The DHHC palmitoyltransferase approximated regulates Fat signaling and Dachs localization and activity

The deubiquitylating enzyme Fat facets promotes Fat signalling and restricts tissue growth

Tissue growth is regulated by many signals, including polarity cues. The Hippo signalling pathway restricts tissue growth and receives inputs from the planar cell polarity-controlling Fat signalling pathway. The atypical cadherin Fat restricts growth via several mechanisms that ultimately control the activity of the pro-growth transcriptional co-activator Yorkie. Fat signalling activates the Yorkie inhibitory kinase Warts, and modulates the function of the FERM protein Expanded, which promotes Hippo signalling and also directly inhibits Yorkie. Although several Fat pathway activity modulators are known to be involved in ubiquitylation, the role of this post-translational modification in the pathway remains unclear. Moreover, no deubiquitylating enzymes have been described in this pathway. Here, using in vivo RNAi screening, we identify the deubiquitylating enzyme Fat facets as a positive regulator of Fat signalling with roles in tissue growth control. Fat facets interacts genetically and physically with Fat signalling components and regulates Yorkie target gene expression. Thus, we uncover a role for reversible ubiquitylation in the control of Fat signalling and tissue growth regulation.

Dachsous and Fat coordinately repress the Dachs-Dlish-Approximated complex to control growth

Two protocadherins, Dachsous and Fat, regulate organ growth in Drosophila via the Hippo pathway. Dachsous and Fat bind heterotypically to regulate the abundance and subcellular localization of a "core complex" consisting of Dachs, Dlish, and Approximated. This complex localizes to the junctional cortex where it represses Warts. Dachsous is believed to promote growth by recruiting and stabilizing this complex, while Fat represses growth by promoting its degradation. Here, we examine the functional relationships between the intracellular domains of Dachsous and Fat and the core complex. While Dachsous promotes the accumulation of core complex proteins in puncta, it is not required for their assembly. Indeed, the core complex accumulates maximally in the absence of both Dachsous and Fat. Furthermore, Dachsous represses growth in the absence of Fat by removing the core complex from the junctional cortex. Fat similarly recruits core complex components but promotes their degradation. Our findings reveal that Dachsous and Fat coordinately constrain tissue growth by repressing the core complex.

Polar targeting of proteins - a green perspective

Cell polarity - the asymmetric distribution of molecules and cell structures within the cell - is a feature that almost all cells possess. Even though the cytoskeleton and other intracellular organelles can have a direction and guide protein distribution, the plasma membrane is, in many cases, essential for the asymmetric localization of proteins because it helps to concentrate proteins and restrict their localization. Indeed, many proteins that exhibit asymmetric or polarized localization are either embedded in the PM or located close to it in the cellular cortex. Such proteins, which we refer to here as 'polar proteins', use various mechanisms of membrane targeting, including vesicle trafficking, direct phospholipid binding, or membrane anchoring mediated by post-translational modifications or binding to other proteins. These mechanisms are often shared with non-polar proteins, yet the unique combinations of several mechanisms or protein-specific factors assure the asymmetric distribution of polar proteins. Although there is a relatively detailed understanding of polar protein membrane targeting mechanisms in animal and yeast models, knowledge in plants is more fragmented and focused on a limited number of known polar proteins in different contexts. In this Review, we combine the current knowledge of membrane targeting mechanisms and factors for known plant transmembrane and cortical proteins and compare these with the mechanisms elucidated in non-plant systems. We classify the known factors as general or polarity specific, and we highlight areas where more knowledge is needed to construct an understanding of general polar targeting mechanisms in plants or to resolve controversies.

Dachsous and Fat coordinately repress the Dachs-Dlish-Approximated complex to control growth

Two protocadherins, Dachsous (Ds) and Fat (Ft), regulate organ growth in Drosophila via the Hippo pathway. Ds and Ft bind heterotypically to regulate the abundance and subcellular localization of a 'core complex' consisting of Dachs, Dlish and Approximated. This complex localizes to the junctional cortex where it promotes growth by repressing the pathway kinase Warts. Ds is believed to promote growth by recruiting and stabilizing the core complex at the junctional cortex, while Ft represses growth by promoting degradation of core complex components. Here, we examine the functions of intracellular domains of Ds and Ft and their relationship to the core complex. While Ds promotes accumulation of the core complex proteins in cortical puncta, it is not required for core complex assembly. Indeed, the core complex assembles maximally in the absence of both Ds and Ft. Furthermore, while Ds promotes growth in the presence of Ft, it represses growth in the absence of Ft by removing the core complex from the junctional cortex. Ft similarly recruits core complex components, however it normally promotes their degradation. Our findings reveal that Ds and Ft constrain tissue growth by repressing the default 'on' state of the core complex.

Expanded directly binds conserved regions of Fat to restrain growth via the Hippo pathway

The Hippo pathway is a conserved and critical regulator of tissue growth. The FERM protein Expanded is a key signaling hub that promotes activation of the Hippo pathway, thereby inhibiting the transcriptional co-activator Yorkie. Previous work identified the polarity determinant Crumbs as a primary regulator of Expanded. Here, we show that the giant cadherin Fat also regulates Expanded directly and independently of Crumbs. We show that direct binding between Expanded and a highly conserved region of the Fat cytoplasmic domain recruits Expanded to the apicolateral junctional zone and stabilizes Expanded. In vivo deletion of Expanded binding regions in Fat causes loss of apical Expanded and promotes tissue overgrowth. Unexpectedly, we find Fat can bind its ligand Dachsous via interactions of their cytoplasmic domains, in addition to the known extracellular interactions. Importantly, Expanded is stabilized by Fat independently of Dachsous binding. These data provide new mechanistic insights into how Fat regulates Expanded, and how Hippo signaling is regulated during organ growth.

The S-palmitoylome and DHHC-PAT interactome of Drosophila melanogaster S2R+ cells indicate a high degree of conservation to mammalian palmitoylomes

Protein S-palmitoylation, the addition of a long-chain fatty acid to target proteins, is among the most frequent reversible protein modifications in Metazoa, affecting subcellular protein localization, trafficking and protein-protein interactions. S-palmitoylated proteins are abundant in the neuronal system and are associated with neuronal diseases and cancer. Despite the importance of this post-translational modification, it has not been thoroughly studied in the model organism Drosophila melanogaster. Here we present the palmitoylome of Drosophila S2R+ cells, comprising 198 proteins, an estimated 3.5% of expressed genes in these cells. Comparison of orthologs between mammals and Drosophila suggests that S-palmitoylated proteins are more conserved between these distant phyla than non-S-palmitoylated proteins. To identify putative client proteins and interaction partners of the DHHC family of protein acyl-transferases (PATs) we established DHHC-BioID, a proximity biotinylation-based method. In S2R+ cells, ectopic expression of the DHHC-PAT dHip14-BioID in combination with Snap24 or an interaction-deficient Snap24-mutant as a negative control, resulted in biotinylation of Snap24 but not the Snap24-mutant. DHHC-BioID in S2R+ cells using 10 different DHHC-PATs as bait identified 520 putative DHHC-PAT interaction partners of which 48 were S-palmitoylated and are therefore putative DHHC-PAT client proteins. Comparison of putative client protein/DHHC-PAT combinations indicates that CG8314, CG5196, CG5880 and Patsas have a preference for transmembrane proteins, while S-palmitoylated proteins with the Hip14-interaction motif are most enriched by DHHC-BioID variants of approximated and dHip14. Finally, we show that BioID is active in larval and adult Drosophila and that dHip14-BioID rescues dHip14 mutant flies, indicating that DHHC-BioID is non-toxic. In summary we provide the first systematic analysis of a Drosophila palmitoylome. We show that DHHC-BioID is sensitive and specific enough to identify DHHC-PAT client proteins and provide DHHC-PAT assignment for ca. 25% of the S2R+ cell palmitoylome, providing a valuable resource. In addition, we establish DHHC-BioID as a useful concept for the identification of tissue-specific DHHC-PAT interactomes in Drosophila.

Emerging Mechanisms of Growth and Patterning Regulation by Dachsous and Fat Protocadherins

Dachsous (Ds) and Fat are evolutionarily conserved cell adhesion molecules that play a critical role in development of multiple organ systems, where they coordinate tissue growth and morphogenesis. Much of our understanding of Ds-Fat signaling pathway comes from studies in Drosophila, where they initiate a signaling pathway that regulate growth by influencing Hippo signaling and morphogenesis by regulating Planar Cell Polarity (PCP). In this review, we discuss recent advances in our understanding of the mechanisms by which Ds-Fat signaling pathway regulates these critical developmental processes. Further, we discuss the progress in our understanding about how they function in mammals.

How do the Fat-Dachsous and core planar polarity pathways act together and independently to coordinate polarized cell behaviours?

Planar polarity describes the coordinated polarization of cells within the plane of a tissue. This is controlled by two main pathways in Drosophila: the Frizzled-dependent core planar polarity pathway and the Fat–Dachsous pathway. Components of both of these pathways become asymmetrically localized within cells in response to long-range upstream cues, and form intercellular complexes that link polarity between neighbouring cells. This review examines if and when the two pathways are coupled, focusing on the Drosophila wing, eye and abdomen. There is strong evidence that the pathways are molecularly coupled in tissues that express a specific isoform of the core protein Prickle, namely Spiny-legs. However, in other contexts, the linkages between the pathways are indirect. We discuss how the two pathways act together and independently to mediate a diverse range of effects on polarization of cell structures and behaviours.

A palmitoyltransferase Approximated gene Bm-app regulates wing development in Bombyx mori

The silkworm Bombyx mori is an important lepidopteran model insect in which many kinds of natural mutants have been identified. However, molecular mechanisms of most of these mutants remain to be explored. Here we report the identification of a gene Bm-app is responsible for the silkworm minute wing (mw) mutation which exhibits exceedingly small wings during pupal and adult stages. Compared with the wild type silkworm, relative messenger RNA expression of Bm-app is significantly decreased in the u11 mutant strain which shows mw phenotype. A 10 bp insertion in the putative promoter region of the Bm-app gene in mw mutant strain was identified and the dual luciferase assay revealed that this insertion decreased Bm-app promoter activity. Furthermore, clustered regularly interspaced short palindromic repeats/RNA-guided Cas9 nucleases-mediated depletion of the Bm-app induced similar wing defects which appeared in the mw mutant, demonstrating that Bm-app controls wing development in B. mori. Bm-app encodes a palmitoyltransferase and is responsible for the palmitoylation of selected cytoplasmic proteins, indicating that it is required for cell mitosis and growth during wing development. We also discuss the possibility that Bm-app regulates wing development through the Hippo signaling pathway in B. mori.

Oriented Cell Divisions Are Not Required for Drosophila Wing Shape

Formation of correctly shaped organs is vital for normal function. The Drosophila wing has an elongated shape, which has been attributed in part to a preferential orientation of mitotic spindles along the proximal-distal axis [1, 2]. Orientation of mitotic spindles is believed to be a fundamental morphogenetic mechanism in multicellular organisms [3-6]. A contribution of spindle orientation to wing shape was inferred from observations that mutation of Dachsous-Fat pathway genes results in both rounder wings and loss of the normal proximal-distal bias in spindle orientation [1, 2, 7]. To directly evaluate the potential contribution of spindle orientation to wing morphogenesis, we assessed the consequences of loss of the Drosophila NuMA homolog Mud, which interacts with the dynein complex and has a conserved role in spindle orientation [8, 9]. Loss of Mud randomizes spindle orientation but does not alter wing shape. Analysis of growth and cell dynamics in developing discs and in ex vivo culture suggests that the absence of oriented cell divisions is compensated for by an increased contribution of cell rearrangements to wing shape. Our results indicate that oriented cell divisions are not required for wing morphogenesis, nor are they required for the morphogenesis of other Drosophila appendages. Moreover, our results suggest that normal organ shape is not achieved through locally specifying and then summing up individual cell behaviors, like oriented cell division. Instead, wing shape might be specified through tissue-wide stresses that dictate an overall arrangement of cells without specifying the individual cell behaviors needed to achieve it.

Early girl is a novel component of the Fat signaling pathway

The Drosophila protocadherins Dachsous and Fat regulate growth and tissue polarity by modulating the levels, membrane localization and polarity of the atypical myosin Dachs. Localization to the apical junctional membrane is critical for Dachs function, and the adapter protein Vamana/Dlish and palmitoyl transferase Approximated are required for Dachs membrane localization. However, how Dachs levels are regulated is poorly understood. Here we identify the early girl gene as playing an essential role in Fat signaling by limiting the levels of Dachs protein. early girl mutants display overgrowth of the wings and reduced cross vein spacing, hallmark features of mutations affecting Fat signaling. Genetic experiments reveal that it functions in parallel with Fat to regulate Dachs. early girl encodes an E3 ubiquitin ligase, physically interacts with Dachs, and regulates its protein stability. Concomitant loss of early girl and approximated results in accumulation of Dachs and Vamana in cytoplasmic punctae, suggesting that it also regulates their trafficking to the apical membrane. Our findings establish a crucial role for early girl in Fat signaling, involving regulation of Dachs and Vamana, two key downstream effectors of this pathway.

During development, organs grow to achieve a consistent final size. The evolutionarily conserved Hippo signaling network plays a central role in organ size control, and when dysregulated can be associated with cancer and other diseases. Fat signaling is one of several upstream pathways that impinge on Hippo signaling to regulate organ growth. We describe here identification of the Drosophila early girl gene as a new component of the Fat signaling pathway. We show that Early girl controls Fat signaling by regulating the levels of the Dachs protein. However Early girl differs from other Fat signaling regulators in that it doesn’t influence planar cell polarity or control the polarity of Dachs localization. early girl encodes a conserved protein that is predicted to influence protein stability, and it can physically associate with Dachs. We also discovered that Early girl acts together with another protein, called Approximated, to regulate the sub-cellular localization of Dachs and a Dachs-interacting protein called Vamana. Altogether, our observations establish Early girl as an essential component of Fat signaling that acts to regulate the levels and localization of Dachs and Vamana.

Fat-regulated adaptor protein Dlish binds the growth suppressor Expanded and controls its stability and ubiquitination

To regulate the growth and size of organs, cells can use information from their neighbors to modify intracellular mediators of cell proliferation. The intracellular Hippo pathway is a widely utilized nexus for growth control in animals, but its regulation by extracellular signals is not fully understood. We here identify a pathway that regulates organ size in Drosophila, triggered by the transmembrane receptor, the giant protocadherin Fat. We show that the Fat-regulated SH3 domain adaptor protein Dlish binds to and reduces the stability of the growth suppressor Expanded, a known regulator of the Hippo pathway. The destabilization of Expanded by Dlish works in parallel to a previously established pathway in which Dlish increases levels of the growth-stimulating protein Dachs.

The Drosophila protocadherin Fat controls organ size through the Hippo pathway, but the biochemical links to the Hippo pathway components are still poorly defined. We previously identified Dlish, an SH3 domain protein that physically interacts with Fat and the type XX myosin Dachs, and showed that Fat’s regulation of Dlish levels and activity helps limit Dachs-mediated inhibition of Hippo pathway activity. We here characterize a parallel growth control pathway downstream of Fat and Dlish. Using immunoprecipitation and mass spectrometry to search for Dlish partners, we find that Dlish binds the FERM domain growth repressor Expanded (Ex); Dlish SH3 domains directly bind sites in the Ex C terminus. We further show that, in vivo, Dlish reduces the subapical accumulation of Ex, and that loss of Dlish blocks the destabilization of Ex caused by loss of Fat. Moreover, Dlish can bind the F-box E3 ubiquitin ligase Slimb and promote Slimb-mediated ubiquitination of Expanded in vitro. Both the in vitro and in vivo effects of Dlish on Ex require Slimb, strongly suggesting that Dlish destabilizes Ex by helping recruit Slimb-containing E3 ubiquitin ligase complexes to Ex.

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.

Reduced methylation downregulates CD39/ENTPD1 and ZDHHC14 to suppress trophoblast cell proliferation and invasion: Implications in preeclampsia

Preeclampsia (PE) is a pregnancy-specific syndrome affecting up to 8% of pregnancies worldwide. While PE is a leading cause of maternal and neonatal mortality and morbidity, the pathophysiology of PE is unclear to date. Here, we have verified that dysregulation of CD39/ectonucleoside triphosphate diphosphohydrolase-1 (ENTPD1) and zinc finger DHHC-type containing 14 (ZDHHC14) via DNA methylation plays a vital role in late-onset PE pathology. Our study confirmed the differentially methylated regions (DMRs) of the CD39 and ZDHHC14 gene bodies that we found previously in clinical samples of preeclamptic placentas by MassARRAY EpiTYPER. Then, we showed that CD39 and ZDHHC14 were restricted to the syncytiotrophoblast of the full-term human placenta and that their gene expression levels were significantly decreased in the late-onset preeclamptic placenta. Because DNA methylation can affect gene expression, treatment of trophoblast cell lines (BeWo and JEG-3) with 5-Aza-2'-deoxycytidine (5-Aza-dC) was performed to deplete global DNA methylation in vitro. Then, we found that gene expression of CD39 and ZDHHC14 was decreased and that secretion of CD39 was also markedly downregulated in the hypomethylated trophoblast cell lines. Moreover, siRNA-mediated knockdown of CD39 or ZDHHC14 significantly inhibited trophoblast cell proliferation and invasion. Collectively, our study shows that downregulation of CD39 and ZDHHC14 via hypomethylation is relevant to late-onset PE through the effects of these genes on trophoblast cell lines. Hence, CD39 and ZDHHC14 may act as potential markers and targets for the clinical diagnosis and treatment of PE.

Correlated S-palmitoylation profiling of Snail-induced epithelial to mesenchymal transition

Epithelial cells form spatially-organized adhesion complexes that establish polarity gradients, regulate cell proliferation, and direct wound healing. As cells accumulate oncogenic mutations, these key tumor suppression mechanisms are disrupted, eliminating many adhesion complexes and bypassing contact inhibition. The transcription factor Snail is often expressed in malignant cancers, where it promotes transcriptional reprogramming to drive epithelial-mesenchymal transition (EMT) and establishes a more invasive state. S-Palmitoylation describes the fatty-acyl post-translational modification of cysteine residues in proteins, and is required for membrane anchoring, trafficking, localization and function of hundreds of proteins involved in cell growth, polarity, and signaling. Since Snail-expression disrupts apico-basolateral cell polarity, we asked if Snail-dependent transformation induces proteome-wide changes in S-palmitoylation. MCF10A breast cancer cells were retrovirally transduced with Snail and correlated proteome-wide changes in protein abundance and S-palmitoylation were profiled by using stable isotope labeling in cell culture with amino acid (SILAC) mass spectrometry. This analysis identified increased levels of proteins involved in migration, glycolysis, and cell junction remodeling, and decreased levels of proteins involved in cell adhesion. Overall, protein S-palmitoylation is highly correlated with protein abundance, yet for a subset of proteins, this correlation is uncoupled. These findings suggest that Snail-overexpression affects the S-palmitoylation cycle of some proteins, which may participate in cell polarity and tumor suppression.

Want to promote tissue growth? There’s an App for that!

Study describes how a palmitoyltransferase regulates the Hippo pathway in flies.

The palmitoyltransferase Approximated promotes growth via the Hippo pathway by palmitoylation of Fat

The palmitoyl transferase Approximated regulates signaling by the protocadherin Fat to control tissue growth upstream of the Hippo pathway in Drosophila. Matakatsu et al. show that palmitoylation of the intracellular domain of Fat by Approximated negatively regulates Fat and its ability to restrict growth.

The large protocadherin Fat functions to promote Hippo pathway activity in restricting tissue growth. Loss of Fat leads to accumulation of the atypical myosin Dachs at the apical junctional region, which in turn promotes growth by inhibiting Warts. We previously identified Approximated (App), a DHHC domain palmitoyltransferase, as a negative regulator of Fat signaling in growth control. We show here that App promotes growth by palmitoylating the intracellular domain of Fat, and that palmitoylation negatively regulates Fat function. Independently, App also recruits Dachs to the apical junctional region through protein–protein association, thereby stimulating Dachs’s activity in promoting growth. Further, we show that palmitoylation by App functions antagonistically to phosphorylation by Discs-overgrown, which activates Fat. Together, these findings suggest a model in which App promotes Dachs activity by simultaneously repressing Fat via posttranslational modification and recruiting Dachs to the apical junctional region, thereby promoting tissue growth.

Vamana Couples Fat Signaling to the Hippo Pathway

The protocadherins Dachsous and Fat initiate a signaling pathway that controls growth and planar cell polarity by regulating the membrane localization of the atypical myosin Dachs. How Dachs is regulated by Fat signaling has remained unclear. Here we identify the vamana gene as playing a crucial role in regulating membrane localization of Dachs and in linking Fat and Dachsous to Dachs regulation. Vamana, an SH3-domain-containing protein, physically associates with and co-localizes with Dachs and promotes its membrane localization. Vamana also associates with the Dachsous intracellular domain and with a region of the Fat intracellular domain that is essential for controlling Hippo signaling and levels of Dachs. Epistasis experiments, structure-function analysis, and physical interaction experiments argue that Fat negatively regulates Dachs in a Vamana-dependent process. Our findings establish Vamana as a crucial component of the Dachsous-Fat pathway that transmits Fat signaling by regulating Dachs.

The novel SH3 domain protein Dlish/CG10933 mediates fat signaling in Drosophila by binding and regulating Dachs

Much of the Hippo and planar cell polarity (PCP) signaling mediated by the Drosophila protocadherin Fat depends on its ability to change the subcellular localization, levels and activity of the unconventional myosin Dachs. To better understand this process, we have performed a structure-function analysis of Dachs, and used this to identify a novel and important mediator of Fat and Dachs activities, a Dachs-binding SH3 protein we have named Dlish. We found that Dlish is regulated by Fat and Dachs, that Dlish also binds Fat and the Dachs regulator Approximated, and that Dlish is required for Dachs localization, levels and activity in both wild type and fat mutant tissue. Our evidence supports dual roles for Dlish. Dlish tethers Dachs to the subapical cell cortex, an effect partly mediated by the palmitoyltransferase Approximated under the control of Fat. Conversely, Dlish promotes the Fat-mediated degradation of Dachs.

DOI:http://dx.doi.org/10.7554/eLife.16624.001

Modulation of junction tension by tumor-suppressors and proto-oncogenes regulates cell-cell contacts

Tumor-suppressor and proto-oncogenes play critical roles in tissue proliferation. Furthermore, deregulation of their functions is deleterious to tissue architecture and can result in the sorting of somatic rounded clones minimizing their contact with surrounding wild-type (wt) cells. Defects in somatic clones shape correlate with defects in proliferation, cell affinity, cell-cell adhesion, oriented cell division and cortical elasticity. Combining genetics, live-imaging, laser ablation and computer simulations, we aim to analyze whether distinct or similar mechanisms can account for the common role of tumor-suppressor and proto-oncogenes in cell-cell contact regulation. In Drosophila epithelia, Fat (Ft) and Dachsous (Ds) tumor-suppressors regulate cell proliferation, tissue morphogenesis, planar cell polarity and junction tension. By analyzing the time evolution of ft mutant cells and clones, we show that ft clones reduce their cell-cell contact with surrounding wt tissue in the absence of concomitant cell divisions and over-proliferation. This contact reduction depends on opposite changes of junction tensions in the clone bulk and its boundary with neighboring wt tissue. More generally, either clone bulk or boundary junction tensions is modulated by the activation of Yorkie, Myc and Ras yielding similar contact reductions with wt cells. Together our data highlight mechanical roles for proto-oncogene and tumor-suppressor pathways in cell-cell interactions.

Coordination of planar cell polarity pathways through Spiny-legs

Morphogenesis and physiology of tissues and organs requires planar cell polarity (PCP) systems that orient and coordinate cells and their behaviors, but the relationship between PCP systems has been controversial. We have characterized how the Frizzled and Dachsous-Fat PCP systems are connected through the Spiny-legs isoform of the Prickle-Spiny-legs locus. Two different components of the Dachsous-Fat system, Dachsous and Dachs, can each independently interact with Spiny-legs and direct its localization in vivo. Through characterization of the contributions of Prickle, Spiny-legs, Dachsous, Fat, and Dachs to PCP in the Drosophila wing, eye, and abdomen, we define where Dachs-Spiny-legs and Dachsous-Spiny-legs interactions contribute to PCP, and provide a new understanding of the orientation of polarity and the basis of PCP phenotypes. Our results support the direct linkage of PCP systems through Sple in specific locales, while emphasizing that cells can be subject to and must ultimately resolve distinct, competing PCP signals.

DOI:http://dx.doi.org/10.7554/eLife.09946.001

Animals have many asymmetric organs. Wings, for example, are aerodynamically shaped and have a clear front, back, top and bottom, and even additions to these organs, such as feathers on the wing, often need to be oriented in a specific manner.

This kind of orientation arises when cells divide and grow asymmetrically in a flat plane. The asymmetry is established at the level of single cells when proteins are not equally spread throughout a cell, but rather asymmetrically distributed. Such cells are said to be ‘planar polarized’; and many experiments addressing this so-called planar cell polarity have been conducted in fruit flies, because they can be genetically altered easily. Previous studies have shown that two signaling pathways—called Frizzled and Dachsous-Fat—regulate how individual cells orient themselves within a flat sheet of cells that forms fruit fly’s wing. The two pathways are not independent, but it is unclear how they are linked. In particular, there has been conflicting evidence as to whether the Dachsous-Fat pathway controls the Frizzled pathway or whether the two act in parallel.

Now, Ambegaonkar and Irvine have discovered new roles for a protein that is involved in both pathways, called 'Spiny-legs'. This protein was known to be important in the Frizzled pathway, but, when it was tracked with a fluorescent tag in developing wing cells it also accumulated in areas where two proteins that make up part of the Dachsous-Fat pathway were located. Biochemical experiments showed that both of these proteins (which are called Dachs or Dachsous) could physically interact with Spiny-legs. Ambegaonkar and Irvine therefore deleted the genes for Dachs or Dachsous in fruit flies and observed that Spiny-legs no longer organized itself in the proper way, implying that Dachs and Dachsous control where Spiny-legs goes within cells.

When this analysis was extended to other fruit fly organs, such as the eyes, Ambegaonkar and Irvine found that Dachsous was more important than Dachs for the correct localization of Spiny-legs. Additionally, the Frizzled and Dachsous-Fat pathways seemed to compete for interactions with Spiny-legs. This connection between the two pathways helps to explain how cells behave when several different signals reach them. It also shows how different organs can reuse conserved components of the pathways to make different end products. Future studies should aim to work out the number of systems that polarize cells and how they are connected in different tissues.

DOI:http://dx.doi.org/10.7554/eLife.09946.002

Mechanisms of planar cell polarity establishment in Drosophila

Correct patterning and polarization of epithelial and mesenchymal cells are essential for morphogenesis and function of all organs and organisms. Epithelial cells are generally polarized in two axes: (a) the ubiquitous apical-basal axis and (b) polarity within the plane of the epithelium. The latter is generally referred to as planar cell polarity (PCP) and also is found in several contexts of mesenchymal cell patterning. In Drosophila, all adult structures display PCP features, and two conserved molecular systems (the Fat [Ft]/Dachsous [Ds] system and the Frizzled [Fz]/PCP pathway) that regulate this process have been identified. Although significant progress has been made in dissecting aspects of PCP signaling within cells, much remains to be discovered about the mechanisms of long-range and local PCP cell-cell interactions. Here, we discuss the current models based on Drosophila studies and incorporate recent insights into this long-standing cell and developmental biology problem.

Drosophila C-terminal Src kinase regulates growth via the Hippo signaling pathway

The Hippo signaling pathway is involved in regulating tissue size by inhibiting cell proliferation and promoting apoptosis. Aberrant Hippo pathway function is often detected in human cancers and correlates with poor prognosis. The Drosophila C-terminal Src kinase (d-Csk) is a genetic modifier of warts (wts), a tumor-suppressor gene in the Hippo pathway, and interacts with the Src oncogene. Reduction in d-Csk expression and the consequent activation of Src are frequently seen in several cancers including hepatocellular and colorectal tumors. Previous studies show that d-Csk regulates cell proliferation and tissue size during development. Given the similarity in the loss-of-function phenotypes of d-Csk and wts, we have investigated the interactions of d-Csk with the Hippo pathway. Here we present multiple lines of evidence suggesting that d-Csk regulates growth via the Hippo signaling pathway. We show that loss of dCsk caused increased Yki activity, and our genetic epistasis places dCsk downstream of Dachs. Furthermore, dCsk requires Yki for its growth regulatory functions, suggesting that dCsk is another upstream member of the network of genes that interact to regulate Wts and its effector Yki in the Hippo signaling pathway.

The Drosophila F-box protein Fbxl7 binds to the protocadherin fat and regulates Dachs localization and Hippo signaling

The Drosophila protocadherin Fat (Ft) regulates growth, planar cell polarity (PCP) and proximodistal patterning. A key downstream component of Ft signaling is the atypical myosin Dachs (D). Multiple regions of the intracellular domain of Ft have been implicated in regulating growth and PCP but how Ft regulates D is not known. Mutations in Fbxl7, which encodes an F-box protein, result in tissue overgrowth and abnormalities in proximodistal patterning that phenocopy deleting a specific portion of the intracellular domain (ICD) of Ft that regulates both growth and PCP. Fbxl7 binds to this same portion of the Ft ICD, co-localizes with Ft to the proximal edge of cells and regulates the levels and asymmetry of D at the apical membrane. Fbxl7 can also regulate the trafficking of proteins between the apical membrane and intracellular vesicles. Thus Fbxl7 functions in a subset of pathways downstream of Ft and links Ft to D localization.

DOI:http://dx.doi.org/10.7554/eLife.03383.001

Multi-cellular organisms are made up of cells that are organized into tissues and organs that reach a predictable size and shape at the end of their development. To do this, cells must be able to sense their position and orientation within the body and know when to stop growing.

Epithelial cells—which make up the outer surface of an animal's body and line the cavities of its internal organs—connect to each other to form flat sheets. These sheets of cells contain structures that are oriented along the plane of the sheet. However, how this so-called ‘planar cell polarity’ coordinates with cell growth in order to build complex tissues and organs remains to be discovered.

A protein called Fat is a major player in both planar cell polarity and the Hippo signaling pathway, which controls cell growth. As such, the Fat protein appears to be crucial for controlling the size and shape of organs. Mutations in the Fat protein cause massive tissue overgrowth, prevent planar cell polarity being established correctly, and stop the legs and wings of fruit flies developing normally.

The Fat protein also plays a role in distributing another protein called Dachs—which is also part of the Hippo signaling pathway. In epithelial cells of the developing wing, Dachs is mostly located on the side of the cell that is closest to the tip of the developing wing (the so-called ‘distal surface’). How Fat and Dachs work together is not understood, but it is known that they do not bind to each other directly.

Now, Bosch et al. show that in the fruit fly Drosophila, the Fat protein binds to another protein called Fbxl7. Flies that cannot produce working Fbxl7 have defects in some aspects of planar cell polarity and a modest increase in tissue growth. Fbxl7 seems to account for part, but not all, of the ability of Fat to restrict tissue growth. Furthermore, a lack of the Fbxl7 protein results in a spreading of Dachs protein across the apical surface—which faces out of the epithelial sheet—of epithelial cells. On the other hand, if Fbxl7 is over-expressed, Dachs is driven to the interior of each cell. Hence, a normal level of Fbxl7 protein restricts the Dachs protein to the correct parts of the cell surface.

Together, the findings of Bosch et al. show that the Fbxl7 protein is a key link between the Fat and Dachs proteins. These results also provide an understanding of how growth and planar cell polarity—two processes that are essential for normal development of all multi-cellular organisms—are coordinated.

DOI:http://dx.doi.org/10.7554/eLife.03383.002

Wnt-signaling and planar cell polarity genes regulate axon guidance along the anteroposterior axis in C. elegans

During the development of the nervous system, neurons encounter signals that inform their outgrowth and polarization. Understanding how these signals combinatorially function to pattern the nervous system is of considerable interest to developmental neurobiologists. The Wnt ligands and their receptors have been well characterized in polarizing cells during asymmetric cell division. The planar cell polarity (PCP) pathway is also critical for cell polarization in the plane of an epithelium. The core set of PCP genes include members of the conserved Wnt-signaling pathway, such as Frizzled and Disheveled, but also the cadherin-domain protein Flamingo. In Drosophila, the Fat and Dachsous cadherins also function in PCP, but in parallel to the core PCP components. C. elegans also have two Fat-like and one Dachsous-like cadherins, at least one of which, cdh-4, contributes to neural development. In C. elegans Wnt ligands and the conserved PCP genes have been shown to regulate a number of different events, including embryonic cell polarity, vulval morphogenesis, and cell migration. As is also observed in vertebrates, the Wnt and PCP genes appear to function to primarily provide information about the anterior to posterior axis of development. Here, we review the recent work describing how mutations in the Wnt and core PCP genes affect axon guidance and synaptogenesis in C. elegans.

Dachsous-dependent asymmetric localization of spiny-legs determines planar cell polarity orientation in Drosophila

In Drosophila, planar cell polarity (PCP) molecules such as Dachsous (Ds) may function as global directional cues directing the asymmetrical localization of PCP core proteins such as Frizzled (Fz). However, the relationship between Ds asymmetry and Fz localization in the eye is opposite to that in the wing, thereby causing controversy regarding how these two systems are connected. Here, we show that this relationship is determined by the ratio of two Prickle (Pk) isoforms, Pk and Spiny-legs (Sple). Pk and Sple form different complexes with distinct subcellular localizations. When the amount of Sple is increased in the wing, Sple induces a reversal of PCP using the Ds-Ft system. A mathematical model demonstrates that Sple is the key regulator connecting Ds and the core proteins. Our model explains the previously noted discrepancies in terms of the differing relative amounts of Sple in the eye and wing.

Overexpression of ZDHHC14 promotes migration and invasion of scirrhous type gastric cancer

Scirrhous type gastric cancer is highly aggressive and has a poorer prognosis than many other types of gastric carcinoma, due to its characteristic rapid cancer cell infiltration and proliferation, extensive stromal fibrosis, and frequent peritoneal dissemination. The aim of the present study was to identify novel prognostic markers or therapeutic targets for scirrhous type gastric cancer. We reviewed a list of genes with upregulated expression in scirrhous type gastric cancer and compared their expression with that in normal stomach from our previous Escherichia coli (E. coli) ampicillin secretion-trap (CAST) analysis. We focused on the ZDHHC14 gene, which encodes zinc finger, DHHC-type containing 14 protein. qRT-PCR analysis of ZDHHC14 in 41 gastric cancer cases revealed that compared to mRNA levels in normal non-neoplastic gastric mucosa, ZDHHC14 mRNA was overexpressed in 27% of gastric cancer tissue samples. The overexpression of ZDHHC14 was significantly associated with depth of tumor invasion, undifferentiated histology and scirrhous pattern. The invasiveness of ZDHHC14-knockdown HSC-44PE and 44As3 gastric cancer cells was decreased in comparison with that of the negative control siRNA-transfected cells, together with downregulation of MMP-17 mRNA. Integrins α5 and β1 were also downregulated in ZDHHC14-knockdown 44As3 cells. Forced expression of ZDHHC14 activated gastric cancer cell migration and invasion in vitro. These results indicate that ZDHHC14 is involved in tumor progression in patients with scirrhous type gastric cancer.

Planar cell polarity signaling: coordination of cellular orientation across tissues

Establishment of Planar Cell Polarity (PCP) in epithelia, in the plane of an epithelium, is an important feature of the development and homeostasis of most organs. Studies in different model organisms have contributed a wealth of information regarding the mechanisms that govern PCP regulation. Genetic studies in Drosophila have identified two signaling systems, the Fz/PCP and Fat/Dachsous system, which are both required for PCP establishment in many different tissues in a largely non-redundant manner. Recent advances in vertebrate PCP studies have added novel factors of PCP regulation and also new cellular features requiring PCP-signaling input, including the positioning and orientation of the primary cilium of many epithelial cells. This review focuses mostly on several recent advances made in the Drosophila and vertebrate PCP field and integrates these within the existing PCP-signaling framework.

Hippo gains weight: added insights and complexity to pathway control

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

The Drosophila selectin furrowed mediates intercellular planar cell polarity interactions via frizzled stabilization

Establishment of planar cell polarity (PCP) in a tissue requires coordination of directional signals from cell to cell. It is thought that this is mediated by the core PCP factors, which include cell-adhesion molecules. Here, we demonstrate that furrowed, the Drosophila selectin, is required for PCP generation. Disruption of PCP in furrowed-deficient flies results from a primary defect in Fz levels and cell adhesion. Furrowed localizes at or near apical junctions, largely colocalizing with Frizzled and Flamingo (Fmi). It physically interacts with and stabilizes Frizzled, and it mediates intercellular Frizzled-Van Gogh (Vang)/Strabismus interactions, similarly to Fmi. Furrowed does so through a homophilic cell-adhesion role that is distinct from its known carbohydrate-binding function described during vertebrate blood-cell/endothelial cell interactions. Importantly, the carbohydrate function is dispensable for PCP establishment. In vivo studies suggest that Furrowed functions partially redundantly with Fmi, mediating intercellular Frizzled-Vang interactions between neighboring cells.

Riquiqui and minibrain are regulators of the hippo pathway downstream of Dachsous

The atypical cadherins Fat (Ft) and Dachsous (Ds) control tissue growth through the Salvador-Warts-Hippo (SWH) pathway, and also regulate planar cell polarity and morphogenesis. Ft and Ds engage in reciprocal signalling as both proteins can serve as receptor and ligand for each other. The intracellular domains (ICDs) of Ft and Ds regulate the activity of the key SWH pathway transcriptional co-activator protein Yorkie (Yki). Signalling from the FtICD is well characterized and controls tissue growth by regulating the abundance of the Yki-repressive kinase Warts (Wts). Here we identify two regulators of the Drosophila melanogaster SWH pathway that function downstream of the DsICD: the WD40 repeat protein Riquiqui (Riq) and the DYRK-family kinase Minibrain (Mnb). Ds physically interacts with Riq, which binds to both Mnb and Wts. Riq and Mnb promote Yki-dependent tissue growth by stimulating phosphorylation-dependent inhibition of Wts. Thus, we describe a previously unknown branch of the SWH pathway that controls tissue growth downstream of Ds.

The Drosophila Cadherin Fat regulates tissue size and planar cell polarity through different domains

The Drosophila Cadherin Fat (Ft) has been identified as a crucial regulator of tissue size and Planar Cell Polarity (PCP). However, the precise mechanism by which Ft regulates these processes remains unclear. In order to advance our understanding of the action of Ft, we have sought to identify the crucial Ft effector domains. Here we report that a small region of the Ft cytoplasmic domain (H2 region) is both necessary and sufficient, when membrane localized, to support viability and prevent tissue overgrowth. Interestingly, the H2 region is dispensable for regulating PCP signaling, whereas the mutant Ft lacking the H2 region is fully capable of directing PCP. This result suggests that Ft's roles in PCP signaling and tissue size control are separable, and each can be carried out independently. Surprisingly, the crucial regions of Ft identified in our structure-function study do not overlap with the previously reported interaction regions with Atrophin, Dco, or Lowfat.

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

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

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.

Fat chance! Getting a grip on a slippery modification

Protein palmitoylation describes the post-translational fatty acyl thioesterification of cellular cysteine residues and is critical for the localization, trafficking, and compartmentalization of a large number of membrane proteins. This labile thioester modification facilitates a dynamic acylation cycle that directionally traffics key signaling complexes, receptors, and channels to select membrane compartments. Chemical enrichment and advanced mass spectrometry-based proteomics methods have highlighted a pervasive role for palmitoylation across all eukaryotes, including animals, plants, and parasites. Emerging chemical tools promise to open new avenues to study the enzymes, substrates, and dynamics of this distinct post-translational modification.

Fat and Dachsous cadherins

Fat and Dachsous (Ds) are very large cell adhesion molecules. They bind each other and have important, highly conserved roles in planar cell polarity (PCP) and growth control. PCP is defined as the directionally coordinated development of cellular structures or behavior. Cellular and tissue growth needs to be modulated in terms of rate and final size, and the Hippo pathway regulates growth in a variety of developmental contexts. Fat and Ds are important upstream regulators of these pathways. There are two Fat proteins in Drosophila, Fat and Fat2, and four in vertebrates, Fat1-4. There is one Ds protein in Drosophila and two in vertebrates, Dachsous1-2. In this chapter, we discuss the roles of Fat and Ds family members, focusing on Drosophila and mouse development.

Hippo and rassf1a Pathways: A Growing Affair

First discovered in Drosophila, the Hippo pathway regulates the size and shape of organ development. Its discovery and study have helped to address longstanding questions in developmental biology. Central to this pathway is a kinase cascade leading from the tumor suppressor Hippo (Mst1 and Mst2 in mammals) to the Yki protein (YAP and TAZ in mammals), a transcriptional coactivator of target genes involved in cell proliferation, survival, and apoptosis. A dysfunction of the Hippo pathway activity is frequently detected in human cancers. Recent studies have highlighted that the Hippo pathway may play an important role in tissue homoeostasis through the regulation of stem cells, cell differentiation, and tissue regeneration. Recently, the impact of RASSF proteins on Hippo signaling potentiating its proapoptotic activity has been addressed, thus, providing further evidence for Hippo's key role in mammalian tumorigenesis as well as other important diseases.

Delineation of the interstitial 6q25 microdeletion syndrome: refinement of the critical causative region

Interstitial deletions of the long arm of chromosome 6 are rare. Clinically, this is a recognizable microdeletion syndrome associated with intellectual disability (ID), acquired microcephaly, typical dysmorphic features, structural anomalies of the brain, and nonspecific multiple organ anomalies. Most of the reported cases have cytogenetically visible interstitial deletions or subtelomeric microdeletions. We report on a boy with global developmental delay, distinct dysmorphic features, dysgenesis of the corpus callosum, limb anomalies, and genital hypoplasia who has a small interstitial deletion of the long arm of chromosome 6 detected by comparative genomic hybridization (CGH). The deleted region spans around 1 Mb of DNA and contains only two coding genes, ARID1B and ZDHHC14. To the best of our knowledge, this case represents the typical phenotype with the smallest deletion reported so far. We discuss the possible role of these genes in the phenotypic manifestations.

Mechanical control of morphogenesis by Fat/Dachsous/Four-jointed planar cell polarity pathway

During animal development, several planar cell polarity (PCP) pathways control tissue shape by coordinating collective cell behavior. Here, we characterize by means of multiscale imaging epithelium morphogenesis in the Drosophila dorsal thorax and show how the Fat/Dachsous/Four-jointed PCP pathway controls morphogenesis. We found that the proto-cadherin Dachsous is polarized within a domain of its tissue-wide expression gradient. Furthermore, Dachsous polarizes the myosin Dachs, which in turn promotes anisotropy of junction tension. By combining physical modeling with quantitative image analyses, we determined that this tension anisotropy defines the pattern of local tissue contraction that contributes to shaping the epithelium mainly via oriented cell rearrangements. Our results establish how tissue planar polarization coordinates the local changes of cell mechanical properties to control tissue morphogenesis.

Planar polarity specification through asymmetric subcellular localization of Fat and Dachsous

Two pathways regulate planar polarity: the core proteins and the Fat-Dachsous-Four-jointed (Ft-Ds-Fj) system. Morphogens specify complementary expression patterns of Ds and Fj that potentially act as polarizing cues. It has been suggested that Ft-Ds-Fj-mediated cues are weak and that the core proteins amplify them. Another view is that the two pathways act independently to generate and propagate polarity: if correct, this raises the question of how gradients of Ft and Ds expression or activity might be interpreted to provide strong cellular polarizing cues and how such cues are propagated from cell to cell. Here, we demonstrate that the complementary expression of Ds and Fj results in biased Ft and Ds protein distribution across cells, with Ft and Ds accumulating on opposite edges. Furthermore, boundaries of Ft and Ds expression result in subcellular asymmetries in protein distribution that are transmitted to neighboring cells, and asymmetric Ds localization results in a corresponding asymmetric distribution of the myosin Dachs. We show that the generation of subcellular asymmetries of Ft and Ds and the core proteins is largely independent in the wing disc and additionally that ommatidial polarity in the eye can be determined without input from the Ft-Ds-Fj system, consistent with the two pathways acting in parallel.

Separating planar cell polarity and Hippo pathway activities of the protocadherins Fat and Dachsous

The giant Drosophila protocadherin Fat (Ft) affects planar cell polarity (PCP). Ft also inhibits the overgrowth of imaginal discs via the Hippo pathway, repressing the activity of the transcription co-factor Yorkie (Yki). Much of Ft activity is likely to be mediated by its intracellular domain (Ft ICD). However, the links between the Ft ICD and either PCP or Hippo activity are poorly understood, and the role of the Hippo pathway in PCP is ambiguous. We have performed a structure-function analysis of the Ft ICD. We found that the effects of the Ft ICD on PCP and the Hippo pathway are largely separable. Surprisingly, the domains required for PCP and Hippo activities do not map to any of the previously identified protein interaction domains, nor, with one exception, to the regions that are highly conserved in mammalian Fat4. We also found that the extracellular domain of Ft can act independently of the Ft ICD in PCP and can trigger dominant-negative and boundary effects on Hippo activity, probably via binding to the protocadherin Dachsous.

The frizzled/stan pathway and planar cell polarity in the Drosophila wing

Drosophila has been the key model system for studies on planar cell polarity (PCP). The rich morphology of the insect exoskeleton contains many structures that display PCP. Among these are the trichomes (cuticular hairs) that cover much of the exoskeleton, sensory bristles, and ommatidia. Many genes have been identified that must function for the development of normal PCP. Among these are the genes that comprise the frizzled/starry night (fz/stan) and dachsous/fat pathways. The mechanisms that underlie the function of the fz/stan pathway are best understood. All of the protein products of these genes accumulate asymmetrically in wing cells and there is good evidence that this involves local intercellular signaling between protein complexes on the distal edge of one cell and the juxtaposed proximal edge of its neighbor. It is thought that a feedback system, directed transport, and stabilizing protein-protein interactions mediate the formation of distal and proximal protein complexes. These complexes appear to recruit downstream proteins that function to spatially restrict the activation of the cytoskeleton in wing cells. This leads to the formation of the array of distally pointing hairs found on wings.

The roles of the cadherins Fat and Dachsous in planar polarity specification in Drosophila

Planar polarity is generated through the activity of two groups of proteins, the "core" system and the Fat (Ft)/Dachsous (Ds) system. Although both are conserved from insects to mammals, vertebrate studies into planar polarity have primarily focussed on core planar polarity proteins and have only recently branched into the study of the Ft/Ds system. In Drosophila, however, years of detailed analysis have started to elucidate some of the mechanisms by which Ft/Ds signalling might set up polarity across a tissue, and how this may impact upon core protein-mediated planar polarity. In this review, we discuss the major findings, models, and controversies that have emerged from Drosophila research into the Ft/Ds system, and indicate some areas for further investigation.

Hippo signaling in Drosophila: recent advances and insights

The Hippo signaling pathway emerged from studies of Drosophila tumor suppressor genes, and is now appreciated as a major growth control pathway in vertebrates as well as arthropods. As a recently discovered pathway, key components of the pathway are continually being identified, and new insights into how the pathway is regulated and deployed are arising at a rapid pace. Over the past year and a half, significant advances have been made in our understanding of upstream regulatory inputs into Hippo signaling, key negative regulators of Hippo pathway activity have been identified, and important roles for the pathway in regeneration have been described. This review describes these and other advances, focusing on recent progress in our understanding of Hippo signaling that has come from continued studies in Drosophila.

Zyxin links fat signaling to the hippo pathway

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