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

PTPN14 forms a complex with Kibra and LATS1 proteins and negatively regulates the YAP oncogenic function

The Hippo signaling pathway regulates cellular proliferation and survival, thus exerting profound effects on normal cell fate and tumorigenesis. Pivotal effectors of this pathway are YAP/TAZ, transcriptional co-activators whose dysfunction contributes to epithelial-to-mesenchymal transition and malignant transformation. Therefore, it is of great importance to decipher the mechanisms underlying the regulations of YAP/TAZ at various levels. Here we report that non-receptor tyrosine phosphatase 14 (PTPN14) interacts with the Kibra protein. The interaction between PTPN14 and Kibra is through the PPXY domain of PTPN14 and WW domain of Kibra. PTPN14 and Kibra can induce the LATS1 activation independently and cooperatively. Interestingly, activation of LATS1 by PTPN14 is dependent on the C terminus of PTPN14 and independent of the upstream mammalian STE20-like kinase (MST) proteins. Furthermore, we demonstrate that PTPN14 increases the LAST1 protein stability. Last, overexpression of Kibra rescues the increased cell migration and aberrant three-dimensional morphogenesis induced by knockdown of PTPN14, and this rescue is mediated through the activation of the upstream LATS1 kinase and subsequent cytoplasmic sequestration of YAP. In summary, our results indicate a potential regulatory role of PTPN14 in the Hippo pathway and demonstrate another layer of regulation in the YAP oncogenic function.

Functions of the Drosophila JAK-STAT pathway: Lessons from stem cells

JAK-STAT signaling has been proposed to act in numerous stem cells in a variety of organisms. Here we provide an overview of its roles in three well characterized stem cell populations in Drosophila, in the intestine, lymph gland and testis. In flies, there is a single JAK and a single STAT, which has made the genetic dissection of pathway function considerably easier and facilitated the analysis of communication between stem cells, their niches and offspring. Studies in flies have revealed roles for this pathway as diverse as regulating bona fide intrinsic self-renewal, integrating response to environmental cues that control quiescence and promoting mitogenic responses to stress.

Crumbs promotes expanded recognition and degradation by the SCF(Slimb/β-TrCP) ubiquitin ligase

In epithelial tissues, growth control depends on the maintenance of proper architecture through apicobasal polarity and cell-cell contacts. The Hippo signaling pathway has been proposed to sense tissue architecture and cell density via an intimate coupling with the polarity and cell contact machineries. The apical polarity protein Crumbs (Crb) controls the activity of Yorkie (Yki)/Yes-activated protein, the progrowth target of the Hippo pathway core kinase cassette, both in flies and mammals. The apically localized Four-point-one, Ezrin, Radixin, Moesin domain protein Expanded (Ex) regulates Yki by promoting activation of the kinase cascade and by directly tethering Yki to the plasma membrane. Crb interacts with Ex and promotes its apical localization, thereby linking cell polarity with Hippo signaling. We show that, as well as repressing Yki by recruiting Ex to the apical membrane, Crb promotes phosphorylation-dependent ubiquitin-mediated degradation of Ex. We identify Skp/Cullin/F-box(Slimb/β-transducin repeats-containing protein) (SCF(Slimb/β-TrCP)) as the E3 ubiquitin ligase complex responsible for Ex degradation. Thus, Crb is part of a homeostatic mechanism that promotes Ex inhibition of Yki, but also limits Ex activity by inducing its degradation, allowing precise tuning of Yki function.

KIBRA: In the brain and beyond

In mammals, the KIBRA locus has been associated with memory performance and cognition by genome-wide single nucleotide polymorphism screening. Genetic studies in Drosophila and human cells have identified KIBRA as a novel regulator of the Hippo signaling pathway, which plays a critical role in human tumorigenesis. Recent studies also indicated that KIBRA is involved in other physiological processes including cell polarity, membrane/vesicular trafficking, mitosis and cell migration. At the biochemical level, KIBRA protein is highly phosphorylated by various kinases in epithelial cells. Here, we discuss the updates concerning the function and regulation of KIBRA in the brain and beyond.

βν integrin inhibits chronic and high level activation of JNK to repress senescence phenotypes in Drosophila adult midgut

Proper control of adult stem cells including their proliferation and differentiation is crucial in maintaining homeostasis of well-organized tissues/organs throughout an organism's life. The Drosophila adult midgut has intestinal stem cells (ISCs), which have been exploited as a simple model system to investigate mechanisms controlling adult tissue homeostasis. Here, we found that a viable mutant of βν integrin (βint-ν), encoding one of two Drosophila integrin β subunits, showed a short midgut and abnormal multilayered epithelia accompanied by an increase in ISC proliferation and misdifferentiation defects. The increase in ISC proliferation and misdifferentiation was due to frequent ISC duplication expanding a pool of ISCs, which was caused by depression of the Notch signalling, and up-regulation of unpaired (upd), a gene encoding an extracellular ligand in the JAK/STAT signalling pathway. In addition, we observed that abnormally high accumulation of filamentous actin (F-actin) was caused in the βint-ν mutant enterocytes. Furthermore, the defects were rescued by suppressing c-Jun N-terminal kinase (JNK) signalling, which was up-regulated in a manner correlated with the defect levels in the above-mentioned βint-ν mutant phenotype. These symptoms observed in young βint-ν mutant midgut were very similar to those in the aged midgut in wild type. Our results suggested that βint-ν has a novel function for the Drosophila adult midgut homeostasis under normal conditions and provided a new insight into possible age-related diseases caused by latent abnormality of an integrin function.

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

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

Bantam is essential for Drosophila intestinal stem cell proliferation in response to Hippo signaling

The Drosophila midgut has emerged as an attractive model system to study stem cell biology. Extensive studies have been carried out to investigate the mechanisms of how the signaling pathways integrate to regulate intestinal stem cells (ISCs), yet, whether the microRNAs are involved in ISC self-renewal and maintenance is unknown. Here we demonstrate that the bantam microRNA is expressed specifically at high levels in Drosophila midgut precursor cells (including ISCs and enteroblasts) and secretory enteroendocrine cells while at extremely low levels in enterocytes. Furthermore, overexpression of bantam microRNA results in increase of the division of the midgut precursor cells, whereas loss of bantam microRNA decreases their proliferation. The mechanical studies show that bantam microRNA is essential for the Hpo pathway induced cell-autonomous ISC self-renewal, while it is disposable for EGFR and Notch pathways mediated ISC proliferation. More interestingly, we find that bantam microRNA is not required for the Hpo pathway mediated non-cell-autonomous ISC proliferation, revealing a novel mechanism by which the Hpo signaling pathway specifies its transcriptional targets in specific tissue to exhibit its biological functions.

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

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

Hippo gains weight: added insights and complexity to pathway control

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

Pseudophosphatases: methods of analysis and physiological functions

Protein tyrosine phosphatases (PTPs) are key enzymes in the regulation of cellular homeostasis and signaling pathways. Strikingly, not all PTPs bear enzymatic activity. A considerable fraction of PTPs are enzymatically inactive and are known as pseudophosphatases. Despite the lack of activity they execute pivotal roles in development, cell biology and human disease. The present review is focused on the methods used to identify pseudophosphatases, their targets, and physiological roles. We present a strategy for detailed enzymatic analysis of inactive PTPs, regulation of inactive PTP domains and identification of binding partners. Furthermore, we provide a detailed overview of human pseudophosphatases and discuss their regulation of cellular processes and functions in human pathologies.

Drosophila as a model system to unravel the layers of innate immunity to infection

Innate immunity relies entirely upon germ-line encoded receptors, signalling components and effector molecules for the recognition and elimination of invading pathogens. The fruit fly Drosophila melanogaster with its powerful collection of genetic and genomic tools has been the model of choice to develop ideas about innate immunity and host–pathogen interactions. Here, we review current research in the field, encompassing all layers of defence from the role of the microbiota to systemic immune activation, and attempt to speculate on future directions and open questions.

Protein tyrosine phosphatase variants in human hereditary disorders and disease susceptibilities

Reversible tyrosine phosphorylation of proteins is a key regulatory mechanism to steer normal development and physiological functioning of multicellular organisms. Phosphotyrosine dephosphorylation is exerted by members of the super-family of protein tyrosine phosphatase (PTP) enzymes and many play such essential roles that a wide variety of hereditary disorders and disease susceptibilities in man are caused by PTP alleles. More than two decades of PTP research has resulted in a collection of PTP genetic variants with corresponding consequences at the molecular, cellular and physiological level. Here we present a comprehensive overview of these PTP gene variants that have been linked to disease states in man. Although the findings have direct bearing for disease diagnostics and for research on disease etiology, more work is necessary to translate this into therapies that alleviate the burden of these hereditary disorders and disease susceptibilities in man.

Drosophila ste-20 family protein kinase, hippo, modulates fat cell proliferation

Background

Evolutionarily conserved Hippo (Hpo) pathway plays a pivotal role in the control of organ size. Although the Hpo pathway regulates proliferation of a variety of epidermal cells, its function in non-ectoderm-derived cells is largely unknown.

Methodology/Principal Findings

Through methods including fat quantification assays, starvation assays, in vivo labeling assays, we show that overexpression of Hpo in Drosophila melanogaster fat body restricts Drosophila body growth and reduces fat storage through regulation of adipocyte proliferation rather than through influencing the size of fat cells and lipid metabolism, whereas compromising Hpo activity results in weight gain and greater fat storage. Furthermore, we provide evidence that Yorkie (Yki, a transcriptional coactivator that functions in the Hpo pathway) antagonizes Hpo to modulate fat storage in Drosophila.

Conclusions/Significance

Our findings specify a role of Hpo in controlling mesoderm-derived cell proliferation. The observed anti-obesity effects of Hpo may indicate great potential for its utilization in anti-obesity therapeutics.

The tyrosine phosphatase PTPN14 is a negative regulator of YAP activity

The Hippo (Hpo) pathway is a novel signaling pathway that controls organ size in Drosophila and mammals and is deregulated in a variety of human cancers. It consists of a set of kinases that, through a number of phosphorylation events, inactivate YAP, a transcriptional co-activator that controls cellular proliferation and apoptosis. We have identified PTPN14 as a YAP-binding protein that negatively regulates YAP activity by controlling its localization. Mechanistically, we find that the interaction of ectopic YAP with PTPN14 can be mediated by the respective WW and PPxY motifs. However, the PTPN14 PPxY motif and phosphatase activity appear to be dispensable for the negative regulation of endogenous YAP, likely suggesting more complex mechanisms of interaction and modulation. Finally, we demonstrate that PTPN14 downregulation can phenocopy YAP activation in mammary epithelial cells and synergize with YAP to induce oncogenic transformation.

Homeostatic epithelial renewal in the gut is required for dampening a fatal systemic wound response in Drosophila

Effective defense responses involve the entire organism. To maintain body homeostasis after tissue damage, a systemic wound response is induced in which the response of each tissue is tightly orchestrated to avoid incomplete recovery or an excessive, damaging response. Here, we provide evidence that in the systemic response to wounding, an apoptotic caspase pathway is activated downstream of reactive oxygen species in the midgut enterocytes (ECs), cells distant from the wound site, in Drosophila. We show that a caspase-pathway mutant has defects in homeostatic gut cell renewal and that inhibiting caspase activity in fly ECs results in the production of systemic lethal factors after wounding. Our results indicate that wounding remotely controls caspase activity in ECs, which activates the tissue stem cell regeneration pathway in the gut to dampen the dangerous systemic wound reaction.

Drosophila melanogaster: a model and a tool to investigate malignancy and identify new therapeutics

For decades, lower-model organisms such as Drosophila melanogaster have often provided the first glimpse into the mechanism of action of human cancer-related proteins, thus making a substantial contribution to elucidating the molecular basis of the disease. More recently, D. melanogaster strains that are engineered to recapitulate key aspects of specific types of human cancer have been paving the way for the future role of this 'workhorse' of biomedical research, helping to further investigate the process of malignancy, and serving as platforms for therapeutic drug discovery.

The Hippo pathway: regulators and regulations

Control of cell number is crucial in animal development and tissue homeostasis, and its dysregulation may result in tumor formation or organ degeneration. The Hippo pathway in both Drosophila and mammals regulates cell number by modulating cell proliferation, cell death, and cell differentiation. Recently, numerous upstream components involved in the Hippo pathway have been identified, such as cell polarity, mechanotransduction, and G-protein-coupled receptor (GPCR) signaling. Actin cytoskeleton or cellular tension appears to be the master mediator that integrates and transmits upstream signals to the core Hippo signaling cascade. Here, we review regulatory mechanisms of the Hippo pathway and discuss potential implications involved in different physiological and pathological conditions.

The Hippo size control pathway--ever expanding

An important regulator of organ size and tumorigenesis is the Hippo pathway. Recent studies have unveiled increasing complexity in regulation of Hippo pathway activity at the level of the oncoprotein Yes-associated protein (YAP). The protein tyrosine phosphatase 14 (PTPN14, known as Pez in Drosophila) was identified as a protein that antagonizes the function of the key Hippo pathway protein YAP by promoting its cytoplasmic localization under high cell density conditions. In Drosophila, Pez was identified as a repressor of epithelial proliferation in vivo. Studies in mammalian cells showed that a family of G protein-coupled receptors, the protease-activated receptors, functioned as activators of YAP. These studies shed light on the intricate regulation of the Hippo pathway and also highlight the importance of investigating these newly discovered regulatory links in physiological and pathological settings to fully appreciate their importance.

The Hippo superhighway: signaling crossroads converging on the Hippo/Yap pathway in stem cells and development

Tissue regeneration is vital to the form and function of an organ. At the core of an organs' ability to self-renew is the stem cell, which maintains homeostasis, and repopulates injured or aged tissue. Tissue damage can dramatically change the dimensions of an organ, and during regeneration, an organ must halt growth once the original tissue dimensions have been restored. Therefore, stem cells must give rise to the appropriate number of differentiated progeny to achieve homeostasis. How this tissue-size checkpoint is regulated and how tissue size information relayed to stem cell compartments is unclear, however, it is likely that these mechanisms are altered during the course of tumorigenesis. An emerging signaling cascade, the Hippo Signaling Pathway, is a broadly conserved potent organ size regulator [1]. However, this pathway does not act alone. A number of examples demonstrate crosstalk between Hippo and other signaling pathways including Wnt, Tgfβ and Notch, with implications for stem cell biology. Here, we focus on these interactions primarily in the context of well characterized stem cell populations.

Autocrine platelet-derived growth factor-vascular endothelial growth factor receptor-related (Pvr) pathway activity controls intestinal stem cell proliferation in the adult Drosophila midgut

A dynamic pool of undifferentiated somatic stem cells proliferate and differentiate to replace dead or dying mature cell types and maintain the integrity and function of adult tissues. Intestinal stem cells (ISCs) in the Drosophila posterior midgut are a well established model to study the complex genetic circuitry that governs stem cell homeostasis. Exposure of the intestinal epithelium to environmental toxins results in the expression of cytokines and growth factors that drive the rapid proliferation and differentiation of ISCs. In the absence of stress signals, ISC homeostasis is maintained through intrinsic pathways. In this study, we uncovered the PDGF- and VEGF-receptor related (Pvr) pathway as an essential regulator of ISC homeostasis under unstressed conditions in the posterior midgut. We found that Pvr is coexpressed with its ligand Pvf2 in ISCs and that hyperactivation of the Pvr pathway distorts the ISC developmental program and drives intestinal dysplasia. In contrast, we show that mutant ISCs in the Pvf/Pvr pathway are defective in homeostatic proliferation and differentiation, resulting in a failure to generate mature cell types. Additionally, we determined that extrinsic stress signals generated by enteropathogenic infection are epistatic to the hypoplasia generated in Pvf/Pvr mutants, making the Pvr pathway unique among all previously studied intrinsic pathways. Our findings illuminate an evolutionarily conserved signal transduction pathway with essential roles in metazoan embryonic development and direct involvement in numerous disease states.

YAP modifies cancer cell sensitivity to EGFR and survivin inhibitors and is negatively regulated by the non-receptor type protein tyrosine phosphatase 14

The Yes-associated protein (YAP) is a transcriptional factor involved in tissue development and tumorigenesis. Although YAP has been recognized as a key element of the Hippo signaling pathway, the mechanisms that regulate YAP activities remain to be fully characterized. In this study, we demonstrate that the non-receptor type protein tyrosine phosphatase 14 (PTPN14) functions as a negative regulator of YAP. We show that YAP forms a protein complex with PTPN14 through the WW domains of YAP and the PPXY motifs of PTPN14. In addition, PTPN14 inhibits YAP-mediated transcriptional activities. Knockdown of YAP sensitizes cancer cells to various anti-cancer agents, such as cisplatin, the EGFR tyrosine kinase inhibitor erlotinib, and the small-molecule antagonist of survivin, S12. YAP-targeted modalities may be used in combination with other cancer drugs to achieve maximal therapeutic effects.

Intestinal stem cell function in Drosophila and mice

Epithelial cells of the digestive tracts of most animals are short-lived, and are constantly replenished by the progeny of long-lived, resident intestinal stem cells. Proper regulation of intestinal stem cell maintenance, proliferation and differentiation is critical for maintaining gut homeostasis. Here we review recent genetic studies of stem cell-mediated homeostatic growth in the Drosophila midgut and the mouse small intestine, highlighting similarities and differences in the mechanisms that control stem cell proliferation and differentiation.

The Hippo pathway regulates stem cell proliferation, self-renewal, and differentiation

Stem cells and progenitor cells are the cells of origin for multi-cellular organisms and organs. They play key roles during development and their dysregulation gives rise to human diseases such as cancer. The recent development of induced pluripotent stem cell (iPSC) technology which converts somatic cells to stem-like cells holds great promise for regenerative medicine. Nevertheless, the understanding of proliferation, differentiation, and self-renewal of stem cells and organ-specific progenitor cells is far from clear. Recently, the Hippo pathway was demonstrated to play important roles in these processes. The Hippo pathway is a newly established signaling pathway with critical functions in limiting organ size and suppressing tumorigenesis. This pathway was first found to inhibit cell proliferation and promote apoptosis, therefore regulating cell number and organ size in both Drosophila and mammals. However, in several organs, disturbance of the pathway leads to specific expansion of the progenitor cell compartment and manipulation of the pathway in embryonic stem cells strongly affects their self-renewal and differentiation. In this review, we summarize current observations on roles of the Hippo pathway in different types of stem cells and discuss how these findings changed our view on the Hippo pathway in organ development and tumorigenesis.

PTPN14 interacts with and negatively regulates the oncogenic function of YAP

The Hippo signaling pathway regulates cellular proliferation and survival, thus exerting profound effects on normal cell fate and tumorigenesis. The pivotal effector of this pathway is YAP, a transcriptional co-activator amplified in mouse and human cancers where it promotes epithelial-to-mesenchymal transition and malignant transformation. Here, we report a novel regulatory mechanism for the YAP oncogenic function via direct interaction with non-receptor tyrosine phosphatase 14 (PTPN14) through the WW domain of YAP and the PPxY domain of PTPN14. We also found that YAP is a direct substrate of PTPN14. In addition, luciferase reporter assay showed that the inhibition of the YAP transcriptional co-activator function by PTPN14 is mediated through their protein interactions and may result from an increase in the inactive cytoplasmic form of YAP. Last, knockdown of PTPN14 induces the nuclear retention of YAP and increases the YAP-dependent cell migration. In summary, our results indicate a potential regulatory role of PTPN14 on YAP and demonstrate a novel mechanism in YAP regulation.