Brk regulates wing disc growth in part via repression of Myc expression

Genomics of a sexually selected sperm ornament and female preference in Drosophila

Our understanding of animal ornaments and the mating preferences driving their exaggeration is limited by knowledge of their genetics. Post-copulatory sexual selection is credited with the rapid evolution of female sperm-storage organ morphology and corresponding sperm quality traits across diverse taxa. In Drosophila, the mechanisms by which longer flagella convey an advantage in the competition among sperm for limited storage space in the female, and by which female sperm-storage organ morphology biases fertilization in favour of longer sperm have been resolved. However, the evolutionary genetics underlying this model post-copulatory ornament and preference system have remained elusive. Here we combined comparative analyses of 149 Drosophila species, a genome-wide association study in Drosophila melanogaster and molecular evolutionary analysis of ~9,400 genes to elucidate how sperm and female sperm-storage organ length co-evolved into one of nature's most extreme ornaments and preferences. Our results reveal a diverse repertoire of pleiotropic genes linking sperm length and seminal receptacle length expression to central nervous system development and sensory biology. Sperm length development appears condition-dependent and is governed by conserved hormonal (insulin/insulin-like growth factor) and developmental (including Notch and Fruitless) pathways. Central developmental pathway genes, including Notch, also comprised the majority of a restricted set of genes contributing to both intraspecific and interspecific variation in sperm length. Our findings support 'good genes' models of female preference evolution.

Psi promotes Drosophila wing growth via direct transcriptional activation of cell cycle targets and repression of growth inhibitors

The first characterised FUSE Binding Protein family member, FUBP1, binds single-stranded DNA to activate MYC transcription. Psi, the sole FUBP protein in Drosophila, binds RNA to regulate P-element and mRNA splicing. Our previous work revealed pro-growth functions for Psi, which depend, in part, on transcriptional activation of Myc. Genome-wide functions for FUBP family proteins in transcriptional control remain obscure. Here, through the first genome-wide binding and expression profiles obtained for a FUBP family protein, we demonstrate that, in addition to being required to activate Myc to promote cell growth, Psi also directly binds and activates stg to couple growth and cell division. Thus, Psi knockdown results in reduced cell division in the wing imaginal disc. In addition to activating these pro-proliferative targets, Psi directly represses transcription of the growth inhibitor tolkin (tok, a metallopeptidase implicated in TGFβ signalling). We further demonstrate tok overexpression inhibits proliferation, while tok loss of function increases mitosis alone and suppresses impaired cell division caused by Psi knockdown. Thus, Psi orchestrates growth through concurrent transcriptional activation of the pro-proliferative genes Myc and stg, in combination with repression of the growth inhibitor tok.

Ecdysone regulates Drosophila wing disc size via a TORC1 dependent mechanism

Most cells in a developing organ stop proliferating when the organ reaches a correct, final size. The underlying molecular mechanisms are not understood. We find that in Drosophila the hormone ecdysone controls wing disc size. To study how ecdysone affects wing size, we inhibit endogenous ecdysone synthesis and feed larvae exogenous ecdysone in a dose-controlled manner. For any given ecdysone dose, discs stop proliferating at a particular size, with higher doses enabling discs to reach larger sizes. Termination of proliferation coincides with a drop in TORC1, but not Dpp or Yki signaling. Reactivating TORC1 bypasses the termination of proliferation, indicating that TORC1 is a main downstream effector causing proliferation termination at the maximal ecdysone-dependent size. Experimental manipulation of Dpp or Yki signaling can bypass proliferation termination in hinge and notum regions, but not the pouch, suggesting that the mechanisms regulating proliferation termination may be distinct in different disc regions.

Forward and feedback control mechanisms of developmental tissue growth

The size and proportions of animals are tightly controlled during development. How this is achieved remains poorly understood. The control of organ size entails coupling of cellular growth and cell division on one hand, and the measure of organ size on the other. In this review we focus on three layers of growth control consisting of genetic patterning, notably chemical gradients, mechanics and energetics which are complemented by a systemic control unit that modulates growth in response to the nutritional conditions and coordinates growth between different organs so as to maintain proportions. Growth factors, often present as concentration dependent chemical gradients, are positive inducers of cellular growth that may be considered as deterministic cues, hence acting as organ-intrinsic controllers of growth. However, the exponential growth dynamics in many developing tissues necessitate more stringent growth control in the form of negative feedbacks. Feedbacks endow biological systems with the capacity to quickly respond to perturbations and to correct the growth trajectory to avoid overgrowth. We propose to integrate chemical, mechanical and energetic control over cellular growth in a framework that emphasizes the self-organizing properties of organ-autonomous growth control in conjunction with systemic organ non-autonomous feedback on growth.

The brinker repressor system regulates injury-induced nociceptive sensitization in Drosophila melanogaster

Chronic pain is a debilitating condition affecting millions of people worldwide, and an improved understanding of the pathophysiology of chronic pain is urgently needed. Nociceptors are the sensory neurons that alert the nervous system to potentially harmful stimuli such as mechanical pressure or noxious thermal temperature. When an injury occurs, the nociceptive threshold for pain is reduced and an increased pain signal is produced. This process is called nociceptive sensitization. This sensitization normally subsides after the injury is healed. However, dysregulation can occur which results in sensitization that persists after the injury has healed. This process is thought to perpetuate chronic pain. The Hedgehog (Hh) signaling pathway has been previously implicated in nociceptive sensitization in response to injury in Drosophila melanogaster. Downstream of Hh signaling, the Bone Morphogenetic Protein (BMP) pathway has also been shown to be necessary for this process. Here, we describe a role for nuclear components of BMP’s signaling pathway in the formation of injury-induced nociceptive sensitization. Brinker (Brk), and Schnurri (Shn) were suppressed in nociceptors using an RNA-interference (RNAi) “knockdown” approach. Knockdown of Brk resulted in hypersensitivity in the absence of injury, indicating that it normally acts to suppress nociceptive sensitivity. Animals in which transcriptional activator Shn was knocked down in nociceptors failed to develop normal allodynia after ultraviolet irradiation injury, indicating that Shn normally acts to promote hypersensitivity after injury. These results indicate that Brk-related transcription regulators play a crucial role in the formation of nociceptive sensitization and may therefore represent valuable new targets for pain-relieving medications.

MYC function and regulation in flies: how Drosophila has enlightened MYC cancer biology

Progress in our understanding of the complex signaling events driving human cancer would have been unimaginably slow without discoveries from Drosophila genetic studies. Significantly, many of the signaling pathways now synonymous with cancer biology were first identified as a result of elegant screens for genes fundamental to metazoan development. Indeed the name given to many core cancer-signaling cascades tells of their history as developmental patterning regulators in flies—e.g. Wingless (Wnt), Notch and Hippo. Moreover, astonishing insight has been gained into these complex signaling networks, and many other classic oncogenic signaling networks (e.g. EGFR/RAS/RAF/ERK, InR/PI3K/AKT/TOR), using sophisticated fly genetics. Of course if we are to understand how these signaling pathways drive cancer, we must determine the downstream program(s) of gene expression activated to promote the cell and tissue over growth fundamental to cancer. Here we discuss one commonality between each of these pathways: they are all implicated as upstream activators of the highly conserved MYC oncogene and transcription factor. MYC can drive all aspects of cell growth and cell cycle progression during animal development. MYC is estimated to be dysregulated in over 50% of all cancers, underscoring the importance of elucidating the signals activating MYC. We also discuss the FUBP1/FIR/FUSE system, which acts as a ‘cruise control’ on the MYC promoter to control RNA Polymerase II pausing and, therefore, MYC transcription in response to the developmental signaling environment. Importantly, the striking conservation between humans and flies within these major axes of MYC regulation has made Drosophila an extremely valuable model organism for cancer research. We therefore discuss how Drosophila studies have helped determine the validity of signaling pathways regulating MYC in vivo using sophisticated genetics, and continue to provide novel insight into cancer biology.

What determines organ size during development and regeneration?

The sizes of living organisms span over 20 orders of magnitude or so. This daunting observation could intimidate researchers aiming to understand the general mechanisms controlling growth. However, recent progress suggests the existence of principles common to organisms as diverse as fruit flies, mice and humans. As we review here, these studies have provided insights into both autonomous and non-autonomous mechanisms controlling organ growth as well as some of the principles underlying growth coordination between organs and across bilaterally symmetrical organisms. This research tackles several aspects of developmental biology and integrates inputs from physics, mathematical modelling and evolutionary biology. Although many open questions remain, this work also helps to shed light on medically related conditions such as tissue and limb regeneration, as well as metabolic homeostasis and cancer.

Overexpressed Gliotactin activates BMP signaling through interfering with the Tkv-Dad association

Epithelial junctions ensure cell-cell adhesion and establish permeability barriers between cells. At the corners of epithelia, the tricellular junction (TCJ) is formed by three adjacent epithelial cells and generates a functional barrier. In Drosophila, a key TCJ protein is Gliotactin (Gli) where loss of Gli disrupts barrier formation and function. Conversely, overexpressed Gli spreads away from the TCJ and triggers apoptosis, delamination, and cell migration. Thus, Gli protein levels are tightly regulated and by two mechanisms, at the protein levels by tyrosine phosphorylation and endocytosis and at the mRNA level through microRNA-184. Regulation of Gli mRNA is mediated through a Gli-BMP-miR184 feedback loop. Excessive Gli triggers BMP signaling pathway through the activation of Tkv type-I BMP receptor and Mad. Elevated level of pMad induces micrRNA-184 expression which in turn targets the Gli 3'UTR and mRNA degradation. Gli activation of Tkv is not through its ligand Dpp but rather through the inhibition of Dad, an inhibitory-Smad. Here, we show that ectopic expression of Gli interferes with Tkv-Dad association by sequestering Dad away from Tkv. The reduced inhibitory effect of Dad on Tkv results in the increased Tkv-pMad signaling activity, and this effect is continuous through larval and pupal wing formation.

The Drosophila MLR COMPASS-like complex regulates bantam miRNA expression differentially in the context of cell fate

The conserved MLR COMPASS-like complexes are histone modifiers that are recruited by a variety of transcription factors to enhancer regions where they act as necessary epigenetic tools for enhancer establishment and function. A critical in vivo target of the Drosophila MLR complex is the bantam miRNA that regulates cell survival and functions in feedback regulation of cellular signaling pathways during development. We determine that loss of Drosophila MLR complex function in developing wing and eye imaginal discs results in growth and patterning defects that are sensitive to bantam levels. Consistent with an essential regulatory role in modulating bantam transcription, the MLR complex binds to tissue-specific bantam enhancers and contributes to fine-tuning expression levels during larval tissue development. In wing imaginal discs, the MLR complex attenuates bantam enhancer activity by negatively regulating expression; whereas, in differentiating eye discs, the complex exerts either positive or negative regulatory activity on bantam transcription depending on cell fate. Furthermore, while the MLR complex is not required to control bantam levels in undifferentiated eye cells anterior to the morphogenetic furrow, it serves to prepare critical enhancer control of bantam transcription for later regulation upon differentiation. Our investigation into the transcriptional regulation of a single target in a developmental context has provided novel insights as to how the MLR complex contributes to the precise timing of gene expression, and how the complex functions to help orchestrate the regulatory output of conserved signaling pathways during animal development.

Anchor Away - A Fast, Reliable and Reversible Technique To Inhibit Proteins in Drosophila melanogaster

Several techniques have been developed to study specific gene function in loss-of-function situations. In Drosophila melanogaster, RNAi and the generation of mutant clones are widely used. However, both techniques have the limitation that there is a significant time lag before gene function is abolished. Given the relatively rapid development of Drosophila, such perdurance is a serious impediment to study gene function. Here we describe the adaptation of the anchor-away technique for use in Drosophila. Anchor-away was originally developed in yeast to quickly and efficiently abrogate the function of nuclear proteins by sequestering - anchoring - them away in a different cellular compartment. The required components are present in the cells, and the system is triggered by the addition of rapamycin, resulting in a rapid generation of a loss-of-function situation. We provide here proof of principle for the system by producing loss-of-function situations for two nuclear proteins – Pygopus and Brinker. The system allows to study the requirement of any protein during any time window, and at the same time circumvents difficulties, such as off-target effects or variable phenotypes, which are inherent in other techniques, for example RNAi.

The transcription factor Spalt and human homologue SALL4 induce cell invasion via the dMyc-JNK pathway in Drosophila

Cancer cell metastasis is a leading cause of mortality in cancer patients. Therefore, revealing the molecular mechanism of cancer cell invasion is of great significance for the treatment of cancer. In human patients, the hyperactivity of transcription factor Spalt-like 4 (SALL4) is sufficient to induce malignant tumorigenesis and metastasis. Here, we found that when ectopically expressing the Drosophila homologue spalt (sal) or human SALL4 in Drosophila, epithelial cells delaminated basally with penetration of the basal lamina and degradation of the extracellular matrix, which are essential properties of cell invasion. Further assay found that sal/SALL4 promoted cell invasion via dMyc-JNK signaling. Inhibition of the c-Jun N-terminal kinase (JNK) signaling pathway through suppressing matrix metalloprotease 1, or basket can achieve suppression of cell invasion. Moreover, expression of dMyc, a suppressor of JNK signaling, dramatically blocked cell invasion induced by sal/SALL4 in the wing disc. These findings reveal a conserved role of sal/SALL4 in invasive cell movement and link the crucial mediator of tumor invasion, the JNK pathway, to SALL4-mediated cancer progression.This article has an associated First Person interview with the first author of the paper.

Exploring MYC relevance to cancer biology from the perspective of cell competition

Cancer has long been regarded and treated as a foreign body appearing by mistake inside a living organism. However, now we know that cancer cells communicate with neighbours, thereby creating modified environments able to support their unusual need for nutrients and space. Understanding the molecular basis of these bi-directional interactions is thus mandatory to approach the complex nature of cancer. Since their discovery, MYC proteins have been showing to regulate a steadily increasing number of processes impacting cell fitness, and are consistently found upregulated in almost all human tumours. Of interest, MYC takes part in cell competition, an evolutionarily conserved fitness comparison strategy aimed at detecting weakened cells, which are then committed to death, removed from the tissue and replaced by fitter neighbours. During physiological development, MYC-mediated cell competition is engaged to eliminate cells with suboptimal MYC levels, so as to guarantee selective growth of the fittest and proper homeostasis, while transformed cells expressing high levels of MYC coopt cell competition to subvert tissue constraints, ultimately disrupting homeostasis. Therefore, the interplay between cells with different MYC levels may result in opposite functional outcomes, depending on the nature of the players. In the present review, we describe the most recent findings on the role of MYC-mediated cell competition in different contexts, with a special emphasis on its impact on cancer initiation and progression. We also discuss the relevance of competition-associated cell death to cancer disease.

The formation of the Thickveins (Tkv) gradient in Drosophila wing discs: A theoretical study

The development of the wing imaginal disc (wing disc) is commonly adopted for the studies of patterning and growth which are two fundamental problems in developmental biology. Decapentaplegic (Dpp) signaling regulates several aspects of wing development, such as the anterior (A)-posterior (P) patterning, cellular growth rate, and cell adhesion. The distribution and activity of Dpp signaling are controlled in part by the expression level of its major type I receptor, Thickveins (Tkv). In this paper, we focus on theoretically investigating mechanisms by which the highly asymmetric pattern of Tkv is established in Drosophila wing discs. To the end, a mathematical model of Hh signaling and Dpp signaling is proposed and validated by comparisons with experimental observations. Our model provides a comprehensive view of the formation of Tkv gradients in wing discs. We found that engrailed (En), Hedgehog (Hh) signaling, and Dpp signaling cooperate to establish the asymmetric gradients of Tkv and pMad in the wing disc. Moreover, our model suggests a Brinker-mediated mechanism of Dpp-dependent repression of Tkv. Based on this mechanism, a couple of predicted experimental observations have been provided for further lab confirmation.

CtBP represses Dpp-dependent Mad activation during Drosophila eye development

Complex networks of signaling pathways maintain the correct balance between positive and negative growth signals, ensuring that tissues achieve proper sizes and differentiation pattern during development. In Drosophila, Dpp, a member of the TGFβ family, plays two main roles during larval eye development. In the early eye primordium, Dpp promotes growth and cell survival, but later on, it switches its function to induce a developmentally-regulated cell cycle arrest in the G1 phase and neuronal photoreceptor differentiation. To advance in the identification and characterization of regulators and targets of Dpp signaling required for retinal development, we carried out an in vivo eye-targeted double-RNAi screen to identify punt (Type II TGFβ receptor) interactors. Using a set of 251 genes associated with eye development, we identified CtBP, Dad, Ago and Brk as punt genetic interactors. Here, we show that downregulation of Ago, or conditions causing increased tissue growth including overexpression of Myc or CyclinD-Cdk4 are sufficient to partially rescue punt-dependent growth and photoreceptor differentiation. Interestingly, we show a novel role for the transcriptional co-repressor CtBP in inhibiting Dpp-dependent Mad activation by phosphorylation, downstream or in parallel to Dad, the inhibitory Smad. Furthermore, CtBP downregulation activates JNK signaling pathway, implying a complex regulation of signaling pathways by CtBP during eye development.

CycD/Cdk4 and Discontinuities in Dpp Signaling Activate TORC1 in the Drosophila Wing Disc

The molecular mechanisms regulating animal tissue size during development are unclear. This question has been extensively studied in the Drosophila wing disc. Although cell growth is regulated by the kinase TORC1, no readout exists to visualize TORC1 activity in situ in Drosophila. Both the cell cycle and the morphogen Dpp are linked to tissue growth, but whether they regulate TORC1 activity is not known. We develop here an anti-phospho-dRpS6 antibody that detects TORC1 activity in situ. We find, unexpectedly, that TORC1 activity in the wing disc is patchy. This is caused by elevated TORC1 activity at the cell cycle G₁/S transition due to CycD/Cdk4 phosphorylating TSC1/2. We find that TORC1 is also activated independently of CycD/Cdk4 when cells with different levels of Dpp signaling or Brinker protein are juxtaposed. We thereby characterize the spatial distribution of TORC1 activity in a developing organ.

Dpp controls growth and patterning in Drosophila wing precursors through distinct modes of action

Dpp, a member of the BMP family, is a morphogen that specifies positional information in Drosophila wing precursors. In this tissue, Dpp expressed along the anterior-posterior boundary forms a concentration gradient that controls the expression domains of target genes, which in turn specify the position of wing veins. Dpp also promotes growth in this tissue. The relationship between the spatio-temporal profile of Dpp signalling and growth has been the subject of debate, which has intensified recently with the suggestion that the stripe of Dpp is dispensable for growth. With two independent conditional alleles of dpp, we find that the stripe of Dpp is essential for wing growth. We then show that this requirement, but not patterning, can be fulfilled by uniform, low level, Dpp expression. Thus, the stripe of Dpp ensures that signalling remains above a pro-growth threshold, while at the same time generating a gradient that patterns cell fates.

DOI:http://dx.doi.org/10.7554/eLife.22546.001

From the wings of a butterfly to the fingers of a human hand, living tissues often have complex and intricate patterns. Developmental biologists have long been fascinated by the signals – called morphogens – that guide how these kinds of pattern develop. Morphogens are substances that are produced by groups of cells and spread to the rest of the tissue to form a gradient. Depending on where they sit along this gradient, cells in the tissue activate different sets of genes, and the resulting pattern of gene activity ultimately defines the position of the different parts of the tissue.

Decades worth of studies into how limbs develop in animals from mice to fruit flies have revealed common principles of morphogen gradients that regulate the development of tissue patterns. Morphogens have been shown to help regulate the growth of tissues in a number of different animals as well. However, how the morphogens regulate tissue size and what role their gradients play in this process remain topics of intense debate in the field of developmental biology.

In the developing wing of a fruit fly, a morphogen called Dpp is expressed in a thin stripe located in the centre and spreads to the rest of the tissue to form a gradient. Bosch, Ziukaite, Alexandre et al. have now characterised where and when the Dpp morphogen must be produced to regulate both the final size of the fly’s wing and the number of cells the wing eventually contains. The experiments involved preventing the production of Dpp in the developing wing in specific cells and at specific stages of development. This approach confirmed that Dpp must be produced in the central stripe for the wing to grow. Matsuda and Affolter and, independently, Barrio and Milán report the same findings in two related studies. Moreover, Bosch et al. and Barrio and Milán also conclude that the gradient of Dpp throughout the wing is not required for growth.

Further work will be needed to explain how the Dpp signal regulates the growth of the wing. The answer to this question will contribute to a better understanding of the role of morphogens in regulating the size of human organs and how a failure to do so might cause developmental disorders.

DOI:http://dx.doi.org/10.7554/eLife.22546.002

Controlling the Master: Chromatin Dynamics at the MYC Promoter Integrate Developmental Signaling

The transcription factor and cell growth regulator MYC is potently oncogenic and estimated to contribute to most cancers. Decades of attempts to therapeutically target MYC directly have not resulted in feasible clinical applications, and efforts have moved toward indirectly targeting MYC expression, function and/or activity to treat MYC-driven cancer. A multitude of developmental and growth signaling pathways converge on the MYC promoter to modulate transcription through their downstream effectors. Critically, even small increases in MYC abundance (<2 fold) are sufficient to drive overproliferation; however, the details of how oncogenic/growth signaling networks regulate MYC at the level of transcription remain nebulous even during normal development. It is therefore essential to first decipher mechanisms of growth signal-stimulated MYC transcription using in vivo models, with intact signaling environments, to determine exactly how these networks are dysregulated in human cancer. This in turn will provide new modalities and approaches to treat MYC-driven malignancy. Drosophila genetic studies have shed much light on how complex networks signal to transcription factors and enhancers to orchestrate Drosophila MYC (dMYC) transcription, and thus growth and patterning of complex multicellular tissue and organs. This review will discuss the many pathways implicated in patterning MYC transcription during development and the molecular events at the MYC promoter that link signaling to expression. Attention will also be drawn to parallels between mammalian and fly regulation of MYC at the level of transcription.

The Drosophila tricellular junction protein Gliotactin regulates its own mRNA levels through BMP-mediated induction of miR-184

Epithelial bicellular and tricellular junctions are essential for establishing and maintaining permeability barriers. Tricellular junctions are formed by the convergence of three bicellular junctions at the corners of neighbouring epithelia. Gliotactin, a member of the Neuroligin family, is located at theDrosophilatricellular junction, and is crucial for the formation of tricellular and septate junctions, as well as permeability barrier function. Gliotactin protein levels are tightly controlled by phosphorylation at tyrosine residues and endocytosis. Blocking endocytosis or overexpressing Gliotactin results in the spread of Gliotactin from the tricellular junction, resulting in apoptosis, delamination and migration of epithelial cells. We show that Gliotactin levels are also regulated at the mRNA level by micro (mi)RNA-mediated degradation and that miRNAs are targeted to a short region in the 3'UTR that includes a conserved miR-184 target site. miR-184 also targets a suite of septate junction proteins, including NrxIV, coracle and Mcr. miR-184 expression is triggered when Gliotactin is overexpressed, leading to activation of the BMP signalling pathway. Gliotactin specifically interferes with Dad, an inhibitory SMAD, leading to activation of the Tkv type-I receptor and activation of Mad to elevate the biogenesis and expression of miR-184.

Dpp spreading is required for medial but not for lateral wing disc growth

Drosophila Decapentaplegic (Dpp) has served as a paradigm to study morphogen-dependent growth control. However, the role of a Dpp gradient in tissue growth remains highly controversial. Two fundamentally different models have been proposed: the 'temporal rule' model suggests that all cells of the wing imaginal disc divide upon a 50% increase in Dpp signalling, whereas the 'growth equalization model' suggests that Dpp is only essential for proliferation control of the central cells. Here, to discriminate between these two models, we generated and used morphotrap, a membrane-tethered anti-green fluorescent protein (GFP) nanobody, which enables immobilization of enhanced (e)GFP::Dpp on the cell surface, thereby abolishing Dpp gradient formation. We find that in the absence of Dpp spreading, wing disc patterning is lost; however, lateral cells still divide at normal rates. These data are consistent with the growth equalization model, but do not fit a global temporal rule model in the wing imaginal disc.

The Spalt Transcription Factors Generate the Transcriptional Landscape of the Drosophila melanogaster Wing Pouch Central Region

The Drosophila genes spalt major (salm) and spalt-related (salr) encode Zn-finger transcription factors regulated by the Decapentaplegic (Dpp) signalling pathway in the wing imaginal disc. The function of these genes is required for cell survival and proliferation in the central region of the wing disc, and also for vein patterning in the lateral regions. The identification of direct Salm and Salr target genes, and the analysis of their functions, are critical steps towards understanding the genetic control of growth and patterning of the Drosophila wing imaginal disc by the Dpp pathway. To identify candidate Salm/Salr target genes, we have compared the expression profile of salm/salr knockdown wing discs with control discs in microarray experiments. We studied by in situ hybridization the expression pattern of the genes whose mRNA levels varied significantly, and uncovered a complex transcription landscape regulated by the Spalt proteins in the wing disc. Interestingly, candidate Salm/Salr targets include genes which expression is turned off and genes which expression is positively regulated by Salm/Salr. Furthermore, loss-of-function phenotypic analysis of these genes indicates, for a fraction of them, a requirement for wing growth and patterning. The identification and analysis of candidate Salm/Salr target genes opens a new avenue to reconstruct the genetic structure of the wing, linking the activity of the Dpp pathway to the development of this epithelial tissue.

How signalling pathways regulate the formation of organs with a precise size and pattern of differentiation is a fundamental question in developmental genetics. One classical example of the link between signalling and organ development is the regulation of wing disc development by the Decapentaplegic/BMP (Dpp) signalling pathway in Drosophila. A key outcome of this pathway is the transcriptional activation of the spalt major (salm) and spalt related (salr) genes, both encoding transcription factors. In this manner, the identification of Salm/Salr target genes is a critical step towards the understanding of the mode of action of these proteins and the genetic logic underlying the regulation of wing development by the Dpp signalling pathway. In order to identify these target genes, we used expression microarrays, in situ hybridization and phenotypic analysis. We identified an unexpected complexity in the transcriptional landscape of the wing disc that includes genes positively and negatively regulated by Salm/Salr. These findings have major implications for the reconstruction of the genetic hierarchy initiated by the Dpp pathway and leading to the formation of a wing with a correct size and pattern, because some of the genes we identified could explain particular aspects of the sal mutant phenotype.

Conservation of transcription factor binding specificities across 600 million years of bilateria evolution

Divergent morphology of species has largely been ascribed to genetic differences in the tissue-specific expression of proteins, which could be achieved by divergence in cis-regulatory elements or by altering the binding specificity of transcription factors (TFs). The relative importance of the latter has been difficult to assess, as previous systematic analyses of TF binding specificity have been performed using different methods in different species. To address this, we determined the binding specificities of 242 Drosophila TFs, and compared them to human and mouse data. This analysis revealed that TF binding specificities are highly conserved between Drosophila and mammals, and that for orthologous TFs, the similarity extends even to the level of very subtle dinucleotide binding preferences. The few human TFs with divergent specificities function in cell types not found in fruit flies, suggesting that evolution of TF specificities contributes to emergence of novel types of differentiated cells.

Coordination of patterning and growth by the morphogen DPP

The elegance of animal body plans derives from an intimate connection between function and form, which during organ formation is linked to patterning and growth. Yet, how patterning and growth are coordinated still remains largely a mystery. To study this question the Drosophila wing imaginal disc, an epithelial primordial organ that later forms the adult wing, has proven to be an invaluable and versatile model. Wing disc development is organized around a coordinate system provided by morphogens such as the TGF-β homolog Decapentaplegic (DPP). The function of DPP has been studied at multiple levels: ranging from the kinetics of gradient formation to the establishment and maintenance of target gene domains as well as DPP's role in growth control. Here, we focus on recent publications that both enrich our view of DPP signaling but also highlight outstanding questions of how DPP coordinates patterning and growth during development.

Mei-P26 mediates tissue-specific responses to the Brat tumor suppressor and the dMyc proto-oncogene in Drosophila

TRIM-NHL proteins are a family of translational regulators that control cell growth, proliferation, and differentiation during development. Drosophila Brat and Mei-P26 TRIM-NHL proteins serve as tumor suppressors in stem cell lineages and have been proposed to exert this action, in part, via the repression of the protooncogene dMyc. Here we analyze the role of Brat, Mei-P26, and dMyc in regulating growth in Drosophila imaginal discs. As in stem cell lineages, Brat and Mei-P26 repress dMyc in epithelial cells by acting at the post-transcriptional and protein level, respectively. Analysis of cell and organ size unravel that Mei-P26 mediates tissue-specific responses to Brat and dMyc activities. Loss-of-function of brat and overexpression of dMyc induce overgrowth in stem cell lineages and eventually can participate in tumor formation. In contrast, an increase in Mei-P26 levels inhibits growth of epithelial cells in these two conditions. Upon depletion of Brat, Mei-P26 up-regulation prevents an increase in dMyc protein levels and leads to tissue undergrowth. This mechanism appears to be tissue-specific since Mei-P26 is not upregulated in brain tumors resulting from brat loss-of-function. Driving Mei-P26 expression in these tumors -mimicking the situation in epithelial cells- is sufficient to prevent dMyc accumulation, thus rescuing the overgrowth. Finally, we show that Mei-P26 upregulation mediates dMyc-induced apoptosis and limits dMyc growth potential in epithelial cells. These findings shed light on the tumor suppressor roles of TRIM-NHL proteins and underscore a new mechanism that maintains tissue homeostasis upon dMyc deregulation.

Dpp/BMP signaling in flies: from molecules to biology

Decapentaplegic (Dpp), the fly homolog of the secreted mammalian BMP2/4 signaling molecules, is involved in almost all aspects of fly development. Dpp has critical functions at all developmental stages, from patterning of the eggshell to the determination of adult intestinal stem cell identity. Here, we focus on recent findings regarding the transcriptional regulatory logic of the pathway, on a new feedback regulator, Pentagone, and on Dpp's roles in scaling and growth of the Drosophila wing.

Large-scale imaginal disc sorting: A protocol for "omics"-approaches

Imaginal discs, especially the wing imaginal disc, are powerful model systems to study organ development. The traditional methods to analyze wing imaginal discs depend on the laborious and time-consuming dissection of larvae. "Omics"-based approaches, such as RNA-seq, ChIP-seq, proteomics and lipidomics, offer new opportunities for the systems-level investigation of organ development. However, it is impractical to manually isolate the required starting material. This is even more problematic when experiments strive for enhanced temporal and spatial resolution. The mass isolation workflow discussed in this review, solves this problem. The semi-automated sorting of 1000 wing imaginal discs in less than 3h forms the basis of a workflow that can be connected to biochemical analyses of organ patterning and growth. In addition to the mass isolation workflow we briefly describe key "omics" technologies and their applications. The combination of mass isolation and "omics"-approaches ensures that the wing imaginal disc will continue to be a key model organ for studying developmental processes, both on the genetic, but increasingly also on the biochemical level.

Socializing with MYC: cell competition in development and as a model for premalignant cancer

Studies in Drosophila and mammals have made it clear that genetic mutations that arise in somatic tissues are rapidly recognized and eliminated, suggesting that cellular fitness is tightly monitored. During development, damaged, mutant, or otherwise unfit cells are prevented from contributing to the tissue and are instructed to die, whereas healthy cells benefit and populate the animal. This cell selection process, known as cell competition, eliminates somatic genetic heterogeneity and promotes tissue fitness during development. Yet cell competition also has a dark side. Super competition can be exploited by incipient cancers to subvert cellular cooperation and promote selfish behavior. Evidence is accumulating that MYC plays a key role in regulation of social behavior within tissues. Given the high number of tumors with deregulated MYC, studies of cell competition promise to yield insight into how the local environment yields to and participates in the early stages of tumor formation.

Myc function in Drosophila

Drosophila contains a single MYC gene. Like its vertebrate homologs, it encodes a transcription factor that activates many targets, including prominently genes involved in ribosome biogenesis and translation. This activity makes Myc a central regulator of growth and/or proliferation of many cell types, such as imaginal disc cells, polyploid cells, stem cells, and blood cells. Importantly, not only does Myc act cell autonomously but it also affects the fate of adjacent cells and tissues. This potential of Myc is harnessed by many different signaling pathways, involving, among others, Wg, Dpp, Hpo, ecdysone, insulin, and mTOR.

Dpp signaling inhibits proliferation in the Drosophila wing by Omb-dependent regional control of bantam

The control of organ growth is a fundamental aspect of animal development but remains poorly understood. The morphogen Dpp has long been considered as a general promoter of cell proliferation during Drosophila wing development. It is an ongoing debate whether the Dpp gradient is required for the uniform cell proliferation observed in the wing imaginal disc. Here, we investigated how the Dpp signaling pathway regulates proliferation during wing development. By systematic manipulation of Dpp signaling we observed that it controls proliferation in a region-specific manner: Dpp, via omb, promoted proliferation in the lateral and repressed proliferation in the medial wing disc. Omb controlled the regional proliferation rate by oppositely regulating transcription of the microRNA gene bantam in medial versus lateral wing disc. However, neither the Dpp nor Omb gradient was essential for uniform proliferation along the anteroposterior axis.

Developmental roles of Drosophila tRNA processing endonuclease RNase ZL as revealed with a conditional rescue system

Drosophila RNase Z(L) (dRNaseZ) belongs to a family of endoribonucleases with a major role in tRNA 3'-end processing. The biochemical function of RNase Z(L) is conserved from yeast to human. Here we present a study of its biological function during Drosophila development. In flies, dRNaseZ provides a non-redundant function, as the RNZ(ED24) knockout (KO) mutation causes early larval lethality. Mosaic and conditional rescue techniques were employed to determine dRNaseZ requirements at later stages. We found that dRNaseZ activity is essential for all phases of fly development that involve cell division, including growth of adult tissue progenitors during larval and metamorphic stages, and gametogenesis in adults. At the cellular level, two major phenotypes were identified-cell growth deficiency in endoreplicating tissues and cell cycle arrest in mitotic tissues. While cell growth and proliferation are both dependant on protein synthesis, the two phenotypes displayed reliance on different dRNaseZ functions. We found that dRNaseZ KO completely blocks tRNA maturation without diminishing the abundance of mature tRNA molecules. Our data indicate that growth arrest of endoreplicating cells is primarily attributed to the relocation of the pool of mature tRNAs into the nuclei causing a decrease in translation efficiency. Mitotically dividing cells appear to be less dependent on translation machinery as they maintain their normal size when deprived of dRNaseZ activity, but rather display a cell cycle arrest at the G2-M transition.