward stem cell biology, drug discovery, disease modeling, and regenerative medicine. hESCs are commonly derived from the inner cell mass of preimplantation embryos and can maintain their pluripotency in appropriate culture media. The Hippo pathway is a major integrator of cell surface-mediated signals and plays an essential role in regulating hESCs function. Yes-associated protein (YAP) and TAZ (PDZ binding motif) are critical downstream transcriptional coactivators in the Hippo pathway. The culture conditions have effects on the cytoplasmic or nuclear YAP/TAZ localization. Also, the activity of Hippo pathway is influenced by cell density, mechanical tension, and biochemical signals. In this review article, we summarize the function of YAP/TAZ and focus on the regulation of YAP/TAZ in self-renewal and differentiation of hESCs.

Systematic analysis of the Hippo pathway organization and oncogenic alteration in evolution

The Hippo pathway is a central regulator of organ size and a key tumor suppressor via coordinating cell proliferation and death. Initially discovered in Drosophila, the Hippo pathway has been implicated as an evolutionarily conserved pathway in mammals; however, how this pathway was evolved to be functional from its origin is still largely unknown. In this study, we traced the Hippo pathway in premetazoan species, characterized the intrinsic functions of its ancestor components, and unveiled the evolutionary history of this key signaling pathway from its unicellular origin. In addition, we elucidated the paralogous gene history for the mammalian Hippo pathway components and characterized their cancer-derived somatic mutations from an evolutionary perspective. Taken together, our findings not only traced the conserved function of the Hippo pathway to its unicellular ancestor components, but also provided novel evolutionary insights into the Hippo pathway organization and oncogenic alteration.

Yorkie Growth-Promoting Activity Is Limited by Atg1-Mediated Phosphorylation

The expression of multiple growth-promoting genes is coordinated by the transcriptional co-activator Yorkie with its major regulatory input provided by the Hippo-Warts kinase cascade. Here, we identify Atg1/ULK1-mediated phosphorylation of Yorkie as an additional inhibitory input independent of the Hippo-Warts pathway. Two serine residues in Yorkie, S74 and S97, are Atg1/ULK1 consensus target sites and are phosphorylated by ULK1 in vitro, thereby preventing its binding to Scalloped. In vivo, gain of function of Atg1, or its activator Acinus, caused elevated Yorkie phosphorylation and inhibited Yorkie's growth-promoting activity. Loss of function of Atg1 or Acinus raised expression of Yorkie target genes and increased tissue size. Unlike Atg1's role in autophagy, Atg1-mediated phosphorylation of Yorkie does not require Atg13. Atg1 is activated by starvation and other cellular stressors and therefore can impose temporary stress-induced constraints on the growth-promoting gene networks under the control of Hippo-Yorkie signaling.

YAP/TAZ functions and their regulation at a glance

YAP and TAZ proteins are transcriptional coactivators encoded by paralogous genes, which shuttle between the cytoplasm and the nucleus in response to multiple inputs, including the Hippo pathway. In the nucleus, they pair with DNA-binding factors of the TEAD family to regulate gene expression. Nuclear YAP/TAZ promote cell proliferation, organ overgrowth, survival to stress and dedifferentiation of post-mitotic cells into their respective tissue progenitors. YAP/TAZ are required for growth of embryonic tissues, wound healing and organ regeneration, where they are activated by cell-intrinsic and extrinsic cues. Surprisingly, this activity is dispensable in many adult self-renewing tissues, where YAP/TAZ are constantly kept in check. YAP/TAZ lay at the center of a complex regulatory network including cell-autonomous factors but also cell- and tissue-level structural features such as the mechanical properties of the cell microenvironment, the establishment of cell-cell junctions and of basolateral tissue polarity. Enhanced levels and activity of YAP/TAZ are observed in many cancers, where they sustain tumor growth, drug resistance and malignancy. In this Cell Science at a Glance article and the accompanying poster, we review the biological functions of YAP/TAZ and their regulatory mechanisms, and highlight their position at the center of a complex signaling network.

The spectraplakin Dystonin antagonizes YAP activity and suppresses tumourigenesis

Aberrant expression of the Spectraplakin Dystonin (DST) has been observed in various cancers, including those of the breast. However, little is known about its role in carcinogenesis. In this report, we demonstrate that Dystonin is a candidate tumour suppressor in breast cancer and provide an underlying molecular mechanism. We show that in MCF10A cells, Dystonin is necessary to restrain cell growth, anchorage-independent growth, self-renewal properties and resistance to doxorubicin. Strikingly, while Dystonin maintains focal adhesion integrity, promotes cell spreading and cell-substratum adhesion, it prevents Zyxin accumulation, stabilizes LATS and restricts YAP activation. Moreover, treating DST-depleted MCF10A cells with the YAP inhibitor Verteporfin prevents their growth. In vivo, the Drosophila Dystonin Short stop also restricts tissue growth by limiting Yorkie activity. As the two Dystonin isoforms BPAG1eA and BPAG1e are necessary to inhibit the acquisition of transformed features and are both downregulated in breast tumour samples and in MCF10A cells with conditional induction of the Src proto-oncogene, they could function as the predominant Dystonin tumour suppressor variants in breast epithelial cells. Thus, their loss could deem as promising prognostic biomarkers for breast cancer.

CDK7 regulates organ size and tumor growth by safeguarding the Hippo pathway effector Yki/Yap/Taz in the nucleus

Hippo signaling controls organ size and tumor progression through a conserved pathway leading to nuclear translocation of the transcriptional effector Yki/Yap/Taz. Most of our understanding of Hippo signaling pertains to its cytoplasmic regulation, but how the pathway is controlled in the nucleus remains poorly understood. Here we uncover an evolutionarily conserved mechanism by which CDK7 promotes Yki/Yap/Taz stabilization in the nucleus to sustain Hippo pathway outputs. We found that a modular E3 ubiquitin ligase complex CRL4^DCAF12 binds and targets Yki/Yap/Taz for ubiquitination and degradation, whereas CDK7 phosphorylates Yki/Yap/Taz at S169/S128/S90 to inhibit CRL4^DCAF12 recruitment, leading to Yki/Yap/Taz stabilization. As a consequence, inactivation of CDK7 reduced organ size and inhibited tumor growth, which could be reversed by restoring Yki/Yap activity. Our study identifies an unanticipated layer of Hippo pathway regulation, defines a novel mechanism by which CDK7 regulates tissue growth, and implies CDK7 as a drug target for Yap/Taz-driven cancer.

Hippo-YAP/TAZ signalling in organ regeneration and regenerative medicine

The Hippo pathway and its downstream effectors, the transcriptional co-activators Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), regulate organ growth and cell plasticity during animal development and regeneration. Remarkably, experimental activation of YAP/TAZ in the mouse can promote regeneration in organs with poor or compromised regenerative capacity, such as the adult heart and the liver and intestine of old or diseased mice. However, therapeutic YAP/TAZ activation may cause serious side effects. Most notably, YAP/TAZ are hyperactivated in human cancers, and prolonged activation of YAP/TAZ triggers cancer development in mice. Thus, can the power of YAP/TAZ to promote regeneration be harnessed in a safe way? Here, we review the role of Hippo signalling in animal regeneration, examine the promises and risks of YAP/TAZ activation for regenerative medicine and discuss strategies to activate YAP/TAZ for regenerative therapy while minimizing adverse side effects.

Hippo-Yap/Taz signaling: Complex network interactions and impact in epithelial cell behavior

The Hippo pathway has emerged as a crucial integrator of signals in biological events from development to adulthood and in diseases. Although extensively studied in Drosophila and in cell cultures, major gaps of knowledge still remain on how this pathway functions in mammalian systems. The pathway consists of a growing number of components, including core kinases and adaptor proteins, which control the subcellular localization of the transcriptional co-activators Yap and Taz through phosphorylation of serines at key sites. When localized to the nucleus, Yap/Taz interact with TEAD transcription factors to induce transcriptional programs of proliferation, stemness, and growth. In the cytoplasm, Yap/Taz interact with multiple pathways to regulate a variety of cellular functions or are targeted for degradation. The Hippo pathway receives cues from diverse intracellular and extracellular inputs, including growth factor and integrin signaling, polarity complexes, and cell-cell junctions. This review highlights the mechanisms of regulation of Yap/Taz nucleocytoplasmic shuttling and their implications for epithelial cell behavior using the lung as an intriguing example of this paradigm. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Signaling Pathways > Cell Fate Signaling Establishment of Spatial and Temporal Patterns > Cytoplasmic Localization.

Toxicological effects of ciprofloxacin exposure to Drosophila melanogaster

The abuse of ciprofloxacin (CIP) may cause serious side effects and the mechanisms underlying these effects remain unclear. Here, we determinate the 48 h, 72 h and 96 h LC₅₀ values of CIP to Drosophila melanogaster and demonstrate a series of adverse effects after D. melanogaster was exposed to CIP at a sublethal concentration (3.2 mg mL^-1). Treated individuals showed shorter lifespan, delayed development and many of the treated larvae failed to pupate or hatch. Smaller body size was observed at every life stage when exposed to CIP and the size of pupae, the weight of third-instar larvae exhibited a perfectly dose-response relationship that the larger concentration exposed to, the smaller body size or lighter weight is. Moreover, reduction in fat body cell viability, elevated oxidative stress markers (SOD and CAT) and down-regulation of diap1, ex, two target genes of Yorkie (Yki), was observed in response to CIP exposure. Most importantly, we found two types of black spot in Drosophila and the proportion of larvae with a black spot was positively related to the treatment dose, which is new in the field. This study provides a scientific basis for the potential harm caused by abuse of quinolones with the goal of urging cautious use of antibiotics.

Usp10 Modulates the Hippo Pathway by Deubiquitinating and Stabilizing the Transcriptional Coactivator Yorkie

The Hippo signaling pathway is an evolutionarily conserved regulator that plays important roles in organ size control, homeostasis, and tumorigenesis. As the key effector of the Hippo pathway, Yorkie (Yki) binds to transcription factor Scalloped (Sd) and promotes the expression of target genes, leading to cell proliferation and inhibition of apoptosis. Thus, it is of great significance to understand the regulatory mechanism for Yki protein turnover. Here, we provide evidence that the deubiquitinating enzyme ubiquitin-specific protease 10 (Usp10) binds Yki to counteract Yki ubiquitination and stabilize Yki protein in Drosophila S2 cells. The results in Drosophila wing discs indicate that silence of Usp10 decreases the transcription of target genes of the Hippo pathway by reducing Yki protein. In vivo functional analysis ulteriorly showed that Usp10 upregulates the Yki activity in Drosophila eyes. These findings uncover Usp10 as a novel Hippo pathway modulator and provide a new insight into the regulation of Yki protein stability and activity.

Mask family proteins ANKHD1 and ANKRD17 regulate YAP nuclear import and stability

Mask family proteins were discovered in Drosophila to promote the activity of the transcriptional coactivator Yorkie (Yki), the sole fly homolog of mammalian YAP (YAP1) and TAZ (WWTR1). The molecular function of Mask, or its mammalian homologs Mask1 (ANKHD1) and Mask2 (ANKRD17), remains unclear. Mask family proteins contain two ankyrin repeat domains that bind Yki/YAP as well as a conserved nuclear localisation sequence (NLS) and nuclear export sequence (NES), suggesting a role in nucleo-cytoplasmic transport. Here we show that Mask acts to promote nuclear import of Yki, and that addition of an ectopic NLS to Yki is sufficient to bypass the requirement for Mask in Yki-driven tissue growth. Mammalian Mask1/2 proteins also promote nuclear import of YAP, as well as stabilising YAP and driving formation of liquid droplets. Mask1/2 and YAP normally colocalise in a granular fashion in both nucleus and cytoplasm, and are co-regulated during mechanotransduction.

The Hippo pathway integrates PI3K-Akt signals with mechanical and polarity cues to control tissue growth

The Hippo signalling pathway restricts cell proliferation in animal tissues by inhibiting Yes-associated protein (YAP or YAP1) and Transcriptional Activator with a PDZ domain (TAZ or WW-domain-containing transcriptional activator [WWTR1]), coactivators of the Scalloped (Sd or TEAD) DNA-binding transcription factor. Drosophila has a single YAP/TAZ homolog named Yorkie (Yki) that is regulated by Hippo pathway signalling in response to epithelial polarity and tissue mechanics during development. Here, we show that Yki translocates to the nucleus to drive Sd-mediated cell proliferation in the ovarian follicle cell epithelium in response to mechanical stretching caused by the growth of the germline. Importantly, mechanically induced Yki nuclear localisation also requires nutritionally induced insulin/insulin-like growth factor 1 (IGF-1) signalling (IIS) via phosphatidyl inositol-3-kinase (PI3K), phosphoinositide-dependent kinase 1 (PDK1 or PDPK1), and protein kinase B (Akt or PKB) in the follicular epithelium. We find similar results in the developing Drosophila wing, where Yki becomes nuclear in the mechanically stretched cells of the wing pouch during larval feeding, which induces IIS, but translocates to the cytoplasm upon cessation of feeding in the third instar stage. Inactivating Akt prevents nuclear Yki localisation in the wing disc, while ectopic activation of the insulin receptor, PI3K, or Akt/PKB is sufficient to maintain nuclear Yki in mechanically stimulated cells of the wing pouch even after feeding ceases. Finally, IIS also promotes YAP nuclear localisation in response to mechanical cues in mammalian skin epithelia. Thus, the Hippo pathway has a physiological function as an integrator of epithelial cell polarity, tissue mechanics, and nutritional cues to control cell proliferation and tissue growth in both Drosophila and mammals.

Yorkie and JNK revert syncytial muscles into myoblasts during Org-1-dependent lineage reprogramming

The formation and differentiation of syncytial muscles is typically considered an irreversible developmental process. Schaub et al. describe molecular events that dedifferentiate syncytial muscle into mononucleate myoblasts during a naturally occurring lineage reprogramming process.

Lineage reprogramming has received increased research attention since it was demonstrated that lineage-restricted transcription factors can be used in vitro for direct reprogramming. Recently, we reported that the ventral longitudinal musculature of the adult Drosophila heart arises in vivo by direct lineage reprogramming from larval alary muscles, a process that starts with the dedifferentiation and fragmentation of syncytial muscle cells into mononucleate myoblasts and depends on Org-1 (Drosophila Tbx1). Here, we shed light on the events occurring downstream of Org-1 in this first step of transdifferentiation and show that alary muscle lineage-specific activation of Yorkie plays a key role in initiating the dedifferentiation and fragmentation of these muscles. An additional necessary input comes from active dJNK signaling, which contributes to the activation of Yorkie and furthermore activates dJun. The synergistic activities of the Yorkie/Scalloped and dJun/dFos transcriptional activators subsequently initiate alary muscle fragmentation as well as up-regulation of Myc and piwi, both crucial for lineage reprogramming.

Yorkie and JNK Control Tumorigenesis in Drosophila Cells with Cytokinesis Failure

Cytokinesis failure may result in the formation of polyploid cells, and subsequent mitosis can lead to aneuploidy and tumor formation. Tumor suppressor mechanisms limiting the oncogenic potential of these cells have been described. However, the universal applicability of these tumor-suppressive barriers remains controversial. Here, we use Drosophila epithelial cells to investigate the consequences of cytokinesis failure in vivo. We report that cleavage defects trigger the activation of the JNK pathway, leading to downregulation of the inhibitor of apoptosis DIAP1 and programmed cell death. Yorkie overcomes the tumor-suppressive role of JNK and induces neoplasia. Yorkie regulates the cell cycle phosphatase Cdc25/string, which drives tumorigenesis in a context of cytokinesis failure. These results highlight the functional significance of the JNK pathway in epithelial cells with defective cytokinesis and elucidate a mechanism used by emerging tumor cells to bypass this tumor-suppressive barrier and develop into tumors.

The CAF-1 complex couples Hippo pathway target gene expression and DNA replication

The Hippo signaling pathway regulates tissue growth and organ development in many animals, including humans. Pathway activity leads to inactivation of Yorkie (Yki), a transcriptional coactivator that drives expression of growth-promoting genes. In addition, Yki has been shown to recruit chromatin modifiers that enhance chromatin accessibility and thereby enhance Yki function. Here, we asked whether changes in chromatin accessibility that occur during DNA replication could also affect Yki function. We found that depletion of the chromatin assembly complex-1 (CAF-1) complex, a histone chaperone that is required for nucleosome assembly after DNA replication, in the wing imaginal epithelium leads to increased Hippo pathway target gene expression but does not affect expression of other genes. Yki shows greater association with target sites when CAF-1 is depleted and misregulation of target gene expression is Yki-dependent, suggesting that nucleosome assembly competes with Yki for pathway targets post-DNA replication. Consistent with this idea, increased target gene expression is DNA replication dependent and newly replicated chromatin at target sites shows marked nucleosome depletion when CAF-1 function is reduced. These observations suggest a connection between cell cycle progression and Hippo pathway target expression, providing insights into functions of the Hippo pathway in normal and abnormal tissue growth.

Deleted in azoospermia-associated protein 2 regulates innate immunity by stimulating Hippo signaling in crab

The Hippo-signaling pathway plays a critical role in both normal animal physiology and pathogenesis. Because pharmacological interventions targeting this pathway have diverse clinical implications, a better understanding of its regulation in various conditions and organisms is crucial. Here, we identified deleted in azoospermia-associated protein 2 (DAZAP2) in the Chinese mitten crab (Eriocheir sinensis), designated EsDAZAP2, as a Hippo-regulatory protein highly similar to proteins in various species of insects, fish, and mammals. We found that a bacterial infection significantly induces EsDAZAP2 expression, and an EsDAZAP2 knockdown both suppresses antimicrobial peptide (AMP) expression in vitro and results in increased viable bacterial counts and mortality in vivo, suggesting that EsDAZAP2 plays a critical role in innate immunity. Using yeast two-hybrid screening and co-immunoprecipitation assays, we found that EsDAZAP2 regulates the Toll pathway rather than the immune deficiency and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathways. Our findings also demonstrate that EsDAZAP2 binds to the Hippo protein, Salvador (Sav). Moreover, by examining the regulation of Dorsal, a transcription factor that regulates AMP expression in E. sinensis, we provide experimental evidence indicating that EsDAZAP2 promotes Hippo pathway activation in innate immunity, with EsDAZAP2 and Hippo binding to different Sav domains. To the best of our knowledge, this is the first report of a DAZAP2-regulated Hippo-signaling pathway operating in animal innate immunity.

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.

YAP mediates the positive regulation of hnRNPK on the lung adenocarcinoma H1299 cell growth

Lung cancer is the leading cause of cancer death worldwide, and non-small cell lung cancer (NSCLC) accounts for 80%-85% of diagnostic cases. The molecular mechanisms of NSCLC pathogenesis are not well understood. Heterogeneous nuclear ribonucleoprotein K (hnRNPK) is a multifunctional protein that regulates gene expression and signal transduction and closely associated with tumorigenesis, but its mechanism of action in the pathogenesis of NSCLC is unclear. In this study, we observed that the expression pattern of hnRNPK in H1299 lung adenocarcinoma cells varied depending on the cell density in culture. Moreover, hnRNPK stimulated the ability of proliferation and colony formation of H1299 cells, which is important for the multilayered cell growth in culture. We further investigated whether there is an association between hnRNPK and the elements involved in the cell contact inhibition pathway. By using quantitative reverse transcriptase-polymerase chain reaction assay and a YAP activity reporter system, we found that hnRNPK upregulated the mRNA and protein levels and transcriptional activity of Yes-associated protein 1 (YAP), a master negative regulator of Hippo contact inhibition pathway. Furthermore, YAP knockdown with siRNA abolished the stimulatory effect of hnRNPK on H1299 cell proliferation. These results suggested that YAP could be one of the effectors of hnRNPK. Our data may provide new clues for further understanding the biological functions of hnRNPK, particularly in the context of lung adenocarcinoma oncogenesis.

Opposing transcriptional and post-transcriptional roles for Scalloped in binary Hippo-dependent neural fate decisions

The Hippo tumor suppressor pathway plays many fundamental cell biological roles during animal development. Two central players in controlling Hippo-dependent gene expression are the TEAD transcription factor Scalloped (Sd) and its transcriptional co-activator Yorkie (Yki). Hippo signaling phosphorylates Yki, thereby blocking Yki-dependent transcriptional control. In post-mitotic Drosophila photoreceptors, a bistable negative feedback loop forms between the Hippo-dependent kinase Warts/Lats and Yki to lock in green vs blue-sensitive neuronal subtype choices, respectively. Previous experiments indicate that sd and yki mutants phenocopy each other's functions, both being required for promoting the expression of the blue photoreceptor fate determinant melted (melt) and the blue-sensitive opsin Rh5. Here, we demonstrate that Sd ensures the robustness of this neuronal fate decision via multiple antagonistic gene regulatory roles. In Hippo-positive (green) photoreceptors, Sd directly represses both melt and Rh5 gene expression through defined TEAD binding sites, a mechanism that is antagonized by Yki in Hippo-negative (blue) cells. Additionally, in blue photoreceptors, Sd is required to promote the translation of the Rh5 protein through a 3'UTR-dependent and microRNA-mediated process. Together, these studies reveal that Sd can drive context-dependent cell fate decisions through opposing transcriptional and post-transcriptional mechanisms.

Quercetin inhibited mesangial cell proliferation of early diabetic nephropathy through the Hippo pathway

Diabetic nephropathy (DN) is one of the most common microvascular complications of diabetes and the leading cause of end-stage renal disease. The proliferation of glomerular mesangial cells (MCs) is a common and prominent pathological change of DN, which takes place at the early stage. Quercetin, a bioflavonoid compound, possesses therapeutic efficacy in cardiovascular and kidney diseases via anti-tumour, anti-oxidation, anti-virus, and anti-proliferation effects. However, the mechanism of quercetin in the proliferation of glomerular MCs in early DN has not been reported. In the present study, we investigated the effect of quercetin on the proliferation of glomerular MCs in high glucose-induced mouse glomerular MCs and in db/db mice. On this basis, we tried to clarify the specific mechanisms underlying these effects. The in vitro results showed that the proliferation of glomerular MCs was induced by high glucose, and the Hippo pathway was highly inactivated in high glucose-cultured MCs. Decreased phosphorylation of MST1 and Lats1 promoted expression and nuclear translocation of Yes-associated protein (YAP) and subsequently increased the combination of YAP and TEA/ATS domain (TEAD), which promoted the expression of the downstream target gene such as cyclinE. Quercetin effectively inhibited the high glucose-induced MC proliferation and reactivated the Hippo pathway. In vivo, the proliferation of glomerular MCs was increased, renal function was decreased, and blood fasting glucose was elevated in db/db mice. Furthermore, the Hippo pathway was inactivated in the renal cortex of db/db mice. Eight-week treatment of quercetin retarded MC proliferation, alleviated the renal function, and reactivated Hippo pathway in the renal cortex of db/db mice at 16 weeks. Our previous study clarified that the Hippo pathway was involved in MC proliferation of DN. The results revealed that quercetin inhibited MC proliferation in high glucose-treated mouse glomerular MCs and in DN via reactivation of the Hippo pathway.

Downregulation of WW domain-containing oxidoreductase leads to tamoxifen-resistance by the inactivation of Hippo signaling

Acquired tamoxifen-resistance is an important cause of death in patients with hormone-dependent breast tumors. Therefore, understanding the molecular mechanisms underlying the development of tamoxifen-resistance is critical for successful endocrine therapy. This study aimed to define the role of WW domain-containing oxidoreductase (WWOX) in acquired tamoxifen-resistance. Our results show that low WWOX expression was significantly related to tamoxifen-resistance. Moreover, WWOX-knockdown increased resistance to tamoxifen, while WWOX overexpression decreased the resistance. Furthermore, WWOX silencing decreased Yes-associated protein (YAP) phosphorylation and increased YAP nuclear translocation. Finally, YAP silencing decreased tamoxifen-resistance in WWOX-knockdown cells. Our findings demonstrate that WWOX downregulation can lead to the development of tamoxifen-resistance by inactivating Hippo signaling. Thus, WWOX might be a valuable target and prognostic marker for tamoxifen-resistance.

Impact statement

Understanding the molecular pathways leading to the development of tamoxifen-resistance is an important research focus as acquired tamoxifen-resistance is the main cause of death in patients with benign primary prognosis. Although WW domain-containing oxidoreductase (WWOX) has been related to breast tumorigenesis, its role in acquired tamoxifen-resistance has not yet been demonstrated. Our findings show that WWOX might be a valuable therapeutic target and prognostic marker for tamoxifen-resistance.

Hippo Signaling in Cancer: Lessons From Drosophila Models

Hippo pathway was initially identified through genetic screens for genes regulating organ size in fruitflies. Recent studies have highlighted the role of Hippo signaling as a key regulator of homeostasis, and in tumorigenesis. Hippo pathway is comprised of genes that act as tumor suppressor genes like hippo (hpo) and warts (wts), and oncogenes like yorkie (yki). YAP and TAZ are two related mammalian homologs of Drosophila Yki that act as effectors of the Hippo pathway. Hippo signaling deficiency can cause YAP- or TAZ-dependent oncogene addiction for cancer cells. YAP and TAZ are often activated in human malignant cancers. These transcriptional regulators may initiate tumorigenic changes in solid tumors by inducing cancer stem cells and proliferation, culminating in metastasis and chemo-resistance. Given the complex mechanisms (e.g., of the cancer microenvironment, and the extrinsic and intrinsic cues) that overpower YAP/TAZ inhibition, the molecular roles of the Hippo pathway in tumor growth and progression remain poorly defined. Here we review recent findings from studies in whole animal model organism like Drosophila on the role of Hippo signaling regarding its connection to inflammation, tumor microenvironment, and other oncogenic signaling in cancer growth and progression.

Identification of the kinase STK25 as an upstream activator of LATS signaling

The Hippo pathway maintains tissue homeostasis by negatively regulating the oncogenic transcriptional co-activators YAP and TAZ. Though functional inactivation of the Hippo pathway is common in tumors, mutations in core pathway components are rare. Thus, understanding how tumor cells inactivate Hippo signaling remains a key unresolved question. Here, we identify the kinase STK25 as an activator of Hippo signaling. We demonstrate that loss of STK25 promotes YAP/TAZ activation and enhanced cellular proliferation, even under normally growth-suppressive conditions both in vitro and in vivo. Notably, STK25 activates LATS by promoting LATS activation loop phosphorylation independent of a preceding phosphorylation event at the hydrophobic motif, which represents a form of Hippo activation distinct from other kinase activators of LATS. STK25 is significantly focally deleted across a wide spectrum of human cancers, suggesting STK25 loss may represent a common mechanism by which tumor cells functionally impair the Hippo tumor suppressor pathway.

Mechanotransduction and Cytoskeleton Remodeling Shaping YAP1 in Gastric Tumorigenesis

The essential role of Hippo signaling pathway in cancer development has been elucidated by recent studies. In the gastrointestinal tissues, deregulation of the Hippo pathway is one of the most important driving events for tumorigenesis. It is widely known that Yes-associated protein 1 (YAP1) and WW domain that contain transcription regulator 1 (TAZ), two transcriptional co-activators with a PDZ-binding motif, function as critical effectors negatively regulated by the Hippo pathway. Previous studies indicate the involvement of YAP1/TAZ in mechanotransduction by crosstalking with the extracellular matrix (ECM) and the F-actin cytoskeleton associated signaling network. In gastric cancer (GC), YAP1/TAZ functions as an oncogene and transcriptionally promotes tumor formation by cooperating with TEAD transcription factors. Apart from the classic role of Hippo-YAP1 cascade, in this review, we summarize the current investigations to highlight the prominent role of YAP1/TAZ as a mechanical sensor and responder under mechanical stress and address its potential prognostic and therapeutic value in GC.

Nerfin-1 represses transcriptional output of Hippo signaling in cell competition

The Hippo tumor suppressor pathway regulates tissue growth in Drosophila by restricting the activity of the transcriptional coactivator Yorkie (Yki), which normally complexes with the TEF/TEAD family DNA-binding transcription factor Scalloped (Sd) to drive the expression of growth-promoting genes. Given its pivotal role as a central hub in mediating the transcriptional output of Hippo signaling, there is great interest in understanding the molecular regulation of the Sd-Yki complex. In this study, we identify Nerfin-1 as a transcriptional repressor that antagonizes the activity of the Sd-Yki complex by binding to the TEA DNA-binding domain of Sd. Consistent with its biochemical function, ectopic expression of Nerfin-1 results in tissue undergrowth in an Sd-dependent manner. Conversely, loss of Nerfin-1 enhances the ability of winner cells to eliminate loser cells in multiple scenarios of cell competition. We further show that INSM1, the mammalian ortholog of Nerfin-1, plays a conserved role in repressing the activity of the TEAD-YAP complex. These findings reveal a novel regulatory mode converging on the transcriptional output of the Hippo pathway that may be exploited for modulating the YAP oncoprotein in cancer and regenerative medicine.

Animals uses a range of mechanisms to stop their organs from growing once they have reached the right shape and size. One of these processes, a set of chemical messages called the Hippo pathway, controls the balance of cell death and cell division. In fruit flies, Hippo works by repressing a complex formed of two proteins, Yorkie and Scalloped, which normally switch genes on to encourage cells to grow. Yorkie is also involved in cell competition, a process in which cells in a tissue compare themselves to each other. Healthier ‘winner’ cells then kill neighboring ‘loser’ cells that are weaker or damaged. This ensures that the tissue keeps working properly.

Despite Yorkie and Scalloped being key to control the growth and health of tissues, how the activity of these proteins is regulated was not well understood. To investigate, Guo et al. conducted a series experiments on fruit flies and found that a protein called Nerfin-1 can bind onto Scalloped to stop the Scalloped-Yorkie complex from switching on genes. As a result, flies with too much Nerfin-1 had stunted tissue growth. In addition, Guo et al. confirmed that the Nerfin-1 equivalent in mammals acts in the same way. Further work revealed that Nerfin-1 also plays a role in cell competition: without this protein, ‘winner’ cells became 'super winners', eliminating even more of the loser cells.

Besides regulating the size of organs, the Hippo pathway is also involved in stopping cells from dividing uncontrollably and becoming cancerous. Further research may therefore focus on Nerfin-1 and its equivalent in mammals to understand how this protein could contribute to the emergence of cancer.

Drosophila Hcf regulates the Hippo signaling pathway via association with the histone H3K4 methyltransferase Trr

Control of organ size is a fundamental aspect in biology and plays important roles in development. The Hippo pathway is a conserved signaling cascade that controls tissue and organ size through the regulation of cell proliferation and apoptosis. Here, we report on the roles of Hcf (host cell factor), the Drosophila homolog of Host cell factor 1, in regulating the Hippo signaling pathway. Loss-of-Hcf function causes tissue undergrowth and the down-regulation of Hippo target gene expression. Genetic analysis reveals that Hcf is required for Hippo pathway-mediated overgrowth. Mechanistically, we show that Hcf associates with the histone H3 lysine-4 methyltransferase Trithorax-related (Trr) to maintain H3K4 mono- and trimethylation. Thus, we conclude that Hcf positively regulates Hippo pathway activity through forming a complex with Trr and controlling H3K4 methylation.

β-Catenin and Yes-Associated Protein 1 Cooperate in Hepatoblastoma Pathogenesis

Hepatoblastoma (HB), the most common pediatric primary liver neoplasm, shows nuclear localization of β-catenin and yes-associated protein 1 (YAP1) in almost 80% of the cases. Co-expression of constitutively active S127A-YAP1 and ΔN90 deletion-mutant β-catenin (YAP1-ΔN90-β-catenin) causes HB in mice. Because heterogeneity in downstream signaling is being identified owing to mutational differences even in the β-catenin gene alone, we investigated if co-expression of point mutants of β-catenin (S33Y or S45Y) with S127A-YAP1 led to similar tumors as YAP1-ΔN90-β-catenin. Co-expression of S33Y/S45Y-β-catenin and S127A-YAP1 led to activation of Yap and Wnt signaling and development of HB, with 100% mortality by 13 to 14 weeks. Co-expression with YAP1-S45Y/S33Y-β-catenin of the dominant-negative T-cell factor 4 or dominant-negative transcriptional enhanced associate domain 2, the respective surrogate transcription factors, prevented HB development. Although histologically similar, HB in YAP1-S45Y/S33Y-β-catenin, unlike YAP1-ΔN90-β-catenin HB, was glutamine synthetase (GS) positive. However, both ΔN90-β-catenin and point-mutant β-catenin comparably induced GS-luciferase reporter in vitro. Finally, using a previously reported 16-gene signature, it was shown that YAP1-ΔN90-β-catenin HB tumors exhibited genetic similarities with more proliferative, less differentiated, GS-negative HB patient tumors, whereas YAP1-S33Y/S45Y-β-catenin HB exhibited heterogeneity and clustered with both well-differentiated GS-positive and proliferative GS-negative patient tumors. Thus, we demonstrate that β-catenin point mutants can also collaborate with YAP1 in HB development, albeit with a distinct molecular profile from the deletion mutant, which may have implications in both biology and therapy.

An ADAMTS Sol narae is required for cell survival in Drosophila

Cell survival is essential for all living organisms to cope against multiple environmental insults. Intercellular signaling between dying and surviving cells plays an important role to ensure compensatory proliferation, preventing tissue loss after environmental stresses. Here, we show that Sol narae (Sona), a Disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) in Drosophila is required for cell survival. sona exhibited a positive genetic interaction with Death-associated inhibitor of apoptosis 1 (Diap1), and a negative genetic interaction with reaper (rpr). Transcription patterns of sona, Diap1, and rpr genes in the pouch region of wing discs were coordinately changed after irradiation. Interestingly, there was a negative correlation in the expression levels of Sona and DIAP1, and both cell types, one with high Sona level and the other with high Diap1 level, were resistant to irradiation-induced cell death. The sona-expressing cells rarely entered into cell cycle themselves but promoted the nearby cells to proliferate in irradiation conditions. We found that these sona-expressing cells are able to upregulate Cyclin D (Cyc D) and increase tissue size. Furthermore, transient Sona overexpression increased survival rate and promoted development of flies in irradiation conditions. We propose that the two types of radiation-resistant cells, one with high Sona level and the other with high Diap1 level, communicate with dying cells and between each other for cell survival and proliferation in response to irradiation.

Usp7 regulates Hippo pathway through deubiquitinating the transcriptional coactivator Yorkie

The Hippo pathway plays an important role in organ development and adult tissue homeostasis, and its deregulation has been implicated in many cancers. The Hippo signaling relies on a core kinase cascade culminating in phosphorylation of the transcription coactivator Yorkie (Yki). Although Yki is the key effector of Hippo pathway, the regulation of its protein stability is still unclear. Here, we show that Hippo pathway attenuates the binding of a ubiquitin-specific protease Usp7 to Yki, which regulates Hippo signaling through deubiquitinating Yki. Furthermore, the mammalian homolog of Usp7, HAUSP plays a conserved role in regulating Hippo pathway by modulating Yap ubiquitination and degradation. Finally, we find that the expression of HAUSP is positively correlated with that of Yap, both showing upregulated levels in clinical hepatocellular carcinoma (HCC) specimens. In summary, our findings demonstrate that Yki/Yap is stabilized by Usp7/HAUSP, and provide HAUSP as a potential therapeutic target for HCC.

Hippo signaling leads to the phosphorylation of the key transcriptional effector, Yap/Yki, although how Yap/Yki stability is regulated has remained unclear. Here, Sun et al. identify HAUSP/Usp7 as a conserved and clinically relevant regulator of the Hippo pathway that increases Yap/Yki stability.

YAPping about and not forgetting TAZ

The Hippo pathway has emerged as a major eukaryotic signalling pathway and is increasingly the subject of intense interest, as are the key effectors of canonical Hippo signalling, YES-associated protein (YAP) and TAZ. The Hippo pathway has key roles in diverse biological processes, including network signalling regulation, development, organ growth, tissue repair and regeneration, cancer, stem cell regulation and mechanotransduction. YAP and TAZ are multidomain proteins and function as transcriptional coactivators of key genes to evoke their biological effects. YAP and TAZ interact with numerous partners and their activities are controlled by a complex set of processes. This review provides an overview of Hippo signalling and its role in growth. In particular, the functional domains of YAP and TAZ and the complex mechanisms that regulate their protein stability and activity are discussed. Notably, the similarities and key differences are highlighted between the two paralogues including which partner proteins interact with which functional domains to regulate their activity.

YAP1 influences differentiation of osteoblastic MC3T3-E1 cells through the regulation of ID1

Yes-associated protein 1 (YAP1) transcriptional coactivator has recently been identified to regulate skeletal lineage cell differentiation and bone development. However, the role and molecular mechanisms of YAP1 in the regulation of osteoblastic differentiation remains to be elucidated. In this study, we demonstrated that YAP1 expression was increased during osteogenic differentiation of rat bone mesenchymal stem cells and MC3T3-E1. YAP1 overexpression MC3T3-E1 showed increased expression of osteogenesis markers, such as runt-related transcription factor 2, osteocalcin, and osteopontin, as well as alkaline phosphatase and alizarin red staining. Conversely, YAP1 knockdown significantly suppressed MC3T3-E1 osteoblastic differentiation. Mechanistically, we found that YAP1 overexpression upregulated the mRNA and protein expression of the inhibitor of differentiation/DNA binding 1 (ID1), which was contrary to the results of YAP1-knockdown group. Moreover, the early osteogenic differentiation of MC3T3-E1 cells was enhanced by ID1 overexpression. Furthermore, transient transfection with exogenous ID1 overexpression plasmid completely recaptured the decreased effects of YAP1 knockdown on MC3T3-E1 cell differentiation. In addition, β-catenin and AMP-activated protein kinase signaling pathways participated in YAP1 regulation processes. Taken together, our study suggests that YAP1 is a crucial modulator of osteoblast differentiation in vitro, and provides insight into the mechanism by which YAP1 regulates osteoblast differentiation.

Evolution and Regulation of Limb Regeneration in Arthropods

Regeneration has fascinated both scientists and non-scientists for centuries. Many organisms can regenerate, and arthropod limbs are no exception although their ability to regenerate is a product shaped by natural and sexual selection. Recent studies have begun to uncover cellular and molecular processes underlying limb regeneration in several arthropod species. Here we argue that an evo-devo approach to the study of arthropod limb regeneration is needed to understand aspects of limb regeneration that are conserved and divergent. In particular, we argue that limbs of different species are comprised of cells at distinct stages of differentiation at the time of limb loss and therefore provide insights into regeneration involving both stem cell-like cells/precursor cells and differentiated cells. In addition, we review recent studies that demonstrate how limb regeneration impacts the development of the whole organism and argue that studies on the link between local tissue damage and the rest of the body should provide insights into the integrative nature of development. Molecular studies on limb regeneration are only beginning to take off, but comparative studies on the mechanisms of limb regeneration across various taxa should not only yield interesting insights into development but also answer how this remarkable ability evolved across arthropods and beyond.

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.

Quantitative Real-Time PCR to Measure YAP/TAZ Activity in Human Cells

Transcription coactivators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ, also known as WWTR1) are homologs of the Drosophila Yorkie (Yki) protein and are major downstream effectors of the evolutionarily conserved Hippo pathway. YAP/TAZ play critical roles in regulation of cell proliferation, apoptosis, and stemness, thus mediate functions of the Hippo pathway in organ size control and tumorigenesis. The Hippo pathway inhibits YAP/TAZ through phosphorylation, which leads to YAP/TAZ cytoplasmic retention and degradation. Dephosphorylated and nuclear-localized YAP/TAZ bind to transcription factors, especially the TEAD family proteins, thus transactivate the expression of specific genes. Therefore, measuring the expression level of YAP/TAZ target genes is a critical approach to assess Hippo pathway activity. Through gene expression profiling in different tissues and cells using techniques such as microarray and RNA-seq, many target genes of YAP/TAZ have been identified. Some of these genes were confirmed to be direct YAP/TAZ targets by chromatin immunoprecipitation (ChIP)-PCR or ChIP-seq. These works made it possible to quickly determine YAP/TAZ activity by measuring the mRNA levels of several YAP/TAZ target genes, such as CTGF, CYR61, and miR-130a by quantitative real-time PCR (qPCR). In this chapter, we demonstrate the use of qPCR to measure YAP/TAZ activity in MCF10A cells.

Molecular regulation of mammalian hepatic architecture

The essential liver exocrine and endocrine functions require a precise spatial arrangement of the hepatic lobule consisting of the central vein, portal vein, hepatic artery, intrahepatic bile duct system, and hepatocyte zonation. This allows blood to be carried through the liver parenchyma sampled by all hepatocytes and bile produced by the hepatocytes to be carried out of the liver through the intrahepatic bile duct system composed of cholangiocytes. The molecular orchestration of multiple signaling pathways and epigenetic factors is required to set up lineage restriction of the bipotential hepatoblast progenitor into the hepatocyte and cholangiocyte cell lineages, and to further refine cell fate heterogeneity within each cell lineage reflected in the functional heterogeneity of hepatocytes and cholangiocytes. In addition to the complex molecular regulation, there is a complicated morphogenetic choreography observed in building the refined hepatic epithelial architecture. Given the multifaceted molecular and cellular regulation, it is not surprising that impairment of any of these processes can result in acute and chronic hepatobiliary diseases. To enlighten the development of potential molecular and cellular targets for therapeutic options, an understanding of how the intricate hepatic molecular and cellular interactions are regulated is imperative. Here, we review the signaling pathways and epigenetic factors regulating hepatic cell lineages, fates, and epithelial architecture.

Cell type-dependent function of LATS1/2 in cancer cell growth

The Hippo pathway controls organ size and tissue homeostasis, and its dysregulation often contributes to tumorigenesis. Extensive studies have shown that the Hippo pathway inhibits cell proliferation, and survival in a cell-autonomous manner. We examined the function of the Hippo pathway kinases LATS1/2 (large tumor suppressor 1 and 2) in cancer cells. As expected, loss of LATS1/2 promotes cancer cell growth in most cell lines. Surprisingly, however, LATS1/2 deletion inhibits the growth of murine MC38 colon cancer cells, especially under detachment conditions. This growth inhibitory effect caused by LATS1/2 deletion is due to uncontrolled activation of Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), the key downstream transcriptional coactivators inhibited by LATS1/2. We identified Wnt inducible signaling pathway protein 2 (Wisp2) and coiled-coil domain containing 80 (Ccdc80) as direct targets of YAP/TAZ. Their expression is selectively induced by LATS1/2 deletion in MC38 cells. Furthermore, deletion of WISP2 and CCDC80 prevents the growth inhibitory effect of LATS1/2 loss in MC38 cells. Our study demonstrates that the function of LATS1/2 in cell growth is cell context dependent, suggesting that LATS1/2 inhibition can be a therapeutic approach for some cancer types.

BmHpo mutation induces smaller body size and late stage larval lethality in the silkworm, Bombyx mori

As a core member of the Hippo signaling pathway, Hpo plays a critical role in regulating growth and development. Previous studies reported that loss of function of Hpo results in increased proliferation, reduced apoptosis and induction of tissue overgrowth in Drosophila. In this study, we used CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats/Cas9) to study Hpo gene (BmHpo) function in the lepidopteran insect Bombyx mori, known commonly as the silkworm. Sequence analysis of BmHpo revealed an array of deletions in mutants. We found that BmHpo knockout resulted in defects in body size regulation, in developmental defects and pigment accumulation and early death. Our data show that BmHpo is essential for regulation of insect growth and development and that CRISPR/Cas9 technology can serve as a basis for functional analysis of target genes in lepidopteran insects.

Genome editing reveals the function of Yorkie during the embryonic and early larval development in silkworm, Bombyx mori

As a transcriptional coactivator, Yorkie (Yki) is a major downstream target of the Hippo signalling pathway to regulate the organ size during animal development and regeneration. Previous microarray analysis in the silkworm, Bombyx mori, has shown that genes associated with the Hippo pathway were primarily expressed in gonads and imaginal discs. The RNA-interference-mediated silencing of Yki at the early wandering stage delayed B. mori development and ovary maturation, whereas baculovirus-mediated overexpression at the late larval instar facilitated organ growth and accelerated metamorphosis. Here, we employed CRISPR/Cas9-mediated mutagenesis to investigate the function of Yki in B. mori (BmYki) at the embryonic and early larval stages. Knocking out of BmYki led to reduced body size, moulting defects and, eventually, larval lethality. Sequence analysis of CRISPR/Cas9 mutants exhibited an array of deletions in BmYki. As a critical downstream effector of the Hippo kinase cassette, silencing of BmYki at the embryonic stage is indispensable and the consequence is lethal. Given that the Hippo signalling pathway is evolutionarily conserved, Yki has the potential to be a novel molecular target for genetic-based pest management practices.

The Ambivalent Function of YAP in Apoptosis and Cancer

Yes-associated protein, a core regulator of the Hippo-YAP signaling pathway, plays a vital role in inhibiting apoptosis. Thus, several studies and reviews suggest that yes-associated protein is a good target for treating cancer. Unfortunately, more and more evidence demonstrates that this protein is also an essential contributor of p73-mediated apoptosis. This questions the concept that yes-associated protein is always a good target for developing novel anti-cancer drugs. Thus, the aim of this review was to evaluate the clinical relevance of yes-associated protein for cancer pathophysiology. This review also summarized the molecules, processes and drugs, which regulate Hippo-YAP signaling and discusses their effect on apoptosis. In addition, issues are defined, which should be addressed in the future in order to provide a solid basis for targeting the Hippo-YAP signaling pathway in clinical trials.

Genetic Control of Early Cell Lineages in the Mammalian Embryo

Establishing the different lineages of the early mammalian embryo takes place over several days and several rounds of cell divisions from the fertilized egg. The resulting blastocyst contains the pluripotent cells of the epiblast, from which embryonic stem cells can be derived, as well as the extraembryonic lineages required for a mammalian embryo to survive in the uterine environment. The dynamics of the cellular and genetic interactions controlling the initiation and maintenance of these lineages in the mouse embryo are increasingly well understood through application of the tools of single-cell genomics, gene editing, and in vivo imaging. Exploring the similarities and differences between mouse and human development will be essential for translation of these findings into new insights into human biology, derivation of stem cells, and improvements in fertility treatments.

Mediated nuclear import and export of TAZ and the underlying molecular requirements

Nucleocytoplasmic distribution of Yap/TAZ is regulated by the Hippo pathway and the cytoskeleton. While interactions with cytosolic and nuclear “retention factors” (14–3–3 and TEAD) are known to control their localization, fundamental aspects of Yap/TAZ shuttling remain undefined. It is unclear if translocation occurs only by passive diffusion or via mediated transport, and neither the potential nuclear localization and efflux signals (NLS, NES) nor their putative regulation have been identified. Here we show that TAZ cycling is a mediated process and identify the underlying NLS and NES. The C-terminal NLS, representing a new class of import motifs, is necessary and sufficient for efficient nuclear uptake via a RAN-independent mechanism. RhoA activity directly stimulates this import. The NES lies within the TEAD-binding domain and can be masked by TEAD, thereby preventing efflux. Thus, we describe a RhoA-regulated NLS, a TEAD-regulated NES and propose an improved model of nucleocytoplasmic TAZ shuttling beyond "retention".

The transcriptional co-factors Yap and TAZ are regulated by Hippo signalling and mechanical forces via their nucleocytoplasmic shuttling. Here the authors identify a RhoA-regulated C-terminal nuclear localization signal and a TEAD-regulated N-terminal nuclear export signal of TAZ in an epithelial cell line.

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.

Cyclopeptide RA-V Inhibits Organ Enlargement and Tumorigenesis Induced by YAP Activation

The Hippo pathway restricts organ size during development and its inactivation plays a crucial role in cancer. Yes-associated protein (YAP) and its paralog transcriptional coactivator with PSD-95/Dlg/ZO-1 (PDZ)-binding motif (TAZ) are transcription co-activators and effectors of the Hippo pathway mediating aberrant enlargement of organs and tumor growth upon Hippo pathway inactivation. It has been demonstrated that genetic inactivation of YAP could be an effective approach to inhibit tumorigenesis. In order to identify pharmacological inhibitors of YAP, we screened a library of 52,683 compounds using a YAP-specific reporter assay. In this screen we identified cyclopeptide RA-V (deoxybouvardin) as a specific inhibitor of YAP and TAZ but not other reporters. Unexpectedly, later experiments demonstrated that RA-V represses the protein but not mRNA levels of YAP target genes. Nevertheless, RA-V strongly blocks liver enlargement induced by Mst1/2 knockout. Furthermore, RA-V not only inhibits liver tumorigenesis induced by YAP activation, but also induces regression of established tumors. We found that RA-V inhibits dedifferentiation and proliferation, while inducing apoptosis of hepatocytes. Furthermore, RA-V also induces apoptosis and inhibits proliferation of macrophages in the microenvironment, which are essential for YAP-induced tumorigenesis. RA-V is thus a drug candidate for cancers involving YAP/TAZ activation.