Differences in levels of the transmembrane protein Crumbs can influence cell survival at clonal boundaries

The cell adhesion molecule Echinoid promotes tissue survival and separately restricts tissue overgrowth in Drosophila imaginal discs

The interactions that cells in Drosophila imaginal discs have with their neighbors are known to regulate their ability to survive. In a screen of genes encoding cell surface proteins for gene knockdowns that affect the size or shape of mutant clones, we found that clones of cells with reduced levels of echinoid (ed) are fewer, smaller, and can be eliminated during development. In contrast, discs composed mostly of ed mutant tissue are overgrown. We find that ed mutant tissue has lower levels of the anti-apoptotic protein Diap1 and has increased levels of apoptosis which is consistent with the observed underrepresentation of ed mutant clones and the slow growth of ed mutant tissue. The eventual overgrowth of ed mutant tissue results not from accelerated growth, but from prolonged growth resulting from a failure to arrest growth at the appropriate final size. Ed has previously been shown to physically interact with multiple Hippo-pathway components and it has been proposed to promote Hippo pathway signaling, to exclude Yorkie (Yki) from the nucleus, and restrain the expression of Yki-target genes. We did not observe changes in Yki localization in ed mutant tissue and found decreased levels of expression of several Yorkie-target genes, findings inconsistent with the proposed effect of Ed on Yki. We did, however, observe increased expression of several Yki-target genes in wild-type cells neighboring ed mutant cells, which may contribute to elimination of ed mutant clones. Thus, ed has two distinct functions: an anti-apoptotic function by maintaining Diap1 levels, and a function to arrest growth at the appropriate final size. Both of these are unlikely to be explained by a simple effect on the Hippo pathway.

Cell polarity and extrusion: How to polarize extrusion and extrude misspolarized cells?

The barrier function of epithelia is one of the cornerstones of the body plan organization of metazoans. It relies on the polarity of epithelial cells which organizes along the apico-basal axis the mechanical properties, signaling as well as transport. This barrier function is however constantly challenged by the fast turnover of epithelia occurring during morphogenesis or adult tissue homeostasis. Yet, the sealing property of the tissue can be maintained thanks to cell extrusion: a series of remodeling steps involving the dying cell and its neighbors leading to seamless cell expulsion. Alternatively, the tissue architecture can also be challenged by local damages or the emergence of mutant cells that may alter its organization. This includes mutants of the polarity complexes which can generate neoplastic overgrowths or be eliminated by cell competition when surrounded by wild type cells. In this review, we will provide an overview of the regulation of cell extrusion in various tissues focusing on the relationship between cell polarity, cell organization and the direction of cell expulsion. We will then describe how local perturbations of polarity can also trigger cell elimination either by apoptosis or by cell exclusion, focusing specifically on how polarity defects can be directly causal to cell elimination. Overall, we propose a general framework connecting the influence of polarity on cell extrusion and its contribution to aberrant cell elimination.

Shaggy regulates tissue growth through Hippo pathway in Drosophila

The evolutionarily conserved Hippo pathway coordinates cell proliferation, differentiation and apoptosis to regulate organ growth and tumorigenesis. Hippo signaling activity is tightly controlled by various upstream signals including growth factors and cell polarity, but the full extent to which the pathway is regulated during development remains to be resolved. Here, we report the identification of Shaggy, the homolog of mammalian Gsk3β, as a novel regulator of the Hippo pathway in Drosophila. Our results show that Shaggy promotes the expression of Hippo target genes in a manner that is dependent on its kinase activity. Loss of Shaggy leads to Yorkie inhibition and downregulation of Hippo pathway target genes. Mechanistically, Shaggy acts upstream of the Hippo pathway and negatively regulates the abundance of the FERM domain containing adaptor protein Expanded. Our results reveal that Shaggy is functionally required for Crumbs/Slmb-mediated downregulation of Expanded in vivo, providing a potential molecular link between cellular architecture and the Hippo signaling pathway.

Apical-basal polarity and the control of epithelial form and function

Epithelial cells are the most common cell type in all animals, forming the sheets and tubes that compose most organs and tissues. Apical-basal polarity is essential for epithelial cell form and function, as it determines the localization of the adhesion molecules that hold the cells together laterally and the occluding junctions that act as barriers to paracellular diffusion. Polarity must also target the secretion of specific cargoes to the apical, lateral or basal membranes and organize the cytoskeleton and internal architecture of the cell. Apical-basal polarity in many cells is established by conserved polarity factors that define the apical (Crumbs, Stardust/PALS1, aPKC, PAR-6 and CDC42), junctional (PAR-3) and lateral (Scribble, DLG, LGL, Yurt and RhoGAP19D) domains, although recent evidence indicates that not all epithelia polarize by the same mechanism. Research has begun to reveal the dynamic interactions between polarity factors and how they contribute to polarity establishment and maintenance. Elucidating these mechanisms is essential to better understand the roles of apical-basal polarity in morphogenesis and how defects in polarity contribute to diseases such as cancer.

Cell autonomous and non-autonomous consequences of deviations in translation machinery on organism growth and the connecting signalling pathways

Translation machinery is responsible for the production of cellular proteins; thus, cells devote the majority of their resources to ribosome biogenesis and protein synthesis. Single-copy loss of function in the translation machinery components results in rare ribosomopathy disorders, such as Diamond-Blackfan anaemia in humans and similar developmental defects in various model organisms. Somatic copy number alterations of translation machinery components are also observed in specific tumours. The organism-wide response to haploinsufficient loss-of-function mutations in ribosomal proteins or translation machinery components is complex: variations in translation machinery lead to reduced ribosome biogenesis, protein translation and altered protein homeostasis and cellular signalling pathways. Cells are affected both autonomously and non-autonomously by changes in translation machinery or ribosome biogenesis through cell-cell interactions and secreted hormones. We first briefly introduce the model organisms where mutants or knockdowns of protein synthesis and ribosome biogenesis are characterized. Next, we specifically describe observations in Caenorhabditis elegans and Drosophila melanogaster, where insufficient protein synthesis in a subset of cells triggers cell non-autonomous growth or apoptosis responses that affect nearby cells and tissues. We then cover the characterized signalling pathways that interact with ribosome biogenesis/protein synthesis machinery with an emphasis on their respective functions during organism development.

Crumbs and the apical spectrin cytoskeleton regulate R8 cell fate in the Drosophila eye

The Hippo pathway is an important regulator of organ growth and cell fate. In the R8 photoreceptor cells of the Drosophila melanogaster eye, the Hippo pathway controls the fate choice between one of two subtypes that express either the blue light-sensitive Rhodopsin 5 (Hippo inactive R8 subtype) or the green light-sensitive Rhodopsin 6 (Hippo active R8 subtype). The degree to which the mechanism of Hippo signal transduction and the proteins that mediate it are conserved in organ growth and R8 cell fate choice is currently unclear. Here, we identify Crumbs and the apical spectrin cytoskeleton as regulators of R8 cell fate. By contrast, other proteins that influence Hippo-dependent organ growth, such as the basolateral spectrin cytoskeleton and Ajuba, are dispensable for the R8 cell fate choice. Surprisingly, Crumbs promotes the Rhodopsin 5 cell fate, which is driven by Yorkie, rather than the Rhodopsin 6 cell fate, which is driven by Warts and the Hippo pathway, which contrasts with its impact on Hippo activity in organ growth. Furthermore, neither the apical spectrin cytoskeleton nor Crumbs appear to regulate the Hippo pathway through mechanisms that have been observed in growing organs. Together, these results show that only a subset of Hippo pathway proteins regulate the R8 binary cell fate decision and that aspects of Hippo signalling differ between growing organs and post-mitotic R8 cells.

Roles and mechanisms of YAP/TAZ in orthodontic tooth movement

Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are transcriptional coactivators encoded by paratactic homologous genes, shuttle-crossing between cytoplasm and nucleus to regulate the gene expression and cell behavior and standing at the center place of the sophisticated regulatory networking of mechanotransduction. Orthodontic tooth movement (OTM) is a process in which extracellular mechanical stimuli are transformed into intracellular biochemical signals to regulate cellular responses and tissue remodeling. Literature studies have confirmed that YAP/TAZ plays an important role not only in embryonic development, homeostasis and tumorigenesis, but also in mechanical-biochemical signal transduction of periodontal tissues under the mediation of various signal molecules in its upstream and downstream. Herein, we review the advances in the roles and mechanisms of YAP/TAZ in OTM to provide insights for better understanding and further study of the OTM and possible targeted clinical intervention in orthodontic treatment.

Application of CRISPR screens to investigate mammalian cell competition

Cell competition is defined as the context-dependent elimination of cells that is mediated by intercellular communication, such as paracrine or contact-dependent cell signaling, and/or mechanical stresses. It is considered to be a quality control mechanism that facilitates the removal of suboptimal cells from both adult and embryonic tissues. Cell competition, however, can also be hijacked by transformed cells to acquire a 'super-competitor' status and outcompete the normal epithelium to establish a precancerous field. To date, many genetic drivers of cell competition have been identified predominately through studies in Drosophila. Especially during the last couple of years, ethylmethanesulfonate-based genetic screens have been instrumental to our understanding of the molecular regulators behind some of the most common competition mechanisms in Drosophila, namely competition due to impaired ribosomal function (or anabolism) and mechanical sensitivity. Despite recent findings in Drosophila and in mammalian models of cell competition, the drivers of mammalian cell competition remain largely elusive. Since the discovery of CRISPR/Cas9, its use in functional genomics has been indispensable to uncover novel cancer vulnerabilities. We envision that CRISPR/Cas9 screens will enable systematic, genome-scale probing of mammalian cell competition to discover novel mutations that not only trigger cell competition but also identify novel molecular components that are essential for the recognition and elimination of less fit cells. In this review, we summarize recent contributions that further our understanding of the molecular mechanisms of cell competition by genetic screening in Drosophila, and provide our perspective on how similar and novel screening strategies made possible by whole-genome CRISPR/Cas9 screening can advance our understanding of mammalian cell competition in the future.

Emerging mechanisms of cell competition

The growth and survival of cells within tissues can be affected by 'cell competition' between different cell clones. This phenomenon was initially recognized between wild-type cells and cells with mutations in ribosomal protein (Rp) genes in Drosophila melanogaster. However, competition also affects D. melanogaster cells with mutations in epithelial polarity genes, and wild-type cells exposed to 'super-competitor' cells with mutation in the Salvador-Warts-Hippo tumour suppressor pathway or expressing elevated levels of Myc. More recently, cell competition and super-competition were recognized in mammalian development, organ homeostasis and cancer. Genetic and cell biological studies have revealed that mechanisms underlying cell competition include the molecular recognition of 'different' cells, signalling imbalances between distinct cell populations and the mechanical consequences of differential growth rates; these mechanisms may also involve innate immune proteins, p53 and changes in translation.

The Hippo Pathway as a Driver of Select Human Cancers

The Hippo pathway regulates myriad biological processes in diverse species and is a key cancer signaling network in humans. Although Hippo has been linked to multiple aspects of cancer, its role in this disease is incompletely understood. Large-scale pan-cancer analyses of core Hippo pathway genes reveal that the pathway is mutated at a high frequency only in select human cancers, including malignant mesothelioma and meningioma. Hippo pathway deregulation is also enriched in squamous epithelial cancers. We discuss cancer-related functions of the Hippo pathway and potential explanations for the cancer-restricted mutation profile of core Hippo pathway genes. Greater understanding of Hippo pathway deregulation in cancers will be essential to guide the imminent use of Hippo-targeted therapies.

Growth control in the Drosophila wing disk

The regulation of size and shape is a fundamental requirement of biological development and has been a subject of scientific study for centuries, but we still lack an understanding of how organisms know when to stop growing. Imaginal wing disks of the fruit fly Drosophila melanogaster, which are precursors of the adult wings, are an archetypal tissue for studying growth control. The growth of the disks is dependent on many inter- and intra-organ factors such as morphogens, mechanical forces, nutrient levels, and hormones that influence gene expression and cell growth. Extracellular signals are transduced into gene-control signals via complex signal transduction networks, and since cells typically receive many different signals, a mechanism for integrating the signals is needed. Our understanding of the effect of morphogens on tissue-level growth regulation via individual pathways has increased significantly in the last half century, but our understanding of how multiple biochemical and mechanical signals are integrated to determine whether or not a cell decides to divide is still rudimentary. Numerous fundamental questions are involved in understanding the decision-making process, and here we review the major biochemical and mechanical pathways involved in disk development with a view toward providing a basis for beginning to understand how multiple signals can be integrated at the cell level, and how this translates into growth control at the level of the imaginal disk. This article is categorized under: Analytical and Computational Methods > Computational Methods Biological Mechanisms > Cell Signaling Models of Systems Properties and Processes > Cellular Models.

Casein kinase 1 family proteins promote Slimb-dependent Expanded degradation

Hippo signalling integrates diverse stimuli related to epithelial architecture to regulate tissue growth and cell fate decisions. The Hippo kinase cascade represses the growth-promoting transcription co-activator Yorkie. The FERM protein Expanded is one of the main upstream Hippo signalling regulators in Drosophila as it promotes Hippo kinase signalling and directly inhibits Yorkie. To fulfil its function, Expanded is recruited to the plasma membrane by the polarity protein Crumbs. However, Crumbs-mediated recruitment also promotes Expanded turnover via a phosphodegron-mediated interaction with a Slimb/β-TrCP SCF E3 ligase complex. Here, we show that the Casein Kinase 1 (CKI) family is required for Expanded phosphorylation. CKI expression promotes Expanded phosphorylation and interaction with Slimb/β-TrCP. Conversely, CKI depletion in S2 cells impairs Expanded degradation downstream of Crumbs. In wing imaginal discs, CKI loss leads to elevated Expanded and Crumbs levels. Thus, phospho-dependent Expanded turnover ensures a tight coupling of Hippo pathway activity to epithelial architecture.

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

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

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

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

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

Drosophila Genetics: The Power of Genetic Mosaic Approaches

Drosophila melanogaster has been a central player in the discovery of the Hippo pathway and in understanding its in vivo functions. From a technique standpoint, the Flp-FRT system for the generation of genetic mosaics has been a principle tool. It has broadly been used in the discovery of Hippo pathway members in mutagenesis screens, in the analysis of target gene expression, and in genetic epistasis. Here we briefly introduce this tool, summarize its use in the Hippo pathway field, and provide a protocol for the generation of Flp-FRT clones in imaginal discs with dissection and staining for reporter gene expression to characterize candidate Hippo pathway genes.

The Power of Drosophila Genetics: The Discovery of the Hippo Pathway

The Hippo Pathway comprises a vast network of components that integrate diverse signals including mechanical cues and cell surface or cell-surface-associated molecules to define cellular outputs of growth, proliferation, cell fate, and cell survival on both the cellular and tissue level. Because of the importance of the regulators, core components, and targets of this pathway in human health and disease, individual components were often identified by efforts in mammalian models or for a role in a specific process such as stress response or cell death. However, multiple components were originally discovered in the Drosophila system, and the breakthrough of conceiving that these components worked together in a signaling pathway came from a series of Drosophila genetic screens and fundamental genetic and phenotypic characterization efforts. In this chapter, we will review the original discoveries leading to the conceptual framework of these components as a tumor suppressor network. We will review chronologically the early efforts that established our initial understanding of the core machinery that then launched the growing and vibrant field to be discussed throughout later chapters of this book.

Drosophila Models of Cell Polarity and Cell Competition in Tumourigenesis

Cell competition is an important surveillance mechanism that measures relative fitness between cells in a tissue during development, homeostasis, and disease. Specifically, cells that are "less fit" (losers) are actively eliminated by relatively "more fit" (winners) neighbours, despite the less fit cells otherwise being able to survive in a genetically uniform tissue. Originally described in the epithelial tissues of Drosophila larval imaginal discs, cell competition has since been shown to occur in other epithelial and non-epithelial Drosophila tissues, as well as in mammalian model systems. Many genes and signalling pathways have been identified as playing conserved roles in the mechanisms of cell competition. Among them are genes required for the establishment and maintenance of apico-basal cell polarity: the Crumbs/Stardust/Patj (Crb/Sdt/Patj), Bazooka/Par-6/atypical Protein Kinase C (Baz/Par-6/aPKC), and Scribbled/Discs large 1/Lethal (2) giant larvae (Scrib/Dlg1/L(2)gl) modules. In this chapter, we describe the concepts and mechanisms of cell competition, with emphasis on the relationship between cell polarity proteins and cell competition, particularly the Scrib/Dlg1/L(2)gl module, since this is the best described module in this emerging field.

P53 and mTOR signalling determine fitness selection through cell competition during early mouse embryonic development

Ensuring the fitness of the pluripotent cells that will contribute to future development is important both for the integrity of the germline and for proper embryogenesis. Consequently, it is becoming increasingly apparent that pluripotent cells can compare their fitness levels and signal the elimination of those cells that are less fit than their neighbours. In mammals the nature of the pathways that communicate fitness remain largely unknown. Here we identify that in the early mouse embryo and upon exit from naive pluripotency, the confrontation of cells with different fitness levels leads to an inhibition of mTOR signalling in the less fit cell type, causing its elimination. We show that during this process, p53 acts upstream of mTOR and is required to repress its activity. Finally, we demonstrate that during normal development around 35% of cells are eliminated by this pathway, highlighting the importance of this mechanism for embryonic development.

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.

Physiological and pathological relevance of cell competition in fly to mammals

In multicellular organisms, incidentally emerging suboptimal cells are removed to maintain homeostasis of tissues. The unfavorable cells are excluded by a process termed cell competition whereby the resident normal cells actively eliminate the unfit cells of the identical lineage. Although the phenomenon of cell competition was originally discovered in Drosophila, a number of recent studies have provided implications of cell competition in tissue regeneration, development and oncogenesis in mammals. Here the roles of cell competition in fly to mammals are discussed.

Crumbs, Moesin and Yurt regulate junctional stability and dynamics for a proper morphogenesis of the Drosophila pupal wing epithelium

The Crumbs (Crb) complex is a key epithelial determinant. To understand its role in morphogenesis, we examined its function in the Drosophila pupal wing, an epithelium undergoing hexagonal packing and formation of planar-oriented hairs. Crb distribution is dynamic, being stabilized to the subapical region just before hair formation. Lack of crb or stardust, but not DPatj, affects hexagonal packing and delays hair formation, without impairing epithelial polarities but with increased fluctuations in cell junctions and perimeter length, fragmentation of adherens junctions and the actomyosin cytoskeleton. Crb interacts with Moesin and Yurt, FERM proteins regulating the actomyosin network. We found that Moesin and Yurt distribution at the subapical region depends on Crb. In contrast to previous reports, yurt, but not moesin, mutants phenocopy crb junctional defects. Moreover, while unaffected in crb mutants, cell perimeter increases in yurt mutant cells and decreases in the absence of moesin function. Our data suggest that Crb coordinates proper hexagonal packing and hair formation, by modulating junction integrity via Yurt and stabilizing cell perimeter via both Yurt and Moesin. The Drosophila pupal wing thus appears as a useful system to investigate the functional diversification of the Crb complex during morphogenesis, independently of its role in polarity.

Mechanisms of cell competition emerging from Drosophila studies

Cell competition was described in Drosophila as the loss from mosaic tissues of otherwise viable cells heterozygous for Ribosomal protein mutations ('Minutes'). Cell competition has now been described to occur between multiple other genotypes, such as cells differing in myc expression levels, or mutated for neoplastic tumor suppressors. Recent studies implicate innate immunity components, and possibly mechanical stress, compression and cell intercalation as a consequence of differential growth rates in competitive cell death. Competition to eliminate pre-neoplastic tumors makes use of signals and receptors also used in patterning the nervous system including Slit/Robo2 and Sas/PTP10D to recognize and extrude clones of mutant cells, at least where local epithelial cyto-architecture is favorable. Cell competition facilitates expansion of Drosophila tumors through host tissue, and in normal development may promote developmental robustness and longevity by selecting for optimal progenitor cells.

The Crumbs_C isoform of Drosophila shows tissue- and stage-specific expression and prevents light-dependent retinal degeneration

Drosophila Crumbs (Crb) is a key regulator of epithelial polarity and fulfils a plethora of other functions, such as growth regulation, morphogenesis of photoreceptor cells and prevention of retinal degeneration. This raises the question how a single gene regulates such diverse functions, which in mammals are controlled by three different paralogs. Here, we show that in Drosophila different Crb protein isoforms are differentially expressed as a result of alternative splicing. All isoforms are transmembrane proteins that differ by just one EGF-like repeat in their extracellular portion. Unlike Crb_A, which is expressed in most embryonic epithelia from early stages onward, Crb_C is expressed later and only in a subset of embryonic epithelia. Flies specifically lacking Crb_C are homozygous viable and fertile. Strikingly, these flies undergo light-dependent photoreceptor degeneration despite the fact that the other isoforms are expressed and properly localised at the stalk membrane. This allele now provides an ideal possibility to further unravel the molecular mechanisms by which Drosophila crb protects photoreceptor cells from the detrimental consequences of light-induced cell stress.

Summary: Loss of Crb_C, one protein isoform encoded by Drosophila crumbs, results in light-dependent retinal degeneration, but does not affect any of the other crumbs-specific functions.

The Hippo Pathway: A Master Regulatory Network Important in Development and Dysregulated in Disease

The Hippo Pathway is a master regulatory network that regulates proliferation, cell growth, stemness, differentiation, and cell death. Coordination of these processes by the Hippo Pathway throughout development and in mature organisms in response to diverse external and internal cues plays a role in morphogenesis, in controlling organ size, and in maintaining organ homeostasis. Given the importance of these processes, the Hippo Pathway also plays an important role in organismal health and has been implicated in a variety of diseases including eye disease, cardiovascular disease, neurodegeneration, and cancer. This review will focus on Drosophila reports that identified the core components of the Hippo Pathway revealing specific downstream biological outputs of this complicated network. A brief description of mammalian reports will complement review of the Drosophila studies. This review will also survey upstream regulation of the core components with a focus on feedback mechanisms.

Drosophila Crumbs prevents ectopic Notch activation in developing wings by inhibiting ligand-independent endocytosis

Many signalling components are apically restricted in epithelial cells, and receptor localisation and abundance is key for morphogenesis and tissue homeostasis. Hence, controlling apicobasal epithelial polarity is crucial for proper signalling. Notch is a ubiquitously expressed, apically localised receptor, which performs a plethora of functions; therefore, its activity has to be tightly regulated. Here, we show that Drosophila Crumbs, an evolutionarily conserved polarity determinant, prevents Notch endocytosis in developing wings through direct interaction between the two proteins. Notch endocytosis in the absence of Crumbs results in the activation of the ligand-independent, Deltex-dependent Notch signalling pathway, and does not require the ligands Delta and Serrate or γ-secretase activity. This function of Crumbs is not due to general defects in apicobasal polarity, as localisation of other apical proteins is unaffected. Our data reveal a mechanism to explain how Crumbs directly controls localisation and trafficking of the potent Notch receptor, and adds yet another aspect of Crumbs regulation in Notch pathway activity. Furthermore, our data highlight a close link between the apical determinant Crumbs, receptor trafficking and tissue homeostasis.

Ebi modulates wing growth by ubiquitin-dependent downregulation of Crumbs in Drosophila

Notch signaling at the dorsoventral (DV) boundary is essential for patterning and growth of wings in Drosophila The WD40 domain protein Ebi has been implicated in the regulation of Notch signaling at the DV boundary. Here we show that Ebi regulates wing growth by antagonizing the function of the transmembrane protein Crumbs (Crb). Ebi physically binds to the extracellular domain of Crb (Crb^ext), and this interaction is specifically mediated by WD40 repeats 7-8 of Ebi and a laminin G domain of Crb^ext Wing notching resulting from reduced levels of Ebi is suppressed by decreasing the Crb function. Consistent with this antagonistic genetic relationship, Ebi knockdown in the DV boundary elevates the Crb protein level. Furthermore, we show that Ebi is required for downregulation of Crb by ubiquitylation. Taken together, we propose that the interplay of Crb expression in the DV boundary and ubiquitin-dependent Crb downregulation by Ebi provides a mechanism for the maintenance of Notch signaling during wing development.

YAP and TAZ in epithelial stem cells: A sensor for cell polarity, mechanical forces and tissue damage

The YAP/TAZ family of transcriptional co‐activators drives cell proliferation in epithelial tissues and cancers. Yet, how YAP and TAZ are physiologically regulated remains unclear. Here we review recent reports that YAP and TAZ act primarily as sensors of epithelial cell polarity, being inhibited when cells differentiate an apical membrane domain, and being activated when cells contact the extracellular matrix via their basal membrane domain. Apical signalling occurs via the canonical Crumbs/CRB‐Hippo/MST‐Warts/LATS kinase cascade to phosphorylate and inhibit YAP/TAZ. Basal signalling occurs via Integrins and Src family kinases to phosphorylate and activate YAP/TAZ. Thus, YAP/TAZ is localised to the nucleus in basal stem/progenitor cells and cytoplasm in differentiated squamous cells or columnar cells. In addition, other signals such as mechanical forces, tissue damage and possibly receptor tyrosine kinases (RTKs) can influence MST‐LATS or Src family kinase activity to modulate YAP/TAZ activity.

The Ski2-family helicase Obelus regulates Crumbs alternative splicing and cell polarity

The conserved Ski2-family helicase Obelus regulates alternative splicing of the Crumbs polarity protein to control epithelial polarity and junctional organization in Drosophila.

Alternative splicing can have profound consequences for protein activity, but the functions of most alternative splicing regulators are not known. We show that Obelus, a conserved Ski2-family helicase, is required for cell polarity and adherens junction organization in the Drosophila melanogaster embryo. In obelus mutants, epithelial cells display an expanded apical domain, aggregation of adherens junctions at the cell membrane, and microtubule-dependent defects in centrosome positioning. Through whole-genome transcriptome analysis, we found that Obelus is required for the alternative splicing of a small number of transcripts in the early embryo, including the pre-mRNA that encodes the apical polarity protein Crumbs. In obelus mutants, inclusion of an alternative exon results in increased expression of a Crumbs isoform that contains an additional epidermal growth factor–like repeat in the extracellular domain. Overexpression of this alternative Crumbs isoform recapitulates the junctional aggregation and centrosome positioning defects of obelus mutants. These results indicate that regulation of Crumbs alternative splicing by the Obelus helicase modulates epithelial polarity during development.

Persistence of RNAi-Mediated Knockdown in Drosophila Complicates Mosaic Analysis Yet Enables Highly Sensitive Lineage Tracing

RNA interference (RNAi) has emerged as a powerful way of reducing gene function in Drosophila melanogaster tissues. By expressing synthetic short hairpin RNAs (shRNAs) using the Gal4/UAS system, knockdown is efficiently achieved in specific tissues or in clones of marked cells. Here we show that knockdown by shRNAs is so potent and persistent that even transient exposure of cells to shRNAs can reduce gene function in their descendants. When using the FLP-out Gal4 method, in some instances we observed unmarked “shadow RNAi” clones adjacent to Gal4-expressing clones, which may have resulted from brief Gal4 expression following recombination but prior to cell division. Similarly, Gal4 driver lines with dynamic expression patterns can generate shadow RNAi cells after their activity has ceased in those cells. Importantly, these effects can lead to erroneous conclusions regarding the cell autonomy of knockdown phenotypes. We have investigated the basis of this phenomenon and suggested experimental designs for eliminating ambiguities in interpretation. We have also exploited the persistence of shRNA-mediated knockdown to design a sensitive lineage-tracing method, i-TRACE, which is capable of detecting even low levels of past reporter expression. Using i-TRACE, we demonstrate transient infidelities in the expression of some cell-identity markers near compartment boundaries in the wing imaginal disc.

How to be in a good shape? The influence of clone morphology on cell competition

Cell competition is a conserved mechanism where slow proliferating cells (so called losers) are eliminated by faster proliferating neighbors (so called winners) through apoptosis.¹ It is an important process which prevents developmental malformations and maintains tissue fitness in aging adults.² Recently, we have shown that the probability of elimination of loser cells correlates with the surface of contact between losers and winners in Myc-induced competition.³ Moreover, we have characterized an active mechanism that increases the surface of contact between losers and winners, hence accelerating the elimination of loser cells. This is the first indication that cell shape and mechanics can influence cell competition. Here, we will discuss the consequence of the relationship between shape and competition, as well as the relevance of this model for other modes of competition.

Organ Size Control: Lessons from Drosophila

Of fundamental interest to biologists is how organs achieve a reproducible size during development. Studies of the developing Drosophila wing have provided many key insights that will help give a conceptual understanding of the process beyond the fly. In the wing, there is evidence for both "top-down" mechanisms, in which signals emanating from small subsets of cells direct global proliferation, and "bottom-up" mechanisms, in which the final size is an emergent property of local cell-cell interactions. Mechanical forces also appear to have an important role along with the Hippo pathway, which may integrate multiple types of inputs to regulate the extent of growth.

AP-2-complex-mediated endocytosis of Drosophila Crumbs regulates polarity by antagonizing Stardust

Maintenance of epithelial polarity depends on the correct localization and levels of polarity determinants. The evolutionarily conserved transmembrane protein Crumbs is crucial for the size and identity of the apical membrane, yet little is known about the molecular mechanisms controlling the amount of Crumbs at the surface. Here, we show that Crumbs levels on the apical membrane depend on a well-balanced state of endocytosis and stabilization. The adaptor protein 2 (AP-2) complex binds to a motif in the cytoplasmic tail of Crumbs that overlaps with the binding site of Stardust, a protein known to stabilize Crumbs on the surface. Preventing endocytosis by mutation of AP-2 causes expansion of the Crumbs-positive plasma membrane domain and polarity defects, which can be partially rescued by removing one copy of crumbs. Strikingly, knocking down both AP-2 and Stardust leads to the retention of Crumbs on the membrane. This study provides evidence for a molecular mechanism, based on stabilization and endocytosis, to adjust surface levels of Crumbs, which are essential for maintaining epithelial polarity.

Non-autonomous overgrowth by oncogenic niche cells: Cellular cooperation and competition in tumorigenesis

Tumor progression is classically viewed as the Darwinian evolution of subclones that sequentially acquire genetic mutations and autonomously overproliferate. However, growing evidence suggests that tumor microenvironment and subclone heterogeneity contribute to non‐autonomous tumor progression. Recent Drosophila studies revealed a common mechanism by which clones of genetically altered cells trigger non‐autonomous overgrowth. Such “oncogenic niche cells” (ONCs) do not overgrow but instead stimulate neighbor overgrowth and metastasis. Establishment of ONCs depends on competition and cooperation between heterogeneous cell populations. This review characterizes diverse ONCs identified in Drosophila and describes the genetic basis of non‐autonomous tumor progression. Similar mechanisms may contribute to mammalian cancer progression and recurrence.

The Mammalian Blood-Testis Barrier: Its Biology and Regulation

Spermatogenesis is the cellular process by which spermatogonia develop into mature spermatids within seminiferous tubules, the functional unit of the mammalian testis, under the structural and nutritional support of Sertoli cells and the precise regulation of endocrine factors. As germ cells develop, they traverse the seminiferous epithelium, a process that involves restructuring of Sertoli-germ cell junctions, as well as Sertoli-Sertoli cell junctions at the blood-testis barrier. The blood-testis barrier, one of the tightest tissue barriers in the mammalian body, divides the seminiferous epithelium into 2 compartments, basal and adluminal. The blood-testis barrier is different from most other tissue barriers in that it is not only comprised of tight junctions. Instead, tight junctions coexist and cofunction with ectoplasmic specializations, desmosomes, and gap junctions to create a unique microenvironment for the completion of meiosis and the subsequent development of spermatids into spermatozoa via spermiogenesis. Studies from the past decade or so have identified the key structural, scaffolding, and signaling proteins of the blood-testis barrier. More recent studies have defined the regulatory mechanisms that underlie blood-testis barrier function. We review here the biology and regulation of the mammalian blood-testis barrier and highlight research areas that should be expanded in future studies.

Control of organ growth by patterning and hippo signaling in Drosophila

Control of organ size is of fundamental importance and is controlled by genetic, environmental, and mechanical factors. Studies in many species have pointed to the existence of both organ-extrinsic and -intrinsic size-control mechanisms, which ultimately must coordinate to regulate organ size. Here, we discuss organ size control by organ patterning and the Hippo pathway, which both act in an organ-intrinsic fashion. The influence of morphogens and other patterning molecules couples growth and patterning, whereas emerging evidence suggests that the Hippo pathway controls growth in response to mechanical stimuli and signals emanating from cell-cell interactions. Several points of cross talk have been reported between signaling pathways that control organ patterning and the Hippo pathway, both at the level of membrane receptors and transcriptional regulators. However, despite substantial progress in the past decade, key questions in the growth-control field remain, including precisely how and when organ patterning and the Hippo pathway communicate to control size, and whether these communication mechanisms are organ specific or general. In addition, elucidating mechanisms by which organ-intrinsic cues, such as patterning factors and the Hippo pathway, interface with extrinsic cues, such as hormones to control organ size, remain unresolved.

Apoptotic mechanisms during competition of ribosomal protein mutant cells: roles of the initiator caspases Dronc and Dream/Strica

Heterozygosity for mutations in ribosomal protein genes frequently leads to a dominant phenotype of retarded growth and small adult bristles in Drosophila (the Minute phenotype). Cells with Minute genotypes are subject to cell competition, characterized by their selective apoptosis and removal in mosaic tissues that contain wild-type cells. Competitive apoptosis was found to depend on the pro-apoptotic reaper, grim and head involution defective genes but was independent of p53. Rp/+ cells are protected by anti-apoptotic baculovirus p35 expression but lacked the usual hallmarks of 'undead' cells. They lacked Dronc activity, and neither expression of dominant-negative Dronc nor dronc knockdown by dsRNA prevented competitive apoptosis, which also continued in dronc null mutant cells or in the absence of the initiator caspases dredd and dream/strica. Only simultaneous knockdown of dronc and dream/strica by dsRNA was sufficient to protect Rp/+ cells from competition. By contrast, Rp/Rp cells were also protected by baculovirus p35, but Rp/Rp death was dronc-dependent, and undead Rp/Rp cells exhibited typical dronc-dependent expression of Wingless. Independence of p53 and unusual dependence on Dream/Strica distinguish competitive cell death from noncompetitive apoptosis of Rp/Rp cells and from many other examples of cell death.

Cell competition in vertebrate organ size regulation

The study of animal organ size determination has provided evidence of the existence of organ-intrinsic mechanisms that 'sense' and adjust organ growth. Cell competition, a form of cell interaction that equalizes cell population growth, has been proposed to play a role in organ size regulation. Cell competition involves a cell-context dependent response triggered by perceived differences in cell growth and/or proliferation rates, resulting in apoptosis in growth-disadvantaged cells and compensatory expansion of the more 'fit' cells. The mechanisms that allow cells to compare growth are not yet understood, but a number of genes and pathways have been implicated in cell competition. These include Myc, the members of the Hippo, JAK/STAT and WNT signaling pathways, and the Dlg/Lgl/Scrib and the Crb/Std/PatJ membrane protein complexes. Cell competition was initially characterized in the Drosophila imaginal disc, but several recent studies have shown that cell competition occurs in mouse embryonic stem cells and in the embryonic epiblast, where it plays a role in the regulation of early embryo size. In addition, competition-like behavior has been described in the adult mouse liver and the hematopoietic stem cell compartment. These data indicate that cell competition plays a more universal role in organ size regulation. In addition, as some authors have suggested that similar types of competitive behavior may operate in during tumorigenesis, there may be additional practical reasons for understanding this fundamental process of intercellular communication.

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

Drosophila Myc: A master regulator of cellular performance

The identification of the Drosophila homolog of the human MYC oncogene has fostered a series of studies aimed to address its functions in development and cancer biology. Due to its essential roles in many fundamental biological processes it is hard to imagine a molecular mechanism in which MYC function is not required. For this reason, the easily manipulated Drosophila system has greatly helped in the dissection of the genetic and molecular pathways that regulate and are regulated by MYC function. In this review, we focus on studies of MYC in the fruitfly with particular emphasis on metabolism and cell competition, highlighting the contributions of this model system in the last decade to our understanding of MYC's complex biological nature. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

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.

Cell competition: how to eliminate your neighbours

A conventional view of development is that cells cooperate to build an organism. However, based on studies of Drosophila, it has been known for years that viable cells can be eliminated by their neighbours through a process termed cell competition. New studies in mammals have revealed that this process is universal and that many factors and mechanisms are conserved. During cell competition, cells with lower translation rates or those with lower levels of proteins involved in signal transduction, polarity and cellular growth can survive in a homogenous environment but are killed when surrounded by cells of higher fitness. Here, we discuss recent advances in the field as well as the mechanistic steps involved in this phenomenon, which have shed light on how and why cell competition exists in developing and adult organisms.

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.

Complexities of Crumbs function and regulation in tissue morphogenesis

Establishing and maintaining epithelial polarity is crucial during development and for adult tissue homeostasis. A complex network of evolutionarily conserved proteins regulates this compartmentalization. One such protein is Crumbs, a type I transmembrane protein initially shown to be an important apical determinant in Drosophila. We discuss recent studies that have advanced our understanding of the function and regulation of Crumbs. New findings obtained in flies and fish, reporting homotypic interactions of the extracellular domain and retromer-mediated recycling, shed light on the regulation of Crumbs levels and activity. These results - obtained in different organisms, tissues and developmental stages - point to more complex functions and regulation than previously assumed.

Sticking together the Crumbs - an unexpected function for an old friend

Cell polarity and cell-cell junctions have pivotal roles in organizing cells into tissues and in mediating cell-cell communication. The transmembrane protein Crumbs has a well-established role in the maintenance of epithelial polarity, and it can also regulate signalling via the Notch and Hippo pathways to influence tissue growth. The functions of Crumbs in epithelial polarity and Hippo-mediated growth depend on its short intracellular domain. Recent evidence now points to a conserved and fundamental role for the extracellular domain of Crumbs in mediating homophilic Crumbs-Crumbs interactions at cell-cell junctions.

Mechanisms of cell competition: themes and variations

Cell competition is the short-range elimination of slow-dividing cells through apoptosis when confronted with a faster growing population. It is based on the comparison of relative cell fitness between neighboring cells and is a striking example of tissue adaptability that could play a central role in developmental error correction and cancer progression in both Drosophila melanogaster and mammals. Cell competition has led to the discovery of multiple pathways that affect cell fitness and drive cell elimination. The diversity of these pathways could reflect unrelated phenomena, yet recent evidence suggests some common wiring and the existence of a bona fide fitness comparison pathway.

Tissue repair through cell competition and compensatory cellular hypertrophy in postmitotic epithelia

In multicellular organisms, tissue integrity and organ size are maintained through removal of aberrant or damaged cells and compensatory proliferation. Little is known, however, about this homeostasis system in postmitotic tissues, where tissue-intrinsic genetic programs constrain cell division and new cells no longer arise from stem cells. Here we show that, in postmitotic Drosophila follicular epithelia, aberrant but viable cells are eliminated through cell competition, and the resulting loss of local tissue volume triggers sporadic cellular hypertrophy to repair the tissue. This "compensatory cellular hypertrophy" is implemented by acceleration of the endocycle, a variant cell cycle composed of DNA synthesis and gap phases without mitosis, dependent on activation of the insulin/IGF-like signaling pathway. These results reveal a remarkable homeostatic mechanism in postmitotic epithelia that ensures not only elimination of aberrant cells through cell competition but also proper organ-size control that involves compensatory cellular hypertrophy induced by physical parameters.