Crumbs3-Mediated Polarity Directs Airway Epithelial Cell Fate through the Hippo Pathway Effector Yap

Control of skeletal morphogenesis by the Hippo-YAP/TAZ pathway

The Hippo-YAP/TAZ pathway is an important regulator of tissue growth, but can also control cell fate or tissue morphogenesis. Here, we investigate the function of the Hippo pathway during the development of cartilage, which forms the majority of the skeleton. Previously, YAP was proposed to inhibit skeletal size by repressing chondrocyte proliferation and differentiation. We find that, in vitro, Yap/Taz double knockout impairs murine chondrocyte proliferation, whereas constitutively nuclear nls-YAP5SA accelerates proliferation, in line with the canonical role of this pathway in most tissues. However, in vivo, cartilage-specific knockout of Yap/Taz does not prevent chondrocyte proliferation, differentiation or skeletal growth, but rather results in various skeletal deformities including cleft palate. Cartilage-specific expression of nls-YAP5SA or knockout of Lats1/2 do not increase cartilage growth, but instead lead to catastrophic malformations resembling chondrodysplasia or achondrogenesis. Physiological YAP target genes in cartilage include Ctgf, Cyr61 and several matrix remodelling enzymes. Thus, YAP/TAZ activity controls chondrocyte proliferation in vitro, possibly reflecting a regenerative response, but is dispensable for chondrocyte proliferation in vivo, and instead functions to control cartilage morphogenesis via regulation of the extracellular matrix.

Research Models and Gene Augmentation Therapy for CRB1 Retinal Dystrophies

Retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA) are inherited degenerative retinal dystrophies with vision loss that ultimately lead to blindness. Several genes have been shown to be involved in early onset retinal dystrophies, including CRB1 and RPE65. Gene therapy recently became available for young RP patients with variations in the RPE65 gene. Current research programs test adeno-associated viral gene augmentation or editing therapy vectors on various disease models mimicking the disease in patients. These include several animal and emerging human-derived models, such as human-induced pluripotent stem cell (hiPSC)-derived retinal organoids or hiPSC-derived retinal pigment epithelium (RPE), and human donor retinal explants. Variations in the CRB1 gene are a major cause for early onset autosomal recessive RP with patients suffering from visual impairment before their adolescence and for LCA with newborns experiencing severe visual impairment within the first months of life. These patients cannot benefit yet from an available gene therapy treatment. In this review, we will discuss the recent advances, advantages and disadvantages of different CRB1 human and animal retinal degeneration models. In addition, we will describe novel therapeutic tools that have been developed, which could potentially be used for retinal gene augmentation therapy for RP patients with variations in the CRB1 gene.

Targeting the Hippo pathway in cancer, fibrosis, wound healing and regenerative medicine

The Hippo pathway is an evolutionarily conserved signalling pathway with key roles in organ development, epithelial homeostasis, tissue regeneration, wound healing and immune modulation. Many of these roles are mediated by the transcriptional effectors YAP and TAZ, which direct gene expression via control of the TEAD family of transcription factors. Dysregulated Hippo pathway and YAP/TAZ-TEAD activity is associated with various diseases, most notably cancer, making this pathway an attractive target for therapeutic intervention. This Review highlights the key findings from studies of Hippo pathway signalling across biological processes and diseases, and discusses new strategies and therapeutic implications of targeting this pathway.

Regulation and functions of the Hippo pathway in stemness and differentiation

The Hippo pathway plays important roles in organ development, tissue regeneration, and human diseases, such as cancer. In the canonical Hippo pathway, the MST1/2-LATS1/2 kinase cascade phosphorylates and inhibits transcription coactivators Yes-associated protein and transcription coactivator with PDZ-binding motif and thus regulates transcription of genes important for cell proliferation and apoptosis. However, recent studies have depicted a much more complicate picture of the Hippo pathway with many new components and regulatory stimuli involving both chemical and mechanical signals. Furthermore, accumulating evidence indicates that the Hippo pathway also plays important roles in the determination of cell fates, such as self-renewal and differentiation. Here, we review regulations of the Hippo pathway and its functions in stemness and differentiation emphasizing recent discoveries.

Targeting Hippo signaling pathway by phytochemicals in cancer therapy

The current era of cancer research has been continuously advancing upon identifying novel aspects of tumorigenesis and the principal mechanisms behind the unleashed proliferation, invasion, drug resistance and immortality of cancer cells in hopes of exploiting these findings to achieve a more effective treatment for cancer. In pursuit of this goal, the identification of the first components of an extremely important regulatory pathway in Drosophila melanogaster that largely determines cell fate during the developmental stages, ended up in the discovery of the highly sophisticated Hippo signaling cascade. Soon after, it was revealed that deregulation of the components of this pathway either via mutations or through epigenetic alterations can be observed in a vast variety of tumors and these alterations greatly contribute to the neoplastic transformation of cells, their survival, growth and resistance to therapy. As more hidden aspects of this pathway such as its widespread entanglement with other major cellular signaling pathways are continuously being uncovered, many researchers have sought over the past decade to find ways of therapeutic interventions targeting the major components of the Hippo cascade. To date, various approaches such as the use of exogenous targeting miRNAs and different molecular inhibitors have been recruited herein, among which naturally occurring compounds have shown a great promise. On such a basis, in the present work we review the current understanding of Hippo pathway and the most recent evidence on targeting its components using natural plant-derived phytochemicals.

The extracellular and intracellular regions of Crb2a play distinct roles in guiding the formation of the apical zonula adherens

The transmembrane protein Crumbs (Crb), a key regulator of apical polarity, has a known involvement in establishment of the apical zonula adherens in epithelia, although the precise mechanism remains elusive. The zonula adherens are required to maintain the integrity and orderly arrangement of epithelia. Loss of the zonula adherens leads to morphogenetic defects in the tissues derived from epithelium. In this study, we revealed that the intracellular tail of Crb2a promoted the apical distribution of adherens junctions (AJs) in zebrafish retinal and lens epithelia, but caused assembly into unstable punctum adherens-like adhesion plaques. The extracellular region of Crb2a guided the transformation of AJs from the punctum adherens into stable zonula adherens. Accordingly, a truncated form of Crb2a lacking the extracellular region (Crb2a^ΔEX) could only partially rescue the retinal patterning defects in crb2a null mutant zebrafish (crb2a^m289). By contrast, constitutive over-expression of Crb2a^ΔEX disrupted the integrity of the outer limiting membrane in photoreceptors, which is derived from the zonula adherens of the retinal neuroepithelium. This study demonstrated that both the extracellular region and the intracellular tail of Crb2a are required to guide the formation of the apical zonula adherens.

Changes in Epithelial and Stromal Corneal Stiffness Occur with Age and Obesity

The cornea is avascular, which makes it an excellent model to study matrix protein expression and tissue stiffness. The corneal epithelium adheres to the basement zone and the underlying stroma is composed of keratocytes and an extensive matrix of collagen and proteoglycans. Our goal was to examine changes in corneas of 8- and 15-week mice and compare them to 15-week pre-Type 2 diabetic obese mouse. Nanoindentation was performed on corneal epithelium in situ and then the epithelium was abraded, and the procedure repeated on the basement membrane and stroma. Confocal imaging was performed to examine the localization of proteins. Stiffness was found to be age and obesity dependent. Young’s modulus was greater in the epithelium from 15-week mice compared to 8-week mice. At 15 weeks, the epithelium of the control was significantly greater than that of the obese mice. There was a difference in the localization of Crb3 and PKCζ in the apical epithelium and a lack of lamellipodial extensions in the obese mouse. In the pre-Type 2 diabetic obese mouse there was a difference in the stiffness slope and after injury localization of fibronectin was negligible. These indicate that age and environmental changes incurred by diet alter the integrity of the tissue with age rendering it stiffer. The corneas from the pre-Type 2 diabetic obese mice were significantly softer and this may be a result of changes both in proteins on the apical surface indicating a lack of integrity and a decrease in fibronectin.

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 Hippo Pathway: Biology and Pathophysiology

The Hippo pathway was initially discovered in Drosophila melanogaster as a key regulator of tissue growth. It is an evolutionarily conserved signaling cascade regulating numerous biological processes, including cell growth and fate decision, organ size control, and regeneration. The core of the Hippo pathway in mammals consists of a kinase cascade, MST1/2 and LATS1/2, as well as downstream effectors, transcriptional coactivators YAP and TAZ. These core components of the Hippo pathway control transcriptional programs involved in cell proliferation, survival, mobility, stemness, and differentiation. The Hippo pathway is tightly regulated by both intrinsic and extrinsic signals, such as mechanical force, cell-cell contact, polarity, energy status, stress, and many diffusible hormonal factors, the majority of which act through G protein-coupled receptors. Here, we review the current understanding of molecular mechanisms by which signals regulate the Hippo pathway with an emphasis on mechanotransduction and the effects of this pathway on basic biology and human diseases.

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.

Crumbs organizes the transport machinery by regulating apical levels of PI(4,5)P2 in Drosophila

An efficient vectorial intracellular transport machinery depends on a well-established apico-basal polarity and is a prerequisite for the function of secretory epithelia. Despite extensive knowledge on individual trafficking pathways, little is known about the mechanisms coordinating their temporal and spatial regulation. Here, we report that the polarity protein Crumbs is essential for apical plasma membrane phospholipid-homeostasis and efficient apical secretion. Through recruiting βHeavy-Spectrin and MyosinV to the apical membrane, Crumbs maintains the Rab6-, Rab11- and Rab30-dependent trafficking and regulates the lipid phosphatases Pten and Ocrl. Crumbs knock-down results in increased apical levels of PI(4,5)P2 and formation of a novel, Moesin- and PI(4,5)P2-enriched apical membrane sac containing microvilli-like structures. Our results identify Crumbs as an essential hub required to maintain the organization of the apical membrane and the physiological activity of the larval salivary gland.

The Hippo Kinase LATS2 Controls Helicobacter pylori-Induced Epithelial-Mesenchymal Transition and Intestinal Metaplasia in Gastric Mucosa

BACKGROUND & AIMS

Gastric carcinoma is related mostly to CagA+-Helicobacter pylori infection, which disrupts the gastric mucosa turnover and elicits an epithelial-mesenchymal transition (EMT) and preneoplastic transdifferentiation. The tumor suppressor Hippo pathway controls stem cell homeostasis; its core, constituted by the large tumor suppressor 2 (LATS2) kinase and its substrate Yes-associated protein 1 (YAP1), was investigated in this context.

METHODS

Hippo, EMT, and intestinal metaplasia marker expression were investigated by transcriptomic and immunostaining analyses in human gastric AGS and MKN74 and nongastric immortalized RPE1 and HMLE epithelial cell lines challenged by H pylori, and on gastric tissues of infected patients and mice. LATS2 and YAP1 were silenced using small interfering RNAs. A transcriptional enhanced associated domain (TEAD) reporter assay was used. Cell proliferation and invasion were evaluated.

RESULTS

LATS2 and YAP1 appear co-overexpressed in the infected mucosa, especially in gastritis and intestinal metaplasia. H pylori via CagA stimulates LATS2 and YAP1 in a coordinated biphasic pattern, characterized by an early transient YAP1 nuclear accumulation and stimulated YAP1/TEAD transcription, followed by nuclear LATS2 up-regulation leading to YAP1 phosphorylation and targeting for degradation. LATS2 and YAP1 reciprocally positively regulate each other's expression. Loss-of-function experiments showed that LATS2 restricts H pylori-induced EMT marker expression, invasion, and intestinal metaplasia, supporting a role of LATS2 in maintaining the epithelial phenotype of gastric cells and constraining H pylori-induced preneoplastic changes.

CONCLUSIONS

H pylori infection engages a number of signaling cascades that alienate mucosa homeostasis, including the Hippo LATS2/YAP1/TEAD pathway. In the host-pathogen conflict, which generates an inflammatory environment and perturbations of the epithelial turnover and differentiation, Hippo signaling appears as a protective pathway, limiting the loss of gastric epithelial cell identity that precedes gastric carcinoma development.

A CRISPR Screen Identifies the Cell Polarity Determinant Crumbs 3 as an Adeno-associated Virus Restriction Factor in Hepatocytes

Adeno-associated viruses (AAV) are helper-dependent parvoviruses that have been developed into promising gene therapy vectors. Many studies, including a recent unbiased genomic screen, have identified host factors essential for AAV cell entry, but no genome-wide screens that address inhibitory host factors have been reported. Here, we utilize a novel CRISPR screen to identify AAV restriction factors in a human hepatocyte cell line. The major hit from our gain-of-function screen is the apical polarity determinant Crumbs 3 (Crb3). Knockout (KO) of Crb3 enhances AAV transduction, while overexpression exerts the opposite effect. Further, Crb3 appears to restrict AAV transduction in a serotype- and cell type-specific manner. Particularly, for AAV serotype 9 and a rationally engineered AAV variant, we demonstrate that increased availability of galactosylated glycans on the surfaces of Crb3 KO cells, but not the universal AAV receptor, leads to increased capsid attachment and enhanced transduction. We postulate that Crb3 could serve as a key molecular determinant that restricts the availability of AAV glycan attachment factors on the cell surface by maintaining apical-basal polarity and tight junction integrity.IMPORTANCE Adeno-associated viruses (AAVs) have recently emerged at the forefront as gene therapy vectors; however, our understanding of host factors that influence AAV transduction in different cell types is still evolving. In the present study, we perform a genome-scale CRISPR knockout screen to identify cellular host factors that restrict AAV infection in hepatocyte cultures. We discover that Crumbs 3, which determines cellular polarity, also influences the distribution of certain carbohydrate attachment factors on the cell surface. This in turn affects the ability of virions to bind and enter the cells. This study underscores the importance of cell polarity in AAV transduction and provides a potential molecular basis for the differential infectious mechanism(s) in cell culture versus organ systems.

Yap/Taz are required for establishing the cerebellar radial glia scaffold and proper foliation

Yap/Taz are well-established downstream effectors of the Hippo pathway, known to regulate organ size by directing proliferation and apoptosis. Although the functions of Yap/Taz have been extensively studied, little is known about their role in brain development. Here, through genetic ablation, we show that Yap/Taz are required for cerebellar morphogenesis. Yap/Taz deletion in neural progenitors causes defects in secondary fissure formation, leading to abnormal folia development. Although they seemed very likely to serve an important function in the development of cerebellar granule cell precursors, Yap/Taz are dispensable for their proliferation. Furthermore, Yap/Taz loss does not rescue the medulloblastoma phenotype caused by constitutively active Smoothened. Importantly, Yap/Taz are highly expressed in radial glia and play a crucial role in establishing the radial scaffold and cellular polarity of neural progenitors during embryogenesis. We found that Yap/Taz are necessary to establish and maintain junctional integrity of cerebellar neuroepithelium as prominent junction proteins are not maintained at the apical junction in the absence of Yap/Taz. Our study identifies a novel function of Yap/Taz in cerebellar foliation and finds that they are required to establish the radial glia scaffold and junctional stability.

Polarity proteins in oncogenesis

Most human cancers arise from epithelial tissues, which are apical-basally polarized and possess intercellular adhesive junctions. Epithelial cells grow to characteristic densities, often from proliferative progenitors, which arrest as they mature. Homeostatic mechanisms can maintain this characteristic density if it is exceeded (crowding) or is too low (e.g. in response to wounding). During tumor initiation and progression this homeostatic mechanism is lost. Some aspects of cell polarity are also lost, although many carcinomas retain intercellular junctions and even apical domains. In other cases, and particularly in recurrent tumors, however, the cells become predominantly mesenchymal. A major question, still only incompletely answered, is whether the proteins that determine cell polarity function as tumor suppressors or tumor promoters. Here we discuss recent advances in understanding the role of polarity proteins and homeostasis in cancer.

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.

LATS1/2 suppress NFκB and aberrant EMT initiation to permit pancreatic progenitor differentiation

The Hippo pathway directs cell differentiation during organogenesis, in part by restricting proliferation. How Hippo signaling maintains a proliferation-differentiation balance in developing tissues via distinct molecular targets is only beginning to be understood. Our study makes the unexpected finding that Hippo suppresses nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) signaling in pancreatic progenitors to permit cell differentiation and epithelial morphogenesis. We find that pancreas-specific deletion of the large tumor suppressor kinases 1 and 2 (Lats1/2PanKO) from mouse progenitor epithelia results in failure to differentiate key pancreatic lineages: acinar, ductal, and endocrine. We carried out an unbiased transcriptome analysis to query differentiation defects in Lats1/2PanKO. This analysis revealed increased expression of NFκB activators, including the pantetheinase vanin1 (Vnn1). Using in vivo and ex vivo studies, we show that VNN1 activates a detrimental cascade of processes in Lats1/2PanKO epithelium, including (1) NFκB activation and (2) aberrant initiation of epithelial-mesenchymal transition (EMT), which together disrupt normal differentiation. We show that exogenous stimulation of VNN1 or NFκB can trigger this cascade in wild-type (WT) pancreatic progenitors. These findings reveal an unexpected requirement for active suppression of NFκB by LATS1/2 during pancreas development, which restrains a cell-autonomous deleterious transcriptional program and thereby allows epithelial differentiation.

Recycling Endosomes in Mature Epithelia Restrain Tumorigenic Signaling

The effects of polarized membrane trafficking in mature epithelial tissue on cell growth and cancer progression have not been fully explored in vivo. A majority of colorectal cancers have reduced and mislocalized Rab11, a small GTPase dedicated to trafficking of recycling endosomes. Patients with low Rab11 protein expression have poor survival rates. Using genetic models across species, we show that intact recycling endosome function restrains aberrant epithelial growth elicited by APC or RAS mutations. Loss of Rab11 protein led to epithelial dysplasia in early animal development and synergized with oncogenic pathways to accelerate tumor progression initiated by carcinogen, genetic mutation, or aging. Transcriptomic analysis uncovered an immediate expansion of the intestinal stem cell pool along with cell-autonomous Yki/Yap activation following disruption of Rab11a-mediated recycling endosomes. Intestinal tumors lacking Rab11a traffic exhibited marked elevation of nuclear Yap, upd3/IL6-Stat3, and amphiregulin-MAPK signaling, whereas suppression of Yki/Yap or upd3/IL6 reduced gut epithelial dysplasia and hyperplasia. Examination of Rab11a function in enteroids or cultured cell lines suggested that this endosome unit is required for suppression of the Yap pathway by Hippo kinases. Thus, recycling endosomes in mature epithelia constitute key tumor suppressors, loss of which accelerates carcinogenesis. SIGNIFICANCE: Recycling endosome traffic in mature epithelia constitutes a novel tumor suppressing mechanism.

DNA methylation of CRB3 is a prognostic biomarker in clear cell renal cell carcinoma

Our previous study revealed that CRB3 protein expression was reduced in clear cell renal cell carcinoma (ccRCC) and was associated with TNM stage, pathological grade, and poor prognosis of ccRCC. This study aimed to investigate if DNA methylation of CRB3 decreases its expression, subsequently leading to the progression and poor prognosis of ccRCC. Data for DNA methylation of CRB3, CRB3 mRNA expression, and ccRCC clinicopathological parameters were extracted from the cancer genome atlas (TCGA) database. The relationships among DNA methylation of CRB3, CRB3 mRNA expression, and ccRCC clinicopathological parameters were analyzed using UALCAN, MethHC, LinkedOmics, and Wanderer. We found that CRB3 mRNA levels were lower in ccRCC compared to normal tissues. Methylation of CRB3 increased in ccRCC, with all probes showing differences between ccRCC and normal tissues. Furthermore, CRB3 DNA methylation negatively correlated with CRB3 mRNA expression. CRB3 DNA methylation was also related to pathologic stage, T stage, N stage, and M stage of ccRCC. Overall survival was shorter in ccRCC patients with high CRB3 DNA methylation compared to ccRCC patients with low CRB3 DNA methylation. Methylation of cg24798010, a CRB3 probe, was related to laterality, pathologic stage, T stage, M stage, neoplasm-histologic-grade without N stage, and race. Furthermore, treatment with the DNA methylation inhibitor Decitabine resulted in the upregulation of CRB3 mRNA in ccRCC cell lines. These results indicate that DNA methylation of CRB3 may be both a prognostic marker and therapeutic target for ccRCC.

The Roles of YAP/TAZ and the Hippo Pathway in Healthy and Diseased Skin

Skin is the largest organ of the human body. Its architecture and physiological functions depend on diverse populations of epidermal cells and dermal fibroblasts. Reciprocal communication between the epidermis and dermis plays a key role in skin development, homeostasis and repair. While several stem cell populations have been identified in the epidermis with distinct locations and functions, there is additional heterogeneity within the mesenchymal cells of the dermis. Here, we discuss the current knowledge of how the Hippo pathway and its downstream effectors Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) contribute to the maintenance, activation and coordination of the epidermal and dermal cell populations during development, homeostasis, wound healing and cancer.

Yap and its subcellular localization have distinct compartment-specific roles in the developing lung

Although the Hippo-yes-associated protein (Yap) pathway has been implicated in lung development, the specific roles for Yap and its nucleocytoplasmic shuttling in the developing airway and alveolar compartments remain elusive. Moreover, conflicting results from expression studies and differences in the lung phenotypes of Yap and Hippo kinase null mutants caused controversy over the dynamics and significance of Yap subcellular localization in the developing lung. Here, we show that the aberrant morphogenesis of Yap-deficient lungs results from the disruption of developmental events specifically in distal epithelial progenitors. We also show that activation of nuclear Yap is enough to fulfill the Yap requirements to rescue abnormalities in these lungs. Remarkably, we found that Yap nucleocytoplasmic shuttling is largely dispensable in epithelial progenitors for both branching morphogenesis and sacculation. However, if maintained transcriptionally active in airways, nuclear Yap profoundly alters proximal-distal identity and halts epithelial differentiation. Taken together, these observations provide novel insights into the crucial importance of Hippo-Yap signaling in the lung prenatally.

ABL kinase inhibition sensitizes primary lung adenocarcinomas to chemotherapy by promoting tumor cell differentiation

Lung cancer is the leading cause of cancer mortality in the United States, with an overall five-year survival rate of ~16%. Non-small cell lung cancer (NSCLC) accounts for ~80% of all lung cancer cases, and the majority (40%) of these are adenocarcinomas. Loss of function point mutations in TP53 (46%) and activating mutations in KRAS (33%) are the most common mutations in human lung adenocarcinomas. Because neither of these genetic alterations are clinically actionable, chemotherapy remains the mainstay of treatment in patients with oncogenic KRAS driver mutations. However, chemoresistance to genotoxic agents such as docetaxel remains a major clinical challenge facing lung cancer patients. Here we show that ABL kinase allosteric inhibitors can be effectively used for the treatment of KrasG12D/+; p53-/- lung adenocarcinomas in an autochthonous mouse model. Unexpectedly, we found that treatment of tumor-bearing mice with an ABL allosteric inhibitor promoted differentiation of lung adenocarcinomas from poorly differentiated tumors expressing basal cell markers to tumors expressing terminal differentiation markers in vivo, which rendered lung adenocarcinomas susceptible to chemotherapy. These findings uncover a novel therapeutic approach for the treatment of lung adenocarcinomas with poor response to chemotherapy.

Building and Regenerating the Lung Cell by Cell

The unique architecture of the mammalian lung is required for adaptation to air breathing at birth and thereafter. Understanding the cellular and molecular mechanisms controlling its morphogenesis provides the framework for understanding the pathogenesis of acute and chronic lung diseases. Recent single-cell RNA sequencing data and high-resolution imaging identify the remarkable heterogeneity of pulmonary cell types and provides cell selective gene expression underlying lung development. We will address fundamental issues related to the diversity of pulmonary cells, to the formation and function of the mammalian lung, and will review recent advances regarding the cellular and molecular pathways involved in lung organogenesis. What cells form the lung in the early embryo? How are cell proliferation, migration, and differentiation regulated during lung morphogenesis? How do cells interact during lung formation and repair? How do signaling and transcriptional programs determine cell-cell interactions necessary for lung morphogenesis and function?

Hippo signaling promotes lung epithelial lineage commitment by curbing Fgf10 and β-catenin signaling

Organ growth and tissue homeostasis rely on the proliferation and differentiation of progenitor cell populations. In the developing lung, localized Fgf10 expression maintains distal Sox9-expressing epithelial progenitors and promotes basal cell differentiation in the cartilaginous airways. Mesenchymal Fgf10 expression is induced by Wnt signaling but inhibited by Shh signaling, and epithelial Fgf10 signaling activates β-catenin signaling. The Hippo pathway is a well-conserved signaling cascade that regulates organ size and stem/progenitor cell behavior. Here, we show that Hippo signaling promotes lineage commitment of lung epithelial progenitors by curbing Fgf10 and β-catenin signaling. Our findings show that both inactivation of the Hippo pathway (nuclear Yap) or ablation of Yap result in increased β-catenin and Fgf10 signaling, suggesting a cytoplasmic role for Yap in epithelial lineage commitment. We further demonstrate redundant and non-redundant functions for the two nuclear effectors of the Hippo pathway, Yap and Taz, during lung development.

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.

Emerging roles of YAP/TAZ in lung physiology and diseases

The YAP and TAZ, as the downstream effectors of Hippo pathway, have emerged as important translational co-activators of a wide variety of biological processes. YAP/TAZ plays a crucial role in the lung development and physiology. Dysregulation of YAP/TAZ signaling pathway contributes to the development and progression of chronic lung diseases, including lung cancer, pulmonary fibrosis, pulmonary hypertension, COPD, asthma, and lung infection. Therefore, owing to its critical functions, delineation of the signaling mechanisms of YAP/TAZ in pathological conditions will shed light on developing strategies for its therapeutic targeting. Currently, the complex regulation of this pathway is under extensive investigation. In this review, we summarize and present recent findings of molecular mechanisms of YAP/TAZ in the lung physiological and pathological conditions, as well as the implications of YAP/TAZ for lung diseases treatment and regeneration.

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.

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.

Lats1/2 inactivation reveals Hippo function in alveolar type I cell differentiation during lung transition to air breathing

Lung growth to its optimal size at birth is driven by reiterative airway branching followed by differentiation and expansion of alveolar cell types. How this elaborate growth is coordinated with the constraint of the chest is poorly understood. Here, we investigate the role of Hippo signaling, a cardinal pathway in organ size control, in mouse lung development. Unexpectedly, we found that epithelial loss of the Hippo kinase genes Lats1 and Lats2 (Lats1/2) leads to a striking reduction of lung size owing to an early arrest of branching morphogenesis. This growth defect is accompanied by abnormalities in epithelial cell polarity, cell division plane and extracellular matrix deposition, as well as precocious and increased expression of markers for type 1 alveolar epithelial cells (AEC1s), an indicator of terminal differentiation. Increased AEC1s were also observed in transgenic mice with overexpression of a constitutive nuclear form of downstream transcriptional effector YAP. Conversely, loss of Yap and Taz led to decreased AEC1s, demonstrating that the canonical Hippo signaling pathway is both sufficient and necessary to drive AEC1 fate. These findings together reveal unique roles of Hippo-LATS-YAP signaling in the developing mouse lung.

Transcriptional regulation of cell shape during organ morphogenesis

The emerging field of transcriptional regulation of cell shape changes aims to address the critical question of how gene expression programs produce a change in cell shape. Together with cell growth, division, and death, changes in cell shape are essential for organ morphogenesis. Whereas most studies of cell shape focus on posttranslational events involved in protein organization and distribution, cell shape changes can be genetically programmed. This review highlights the essential role of transcriptional regulation of cell shape during morphogenesis of the heart, lungs, gastrointestinal tract, and kidneys. We emphasize the evolutionary conservation of these processes across different model organisms and discuss perspectives on open questions and research avenues that may provide mechanistic insights toward understanding birth defects.

Expression and localization of the polarity protein CRB2 in adult mouse brain: a comparison with the CRB1rd8 mutant mouse model

Acquisition of cell polarization is essential for the performance of crucial functions, like a successful secretion and appropriate cell signaling in many tissues, and it depends on the correct functioning of polarity proteins, including the Crumbs complex. The CRB proteins, CRB1, CRB2 and CRB3, identified in mammals, are expressed in epithelial-derived tissues like brain, kidney and retina. CRB2 has a ubiquitous expression and has been detected in embryonic brain tissue; but currently there is no data regarding its localization in the adult brain. In our study, we characterized the presence of CRB2 in adult mice brain, where it is particularly enriched in cortex, hippocampus, hypothalamus and cerebellum. Double immunofluorescence analysis confirmed that CRB2 is a neuron-specific protein, present in both soma and projections where colocalizes with certain populations of exocytic and endocytic vesicles and with other members of the Crumbs complex. Finally, in the cortex of CRB1^rd8 mutant mice that contain a mutation in the Crb1 gene generating a truncated CRB1 protein, there is an abnormal increase in the expression levels of the CRB2 protein which suggests a possible compensatory mechanism for the malfunction of CRB1 in this mutant background.

A guide to polarized airway epithelial models for studies of host-pathogen interactions

Mammalian lungs are organs exhibiting the cellular and spatial complexity required for gas exchange to support life. The respiratory epithelium internally lining the airways is susceptible to infections due to constant exposure to inhaled microbes. Biomedical research into respiratory bacterial infections in humans has been mostly carried out using small mammalian animal models or two-dimensional, submerged cultures of undifferentiated epithelial cells. These experimental model systems have considerable limitations due to host specificity of bacterial pathogens and lack of cellular and morphological complexity. This review describes the in vitro differentiated and polarized airway epithelial cells of human origin that are used as a model to study respiratory bacterial infections. Overall, these models recapitulate key aspects of the complexity observed in vivo and can help in elucidating the molecular details of disease processes observed during respiratory bacterial infections.

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.

YAP and TAZ in Lung Cancer: Oncogenic Role and Clinical Targeting

Lung cancer is the leading cause of cancer death in the world and there is no current treatment able to efficiently treat the disease as the tumor is often diagnosed at an advanced stage. Moreover, cancer cells are often resistant or acquire resistance to the treatment. Further knowledge of the mechanisms driving lung tumorigenesis, aggressiveness, metastasization, and resistance to treatments could provide new tools for detecting the disease at an earlier stage and for a better response to therapy. In this scenario, Yes Associated Protein (YAP) and Trascriptional Coactivator with PDZ-binding motif (TAZ), the final effectors of the Hippo signaling transduction pathway, are emerging as promising therapeutic targets. Here, we will discuss the most recent advances made in YAP and TAZ biology in lung cancer and, more importantly, on the newly discovered mechanisms of YAP and TAZ inhibition in lung cancer as well as their clinical implications.

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.

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.

Active epithelial Hippo signaling in idiopathic pulmonary fibrosis

Hippo/YAP signaling plays pleiotropic roles in the regulation of cell proliferation and differentiation during organogenesis and tissue repair. Herein we demonstrate increased YAP activity in respiratory epithelial cells in lungs of patients with idiopathic pulmonary fibrosis (IPF), a common, lethal form of interstitial lung disease (ILD). Immunofluorescence staining in IPF epithelial cells demonstrated increased nuclear YAP and loss of MST1/2. Bioinformatic analyses of epithelial cell RNA profiles predicted increased activity of YAP and increased canonical mTOR/PI3K/AKT signaling in IPF. Phospho-S6 (p-S6) and p-PTEN were increased in IPF epithelial cells, consistent with activation of mTOR signaling. Expression of YAP (S127A), a constitutively active form of YAP, in human bronchial epithelial cells (HBEC3s) increased p-S6 and p-PI3K, cell proliferation and migration, processes that were inhibited by the YAP-TEAD inhibitor verteporfin. Activation of p-S6 was required for enhancing and stabilizing YAP, and the p-S6 inhibitor temsirolimus blocked nuclear YAP localization and suppressed expression of YAP target genes CTGF, AXL, and AJUBA (JUB). YAP and mTOR/p-S6 signaling pathways interact to induce cell proliferation and migration, and inhibit epithelial cell differentiation that may contribute to the pathogenesis of IPF.

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.

The Crumbs3 Polarity Protein

The Crumbs proteins are evolutionarily conserved apical transmembrane proteins. Drosophila Crumbs was discovered via its crucial role in epithelial polarity during fly embryogenesis. Crumbs proteins have variable extracellular domains but a highly conserved intracellular domain that can bind FERM and PDZ domain proteins. Mammals have three Crumbs genes and this review focuses on Crumbs3, the major Crumbs isoform expressed in mammalian epithelial cells. Although initial work has highlighted the role of Crumbs3 in polarity, more recent studies have found it has an important role in tissue morphogenesis functioning as a linker between the apical membrane and the actin cytoskeleton. In addition, recent publications have linked Crumbs3 to growth control via regulation of the Hippo/Yap pathway.

Evolution of mechanotransduction via YAP/TAZ in animal epithelia

Mechanical stretch forces can control the growth of epithelial tissues such as mammalian skin, whose surface area is precisely coordinated with body size. In skin keratinocytes cultured in vitro, mechanical forces acting via Integrin adhesions and the actin cytoskeleton have been shown to induce nuclear translocation of YAP/TAZ co-activators to induce cell proliferation. Furthermore, conditional knockouts of both YAP (also called YAP1) and TAZ (also called WWTR1) in mouse skin resemble the phenotype of skin-specific loss of Integrin beta1 (ITGB1), indicating that this signalling mechanism is important in vivo. Curiously, Integrins are dispensable in Drosophila to activate the sole YAP/TAZ homolog Yorkie (Yki), which has lost the C-terminal PDZ-binding motif needed to promote nuclear localization of YAP/TAZ in mammalian cells. Differences in the structure of the epidermis between deuterostomes (e.g.: stratified squamous skin of mammals) and protostomes (e.g.: monolayered columnar epidermis of Drosophila) may explain this evolutionary divergence. Monolayered columnar epithelia feature a well-differentiated apical membrane domain, where proteins such as Crumbs, Expanded, Merlin and Kibra activate the Hippo pathway to repress Drosophila Yki. Stratified squamous epithelia lack an apical domain and thus depend primarily on basal Integrin adhesions to activate YAP/TAZ in basal layer stem cells via multiple postulated signalling mechanisms. Finally, YAP and TAZ retain the ability to sense the apical domain in the columnar epithelial cells lining internal organs such as the lung bronchus, where YAP/TAZ localize to the nucleus in proliferating basal layer stem cells but translocate to the cytoplasm in differentiated columnar cells.

Cadherin-26 (CDH26) regulates airway epithelial cell cytoskeletal structure and polarity

Polarization of the airway epithelial cells (AECs) in the airway lumen is critical to the proper function of the mucociliary escalator and maintenance of lung health, but the cellular requirements for polarization of AECs are poorly understood. Using human AECs and cell lines, we demonstrate that cadherin-26 (CDH26) is abundantly expressed in differentiated AECs, localizes to the cell apices near ciliary membranes, and has functional cadherin domains with homotypic binding. We find a unique and non-redundant role for CDH26, previously uncharacterized in AECs, in regulation of cell-cell contact and cell integrity through maintaining cytoskeletal structures. Overexpression of CDH26 in cells with a fibroblastoid phenotype increases contact inhibition and promotes monolayer formation and cortical actin structures. CDH26 expression is also important for localization of planar cell polarity proteins. Knockdown of CDH26 in AECs results in loss of cortical actin and disruption of CRB3 and other proteins associated with apical polarity. Together, our findings uncover previously unrecognized functions for CDH26 in the maintenance of actin cytoskeleton and apicobasal polarity of AECs.

Claudin-18: unexpected regulator of lung alveolar epithelial cell proliferation

Claudin 18 (CLDN18) is a tight junction protein that is highly expressed in the lung. While mice lacking CLDN18 exhibit the expected loss of epithelial integrity in the lung, these animals also have unexpectedly large lungs. In this issue of the JCI, Zhou, Flodby, and colleagues reveal that the increased lung size of Cldn18-/- mice is the result of increased type 2 alveolar epithelial (AT2) cell proliferation. This increase in proliferation was shown to be driven by translocation of the transcriptional regulator Yes-associated protein (YAP) to the nucleus and subsequent induction of proliferative pathways. CLDN18-deficent mice also had increased frequency of lung adenocarcinomas. Together, the results of this study advance our understanding of the mechanisms that likely regulate homeostasis of the normal lung as well as promote the proliferative state of malignant cells found in lung adenocarcinomas thought to originate from AT2 cells.

Claudin-18-mediated YAP activity regulates lung stem and progenitor cell homeostasis and tumorigenesis

Claudins, the integral tight junction (TJ) proteins that regulate paracellular permeability and cell polarity, are frequently dysregulated in cancer; however, their role in neoplastic progression is unclear. Here, we demonstrated that knockout of Cldn18, a claudin family member highly expressed in lung alveolar epithelium, leads to lung enlargement, parenchymal expansion, increased abundance and proliferation of known distal lung progenitors, the alveolar epithelial type II (AT2) cells, activation of Yes-associated protein (YAP), increased organ size, and tumorigenesis in mice. Inhibition of YAP decreased proliferation and colony-forming efficiency (CFE) of Cldn18-/- AT2 cells and prevented increased lung size, while CLDN18 overexpression decreased YAP nuclear localization, cell proliferation, CFE, and YAP transcriptional activity. CLDN18 and YAP interacted and colocalized at cell-cell contacts, while loss of CLDN18 decreased YAP interaction with Hippo kinases p-LATS1/2. Additionally, Cldn18-/- mice had increased propensity to develop lung adenocarcinomas (LuAd) with age, and human LuAd showed stage-dependent reduction of CLDN18.1. These results establish CLDN18 as a regulator of YAP activity that serves to restrict organ size, progenitor cell proliferation, and tumorigenesis, and suggest a mechanism whereby TJ disruption may promote progenitor proliferation to enhance repair following injury.

The Hippo pathway in organ development, homeostasis, and regeneration

The Hippo pathway is a universal governor of organ size, tissue homeostasis, and regeneration. A growing body of work has advanced our understanding of Hippo pathway regulation of cell proliferation, differentiation, and spatial patterning not only in organ development but also upon injury-induced regeneration. The pathway's central role in stem cell biology thus implicates its potential for therapeutic manipulation in mammalian organ regeneration. In this review, we survey recent literature linking the Hippo pathway to the development, homeostasis, and regeneration of various organs, including Hippo-independent roles for YAP, defined here as YAP functions that are not regulated by the Hippo pathway kinases LATS1/2.

SHIP2 Regulates Lumen Generation, Cell Division, and Ciliogenesis through the Control of Basolateral to Apical Lumen Localization of Aurora A and HEF 1

Lumen formation during epithelial morphogenesis requires the creation of a luminal space at cell interfaces named apical membrane-initiation sites (AMISs). This is dependent upon integrated signaling from mechanical and biochemical cues, vesicle trafficking, cell division, and processes tightly coupled to ciliogenesis. Deciphering relationships between polarity determinants and lumen or cilia generation remains a fundamental issue. Here, we report that Src homology 2 domain-containing inositol 5-phosphatase 2 (SHIP2), a basolateral determinant of polarity, regulates RhoA-dependent actin contractility and cell division to form AMISs. SHIP2 regulates mitotic spindle alignment. SHIP2 is expressed in G1 phase, whereas Aurora A kinase is enriched in mitosis. SHIP2 binds Aurora A kinase and the scaffolding protein HEF1 and promotes their basolateral localization at the expense of their luminal expression connected with cilia resorption. Furthermore, SHIP2 expression increases cilia length. Thus, our findings offer new insight into the relationships among basolateral proteins, lumen generation, and ciliogenesis.

Role of the Crumbs proteins in ciliogenesis, cell migration and actin organization

Epithelial cell organization relies on a set of proteins that interact in an intricate way and which are called polarity complexes. These complexes are involved in the determination of the apico-basal axis and in the positioning and stability of the cell-cell junctions called adherens junctions at the apico-lateral border in invertebrates. Among the polarity complexes, two are present at the apical side of epithelial cells. These are the Par complex including aPKC, PAR3 and PAR6 and the Crumbs complex including, CRUMBS, PALS1 and PATJ/MUPP1. These two complexes interact directly and in addition to their already well described functions, they play a role in other cellular processes such as ciliogenesis and polarized cell migration. In this review, we will focus on these aspects that involve the apical Crumbs polarity complex and its relation with the cortical actin cytoskeleton which might provide a more comprehensive hypothesis to explain the many facets of Crumbs cell and tissue properties.

EGFR signalling controls cellular fate and pancreatic organogenesis by regulating apicobasal polarity

Apicobasal polarity is known to affect epithelial morphogenesis and cell differentiation, but it remains unknown how these processes are mechanistically orchestrated. We find that ligand-specific EGFR signalling via PI(3)K and Rac1 autonomously modulates apicobasal polarity to enforce the sequential control of morphogenesis and cell differentiation. Initially, EGF controls pancreatic tubulogenesis by negatively regulating apical polarity induction. Subsequently, betacellulin, working via inhibition of atypical protein kinase C (aPKC), causes apical domain constriction within neurogenin3⁺ endocrine progenitors, which results in reduced Notch signalling, increased neurogenin3 expression, and β-cell differentiation. Notably, the ligand-specific EGFR output is not driven at the ligand level, but seems to have evolved in response to stage-specific epithelial influences. The EGFR-mediated control of β-cell differentiation via apical polarity is also conserved in human neurogenin3⁺ cells. We provide insight into how ligand-specific EGFR signalling coordinates epithelial morphogenesis and cell differentiation via apical polarity dynamics.

Integrin-FAK-CDC42-PP1A signaling gnaws at YAP/TAZ activity to control incisor stem cells

How epithelial tissues are able to self-renew to maintain homeostasis and regenerate in response to injury remains a persistent question. The transcriptional effectors YAP and TAZ are increasingly being recognized as central mediators of epithelial stem cell biology, and a wealth of recent studies have been directed at understanding the control and activity of these factors. Recent work by Hu et al. has added to this knowledge, as they identify an Integrin-FAK-CDC42-PP1A signaling cascade that directs nuclear YAP/TAZ activity in stem cell populations of the mouse incisor, and define convergence on mTORC1 signaling as an important mediator of the proliferation of these cells. Here, we review recent studies on YAP/TAZ function and regulation in epithelial tissue-specific stem cells, merging the Hu et al. study together with our current knowledge of YAP/TAZ.

Melatonin and Hippo Pathway: Is There Existing Cross-Talk?

Melatonin is an indolic hormone that regulates a plethora of functions ranging from the regulation of circadian rhythms and antioxidant properties to the induction and maintenance of tumor suppressor pathways. It binds to specific receptors as well as to some cytosolic proteins, leading to several cellular signaling cascades. Recently, the involvement of melatonin in cancer insurgence and progression has clearly been demonstrated. In this review, we will first describe the structure and functions of melatonin and its receptors, and then discuss both molecular and epidemiological evidence on melatonin anticancer effects. Finally, we will shed light on potential cross-talk between melatonin signaling and the Hippo signaling pathway, along with the possible implications for cancer therapy.