Roles for scalloped and vestigial in regulating cell affinity and interactions between the wing blade and the wing hinge

A mismatch in the expression of cell surface molecules induces tissue-intrinsic defense against aberrant cells

Tissue-intrinsic error correction enables epithelial cells to detect abnormal neighboring cells and facilitate their removal from the tissue. One of these pathways, "interface surveillance," is triggered by cells with aberrant developmental and cell-fate-patterning pathways. It remains unknown which molecular mechanisms provide cells with the ability to compare fate between neighboring cells. We demonstrate that Drosophila imaginal discs express an array of cell surface molecules previously implicated in neuronal axon guidance processes. They include members of the Robo, Teneurin, Ephrin, Toll-like, or atypical cadherin families. Importantly, a mismatch in expression levels of these cell surface molecules between adjacent cells is sufficient to induce interface surveillance, indicating that differences in expression levels between neighboring cells, rather than their absolute expression levels, are crucial. Specifically, a mismatch in Robo2 and Robo3, but not Robo1, induces enrichment of actin, myosin II, and Ena/Vasp, as well as activation of JNK and apoptosis at clonal interfaces. Moreover, Robo2 can induce interface surveillance independently of its cytosolic domain and without the need for the Robo-ligand Slit. The expression of Robo2 and other cell surface molecules, such as Teneurins or the Ephrin receptor is regulated by fate-patterning pathways intrinsic and extrinsic to the wing disc, as well as by expression of oncogenic RasV12. Combined, we demonstrate that neighboring cells respond to a mismatch in surface code patterns mediated by specific transmembrane proteins and reveal a novel function for these cell surface proteins in cell fate recognition and removal of aberrant cells during development and homeostasis of epithelial tissues.

TnaA, a trithorax group protein, modulates wingless expression in different regions of the Drosophila wing imaginal disc

wingless expression is exquisitely regulated by different factors and enhancers in the imaginal wing discs of Drosophila melanogaster in four domains: the dorsal band, the dorso-ventral boundary, and the inner and outer ring domains. tonalli is a trithorax group gene that encodes a putative SUMO E3 ligase that binds to chromatin to regulate the expression of its targets, including the Hox genes. However, its role in modulating gene expression is barely known. Here, we show that TnaA modulates the wingless expression at two domains of the wing disc, the dorso-ventral boundary and the inner ring. At first, tonalli interacts genetically with Notch to form the wing margin. In the inner ring domain, TnaA modulates wingless transcription. When the dosage of TnaA increases in or near the inner ring since early larval stages, this domain expands with a rapid increase in wingless expression. TnaA occupies the wingless Inner Ring Enhancer at the wing disc, meanwhile it does not affect wingless expression directed by the Ventral Disc Enhancer in leg discs, suggesting that TnaA acts as a wingless enhancer-specific factor. We describe for the first time the presence of TnaA at the Inner Ring Enhancer as a specific regulator of wingless in the development of wing boundaries.

Bilateral JNK activation is a hallmark of interface surveillance and promotes elimination of aberrant cells

Tissue-intrinsic defense mechanisms eliminate aberrant cells from epithelia and thereby maintain the health of developing tissues or adult organisms. 'Interface surveillance' comprises one such distinct mechanism that specifically guards against aberrant cells which undergo inappropriate cell fate and differentiation programs. The cellular mechanisms which facilitate detection and elimination of these aberrant cells are currently unknown. We find that in Drosophila imaginal discs, clones of cells with inappropriate activation of cell fate programs induce bilateral JNK activation at clonal interfaces, where wild type and aberrant cells make contact. JNK activation is required to drive apoptotic elimination of interface cells. Importantly, JNK activity and apoptosis are highest in interface cells within small aberrant clones, which likely supports the successful elimination of aberrant cells when they arise. Our findings are consistent with a model where clone size affects the topology of interface contacts and thereby the strength of JNK activation in wild type and aberrant interface cells. Bilateral JNK activation is unique to 'interface surveillance' and is not observed in other tissue-intrinsic defense mechanisms, such as classical 'cell-cell competition'. Thus, bilateral JNK interface signaling provides an independent tissue-level mechanism to eliminate cells with inappropriate developmental fate but normal cellular fitness. Finally, oncogenic Ras-expressing clones activate 'interface surveillance' but evade elimination by bilateral JNK activation. Combined, our work establishes bilateral JNK interface signaling and interface apoptosis as a new hallmark of interface surveillance and highlights how oncogenic mutations evade tumor suppressor function encoded by this tissue-intrinsic surveillance system.

The wing imaginal disc

The Drosophila wing imaginal disc is a tissue of undifferentiated cells that are precursors of the wing and most of the notum of the adult fly. The wing disc first forms during embryogenesis from a cluster of ∼30 cells located in the second thoracic segment, which invaginate to form a sac-like structure. They undergo extensive proliferation during larval stages to form a mature larval wing disc of ∼35,000 cells. During this time, distinct cell fates are assigned to different regions, and the wing disc develops a complex morphology. Finally, during pupal stages the wing disc undergoes morphogenetic processes and then differentiates to form the adult wing and notum. While the bulk of the wing disc comprises epithelial cells, it also includes neurons and glia, and is associated with tracheal cells and muscle precursor cells. The relative simplicity and accessibility of the wing disc, combined with the wealth of genetic tools available in Drosophila, have combined to make it a premier system for identifying genes and deciphering systems that play crucial roles in animal development. Studies in wing imaginal discs have made key contributions to many areas of biology, including tissue patterning, signal transduction, growth control, regeneration, planar cell polarity, morphogenesis, and tissue mechanics.

Elucidate growth control mechanisms using reconstructed spatiotemporal distributions of signaling events

A fundamental biological question is how diverse and complex signaling and patterning is controlled correctly to generate distinct tissues, organs, and body plans, but incorrectly in diseased cells and tissues. Signaling pathways important for growth control have been identified, but to identify the mechanisms their transient and context-dependent interactions encode is more difficult. Currently computational systems biology aims to infer the control mechanisms by investigating quantitative changes of gene expression and protein concentrations, but this inference is difficult in nature. We propose it is desirable to explicitly simulate events and orders of gene regulation and protein interactions, which better elucidate control mechanisms, and report a method and tool with three examples. The Drosophila wing model includes Wnt, PCP, and Hippo pathways and mechanical force, incorporates well-confirmed experimental findings, and generates novel results. The other two examples illustrate the building of three-dimensional and large-scale models. These examples support that reconstructed spatiotemporal distributions of key signaling events help elucidate growth control mechanisms. As biologists pay increasing attention to disordered signaling in diseased cells, to develop new modeling methods and tools for conducting new computational studies is important.

TEAD family transcription factors in development and disease

The balance between stem cell potency and lineage specification entails the integration of both extrinsic and intrinsic cues, which ultimately influence gene expression through the activity of transcription factors. One example of this is provided by the Hippo signalling pathway, which plays a central role in regulating organ size during development. Hippo pathway activity is mediated by the transcriptional co-factors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), which interact with TEA domain (TEAD) proteins to regulate gene expression. Although the roles of YAP and TAZ have been intensively studied, the roles played by TEAD proteins are less well understood. Recent studies have begun to address this, revealing that TEADs regulate the balance between progenitor self-renewal and differentiation throughout various stages of development. Furthermore, it is becoming apparent that TEAD proteins interact with other co-factors that influence stem cell biology. This Primer provides an overview of the role of TEAD proteins during development, focusing on their role in Hippo signalling as well as within other developmental, homeostatic and disease contexts.

A dynamic cell recruitment process drives growth of the Drosophila wing by overscaling the vestigial expression pattern

Organs mainly attain their size by cell growth and proliferation, but sometimes also grow through recruitment of undifferentiated cells. Here we investigate the participation of cell recruitment in establishing the pattern of Vestigial (Vg), the product of the wing selector gene in Drosophila. We find that the Vg pattern overscales along the dorsal-ventral (DV) axis of the wing imaginal disc, i.e., it expands faster than the DV length of the pouch. The overscaling of the Vg pattern cannot be explained by differential proliferation, apoptosis, or oriented-cell divisions, but can be recapitulated by a mathematical model that explicitly considers cell recruitment. When impairing cell recruitment genetically, we find that the Vg pattern almost perfectly scales and adult wings are approximately 20% smaller. Conversely, impairing cell proliferation results in very small wings, suggesting that cell recruitment and cell proliferation additively contribute to organ growth in this system. Furthermore, using fluorescent reporter tools, we provide direct evidence that cell recruitment is initiated between early and mid third-instar larval development. Altogether, our work quantitatively shows when, how, and by how much cell recruitment shapes the Vg pattern and drives growth of the Drosophila wing.

The Hippo Signaling Network and Its Biological Functions

Hippo signaling is an evolutionarily conserved network that has a central role in regulating cell proliferation and cell fate to control organ growth and regeneration. It promotes activation of the LATS kinases, which control gene expression by inhibiting the activity of the transcriptional coactivator proteins YAP and TAZ in mammals and Yorkie in Drosophila. Diverse upstream inputs, including both biochemical cues and biomechanical cues, regulate Hippo signaling and enable it to have a key role as a sensor of cells' physical environment and an integrator of growth control signals. Several components of this pathway localize to cell-cell junctions and contribute to regulation of Hippo signaling by cell polarity, cell contacts, and the cytoskeleton. Downregulation of Hippo signaling promotes uncontrolled cell proliferation, impairs differentiation, and is associated with cancer. We review the current understanding of Hippo signaling and highlight progress in the elucidation of its regulatory mechanisms and biological functions.

Interface Contractility between Differently Fated Cells Drives Cell Elimination and Cyst Formation

Although cellular tumor-suppression mechanisms are widely studied, little is known about mechanisms that act at the level of tissues to suppress the occurrence of aberrant cells in epithelia. We find that ectopic expression of transcription factors that specify cell fates causes abnormal epithelial cysts in Drosophila imaginal discs. Cysts do not form cell autonomously but result from the juxtaposition of two cell populations with divergent fates. Juxtaposition of wild-type and aberrantly specified cells induces enrichment of actomyosin at their entire shared interface, both at adherens junctions as well as along basolateral interfaces. Experimental validation of 3D vertex model simulations demonstrates that enhanced interface contractility is sufficient to explain many morphogenetic behaviors, which depend on cell cluster size. These range from cyst formation by intermediate-sized clusters to segregation of large cell populations by formation of smooth boundaries or apical constriction in small groups of cells. In addition, we find that single cells experiencing lateral interface contractility are eliminated from tissues by apoptosis. Cysts, which disrupt epithelial continuity, form when elimination of single, aberrantly specified cells fails and cells proliferate to intermediate cell cluster sizes. Thus, increased interface contractility functions as error correction mechanism eliminating single aberrant cells from tissues, but failure leads to the formation of large, potentially disease-promoting cysts. Our results provide a novel perspective on morphogenetic mechanisms, which arise from cell-fate heterogeneities within tissues and maintain or disrupt epithelial homeostasis.

JAK/STAT signaling is required for hinge growth and patterning in the Drosophila wing disc

JAK/STAT signaling is localized to the wing hinge, but its function there is not known. Here we show that the Drosophila STAT Stat92E is downstream of Homothorax and is required for hinge development by cell-autonomously regulating hinge-specific factors. Within the hinge, Stat92E activity becomes restricted to gap domain cells that lack Nubbin and Teashirt. While gap domain cells lacking Stat92E have significantly reduced proliferation, increased JAK/STAT signaling there does not expand this domain. Thus, this pathway is necessary but not sufficient for gap domain growth. We show that reduced Wingless (Wg) signaling dominantly inhibits Stat92E activity in the hinge. However, ectopic JAK/STAT signaling does not perturb Wg expression in the hinge. We report negative interactions between Stat92E and the notum factor Araucan, resulting in restriction of JAK/STAT signaling from the notum. In addition, we find that the distal factor Nub represses the ligand unpaired as well as Stat92E activity. These data suggest that distal expansion of JAK/STAT signaling is deleterious to wing blade development. Indeed, mis-expression of Unpaired within the presumptive wing blade causes small, stunted adult wings. We conclude that JAK/STAT signaling is critical for hinge fate specification and growth of the gap domain and that its restriction to the hinge is required for proper wing development.

The Hippo kinase promotes Scalloped cytoplasmic localization independently of Warts in a CRM1/Exportin1-dependent manner in Drosophila

Scalloped (SD) is a transcription factor characterized by a TEA/ATTS DNA binding domain. To activate transcription, SD must interact with its coactivators, including Yorkie (YKI) or Vestigial (VG). YKI is the downstream effector of the Hippo signaling pathway that plays a key role in the control of tissue growth. The core components of this pathway are two kinases, Hippo (HPO) and Warts (WTS), which negatively regulate the activity of the SD/YKI complex, retaining YKI in the cytoplasm. We previously showed that HPO kinase can also reduce SD/VG transcriptional activity in Drosophila S2 cells. We further investigated the relationship between the SD/VG complex and the Hippo pathway. We show here that HPO overexpression suppresses overgrowth induced by SD/VG in vivo during Drosophila development. Using S2 cells, we show that HPO promotes the translocation of SD to the cytoplasm in a CRM1-dependent manner, thereby inhibiting the induction of SD/VG target genes. Using RNAi-mediated depletion of yki and a mutant SD protein unable to interact with YKI, we demonstrate that HPO regulates SD localization independently of YKI. This function requires HPO kinase activity, yet surprisingly, not its downstream effector kinase WTS. Taken together, these observations reveal a new and unexpected role of HPO kinase in the regulation of a transcription factor independently of YKI.

The Dorsocross T-box transcription factors promote tissue morphogenesis in the Drosophila wing imaginal disc

The Drosophila wing imaginal disc is subdivided into notum, hinge and blade territories during the third larval instar by formation of several deep apical folds. The molecular mechanisms of these subdivisions and the subsequent initiation of morphogenic processes during metamorphosis are poorly understood. Here, we demonstrate that the Dorsocross (Doc) T-box genes promote the progression of epithelial folds that not only separate the hinge and blade regions of the wing disc but also contribute to metamorphic development by changing cell shapes and bending the wing disc. We found that Doc expression was restricted by two inhibitors, Vestigial and Homothorax, leading to two narrow Doc stripes where the folds separating hinge and blade are forming. Doc mutant clones prevented the lateral extension and deepening of these folds at the larval stage and delayed wing disc bending in the early pupal stage. Ectopic Doc expression was sufficient to generate deep apical folds by causing a basolateral redistribution of the apical microtubule web and a shortening of cells. Cells of both the endogenous blade/hinge folds and of folds elicited by ectopic Doc expression expressed Matrix metalloproteinase 2 (Mmp2). In these folds, integrins and extracellular matrix proteins were depleted. Overexpression of Doc along the blade/hinge folds caused precocious wing disc bending, which could be suppressed by co-expressing MMP2RNAi.

Integration of morphogen signalling within the growth regulatory network

The need to coordinate patterning and growth has been appreciated for many years. The logic that enables seamless integration of the relevant inputs is beginning to be elucidated, particularly in wing imaginal discs of Drosophila. In this tissue, multiple regulatory layers involving the two morphogens Wingless and Dpp, the wing-specific determinant, Vestigial, and the Hippo pathway, converge to regulate growth. Intricate cross-regulation between these components may explain why, at the local level, there is no direct correlation between growth and the graded signalling activity of Wingless and Dpp, despite the requirement of these two pathways for growth.

Hippo signaling in Drosophila: recent advances and insights

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

Spatiotemporally modulated Vestigial gradient by Wingless signaling adaptively regulates cell division for precise wing size control

In animal development, the growth of a tissue or organ is timely arrested when it reaches the stereotyped correct size. How this is robustly controlled remains poorly understood. The prevalent viewpoint, which is that morphogen gradients, due to their organizing roles in development, are directly responsible for growth arrest, cannot explain a number of observations. Recent findings from studies of the Drosophila wing have revealed that the interpretation of the Wingless gradient requires signaling-induced self-inhibition and that cell proliferation is controlled by graded vestigial expression. These findings highlight a growth control mechanism that involves Wingless regulated vestigial expression, but a question is whether they can quantitatively explain the observed precision and robustness of wing size control. Quantitative and systematic investigation into Wingless signaling using a mathematical model has elucidated two points. First, negative regulation of the Vestigial gradient by Wingless signaling makes vestigial expression precise and robust. Second, weak Wingless signaling in a primarily small wing pouch causes a short and steep Vestigial gradient, which stimulates more cell divisions and leads to a significant expansion of the wing pouch; however, strong Wingless signaling in a primarily large wing pouch causes a long and smooth Vestigial gradient, which stimulates fewer cell divisions and results in a slight expansion of the wing pouch. These results substantially decipher an inherent mechanism of tissue and organ size control. Our model explains, and is supported by, a number of experimental observations.

Hippo signaling: growth control and beyond

The Hippo pathway has emerged as a conserved signaling pathway that is essential for the proper regulation of organ growth in Drosophila and vertebrates. Although the mechanisms of signal transduction of the core kinases Hippo/Mst and Warts/Lats are relatively well understood, less is known about the upstream inputs of the pathway and about the downstream cellular and developmental outputs. Here, we review recently discovered mechanisms that contribute to the dynamic regulation of Hippo signaling during Drosophila and vertebrate development. We also discuss the expanding diversity of Hippo signaling functions during development, discoveries that shed light on a complex regulatory system and provide exciting new insights into the elusive mechanisms that regulate organ growth and regeneration.

Evidence for a growth-stabilizing regulatory feedback mechanism between Myc and Yorkie, the Drosophila homolog of Yap

An understanding of how animal size is controlled requires knowledge of how positive and negative growth regulatory signals are balanced and integrated within cells. Here we demonstrate that the activities of the conserved growth-promoting transcription factor Myc and the tumor-suppressing Hippo pathway are codependent during growth of Drosophila imaginal discs. We find that Yorkie (Yki), the Drosophila homolog of the Hippo pathway transducer, Yap, regulates the transcription of Myc, and that Myc functions as a critical cellular growth effector of the pathway. We demonstrate that in turn, Myc regulates the expression of Yki as a function of its own cellular level, such that high levels of Myc repress Yki expression through both transcriptional and posttranscriptional mechanisms. We propose that the codependent regulatory relationship functionally coordinates the cellular activities of Yki and Myc and provides a mechanism of growth control that regulates organ size and has broad implications for cancer.

Reexamining the DNA target selectivity of Scalloped

Selector proteins are transcription factors that coordinate the formation and identity of organs and appendages. The proper formation of these tissues requires the selector proteins to regulate the expression of a large set of genes. Many selector proteins are involved in regulating multiple developmental processes, yet it is not completely clear how they are able to activate different sets of genes in a tissue-specific manner. An association with cofactors is thought to be one method by which enhancer selectivity is achieved. During wing development the selector protein Scalloped (SD) interacts with the cofactor Vestigial (VG). This interaction leads to the activation of a specific set of downstream wing genes. Herein, data are presented indicating that the switch in binding selectivity is likely achieved by VG altering the general affinity that the SD protein has for DNA. The decreased affinity for DNA is compensated for by the fact that the VG protein forms a complex containing two SD proteins. These two properties ensure that the SD–VG complex is able to bind only to enhancers that have two consecutive binding sites. Furthermore, data are presented that indicate that the function of the two terminal domains of the VG protein is not restricted to activating transcription and promoting the recruitment of two SD proteins.

Yorkie: the final destination of Hippo signaling

The Hippo signaling pathway is a key regulator of growth during animal development, whereas loss of normal Hippo pathway activity is associated with a wide range of cancers. Hippo signaling represses growth by inhibiting the activity of a transcriptional co-activator protein, known as Yorkie in Drosophila and Yap in vertebrates. In the 5 years since the first report linking Yorkie to Hippo signaling, intense interest in this pathway has led to rapid increases in our understanding of the action and regulation of Yorkie/Yap, which we review here. These studies have also emphasized the complexity of Yorkie/Yap regulation, including multiple, distinct mechanisms for repressing its transcriptional activity, and multiple DNA-binding partner proteins that can direct Yorkie to distinct downstream target genes.

Spatial discontinuity of optomotor-blind expression in the Drosophila wing imaginal disc disrupts epithelial architecture and promotes cell sorting

Background

Decapentaplegic (Dpp) is one of the best characterized morphogens, required for dorso-ventral patterning of the Drosophila embryo and for anterior-posterior (A/P) patterning of the wing imaginal disc. In the larval wing pouch, the Dpp target gene optomotor-blind (omb) is generally assumed to be expressed in a step function above a certain threshold of Dpp signaling activity.

Results

We show that the transcription factor Omb forms, in fact, a symmetrical gradient on both sides of the A/P compartment boundary. Disruptions of the Omb gradient lead to a re-organization of the epithelial cytoskeleton and to a retraction of cells toward the basal membrane suggesting that the Omb gradient is required for correct epithelial morphology. Moreover, by analysing the shape of omb gain- and loss-of-function clones, we find that Omb promotes cell sorting along the A/P axis in a concentration-dependent manner.

Conclusions

Our findings show that Omb distribution in the wing imaginal disc is described by a gradient rather than a step function. Graded Omb expression is necessary for normal cell morphogenesis and cell affinity and sharp spatial discontinuities must be avoided to allow normal wing development.

Mechanisms of growth and homeostasis in the Drosophila wing

Animal shape and size is controlled with amazing precision during development. External factors such as nutrient availability and crowding can alter overall animal size, but individual body parts scale reproducibly to match the body even with challenges from a changing environment. How is such precision achieved? Here, we review selected research from the last few years in Drosophila--arguably the premier genetic model for the study of animal growth--that sheds light on how body and tissue size are regulated by forces intrinsic to individual organs. We focus on two topics currently under intense study: the influence of pattern regulators on organ and tissue growth and the role of local competitive interactions between cells in tissue homeostasis and final size.

Temporal and spatial windows delimit activation of the outer ring of wingless in the Drosophila wing

Extracellular signalling molecules play many roles in the development of higher organisms. They are used reiteratively in different tissues and stages, but the response of the receiving cells is controlled in a context dependent manner. The pattern of expression of the signalling molecule Wingless/WNT in Drosophila is extraordinarily complex. We have studied the mechanism that controls its expression and function in the outer ring of the Drosophila wing hinge. Our findings indicate that wingless expression is controlled by a dual mechanism: its initial activation requires the product of zinc finger homeodomain 2 and is subsequently repressed by the product of the gene complex elbow/no ocelli. This tight regulation restricts the activation of wingless temporally and spatially. Later in development, wingless expression is maintained by an autoregulatory loop that involves the product of homothorax. We have analyzed the phenotype of a wingless allelic combination that specifically removes the outer ring, and our results show that Wingless is required to promote local proliferation of the wing base cells. Thus, cell proliferation in the proximal-distal axis is controlled by the sequential activation of wingless in the inner ring and the outer ring at different stages of development.

The Fat and Warts signaling pathways: new insights into their regulation,mechanism and conservation

A cassette of cytoplasmic Drosophila tumor suppressors, including the kinases Hippo and Warts, has recently been linked to the transmembrane tumor suppressor Fat. These proteins act within interconnected signaling pathways, the principal functions of which are to control the growth and polarity of developing tissues. Recent studies have enhanced our understanding of the basis for signal transduction by Fat and Warts pathways, including the identification of a DNA-binding protein at the end of the pathway, have established the conservation of Fat and Warts signaling from flies to mammals,and have given us new insights into their regulation and biological functions.

The Drosophila gene zfh2 is required to establish proximal-distal domains in the wing disc

Three main events characterize the development of the proximal-distal axis of the Drosophila wing disc: first, generation of nested circular domains defined by different combinations of gene expression; second, activation of wingless (wg) gene expression in a ring of cells; and third, an increase of cell number in each domain in response to Wg. The mechanisms by which these domains of gene expression are established and maintained are unknown. We have analyzed the role of the gene zinc finger homeodomain 2 (zfh2). We report that in discs lacking zfh2 the limits of the expression domains of the genes tsh, nub, rn, dve and nab coincide, and expression of wg in the wing hinge, is lost. We show that zfh2 expression is delimited distally by Vg, Nub and Dpp signalling, and proximally by Tsh and Dpp. Distal repression of zfh2 permits activation of nab in the wing blade and wg in the wing hinge. We suggest that the proximal-most wing fate, the hinge, is specified first and that later repression of zfh2 permits specification of the distal-most fate, the wing blade. We propose that proximal-distal axis development is achieved by a combination of two strategies: on one hand a process involving proximal to distal specification, with the wing hinge specified first followed later by the distal wing blade; on the other hand, early specification of the proximal-distal domains by different combinations of gene expression. The results we present here indicate that Zfh2 plays a critical role in both processes.

SCALLOPED interacts with YORKIE, the nuclear effector of the hippo tumor-suppressor pathway in Drosophila

In Drosophila, SCALLOPED (SD) belongs to a family of evolutionarily conserved proteins characterized by the presence of a TEA/ATTS DNA-binding domain [1, 2]. SD physically interacts with the product of the vestigial (vg) gene, where the dimer functions as a master gene controlling wing formation [3, 4]. The VG-SD dimer activates the transcription of several specific wing genes, including sd and vg themselves [5, 6]. The dimer drives cell-cycle progression by inducing expression of the dE2F1 transcription factor [7], which regulates genes involved in DNA replication and cell-cycle progression. Recently, YORKIE (YKI) was identified as a transcriptional coactivator that is the downstream effector of the Hippo signaling pathway, which controls cell proliferation and apoptosis in Drosophila[8]. We identified SD as a partner for YKI. We show that interaction between YKI and SD increases SD transcriptional activity both ex vivo in Drosophila S2 cells and in vivo in Drosophila wing discs and promotes YKI nuclear localization. We also show that YKI overexpression induces vg and dE2F1 expression and that proliferation induced by YKI or by a dominant-negative form of FAT in wing disc is significantly reduced in a sd hypomorphic mutant context. Contrary to YKI, SD is not required in all imaginal tissues. This indicates that YKI-SD interaction acts in a tissue-specific fashion and that other YKI partners must exist.

The TEAD/TEF family of transcription factor Scalloped mediates Hippo signaling in organ size control

The Hippo (Hpo) signaling pathway governs cell growth, proliferation, and apoptosis by controlling key regulatory genes that execute these processes; however, the transcription factor of the pathway has remained elusive. Here we provide evidence that the TEAD/TEF family transcription factor Scalloped (Sd) acts together with the coactivator Yorkie (Yki) to regulate Hpo pathway-responsive genes. Sd and Yki form a transcriptional complex whose activity is inhibited by Hpo signaling. Sd overexpression enhances, whereas its inactivation suppresses, tissue overgrowth caused by Yki overexpression or tumor suppressor mutations in the Hpo pathway. Inactivation of Sd diminishes Hpo target gene expression and reduces organ size, whereas a constitutively active Sd promotes tissue overgrowth. Sd promotes Yki nuclear localization, whereas Hpo signaling retains Yki in the cytoplasm by phosphorylating Yki at S168. Finally, Sd recruits Yki to the enhancer of the pathway-responsive gene diap1, suggesting that diap1 is a direct transcriptional target of the Hpo pathway.

Drosophila T-box transcription factor Optomotor-blind prevents pathological folding and local overgrowth in wing epithelium through confining Hh signal

Aberration of morphogen signaling leads directly to inappropriate cell differentiation and secondarily causes various pathological phenotypes such as abnormal morphogenesis and tumorigenesis. However, mechanisms for linking morphogen signaling and the higher order phenotypes have not been fully elucidated. Here we focus on the Drosophila T-box gene optomotor-blind (omb), a transcriptional target of a long-range morphogen Decapentaplegic (Dpp). Genetic analyses of omb function revealed that a negative feedback loop, where omb plays a crucial role, exists between Dpp and its upstream regulator Hedgehog (Hh), a short-range morphogen. Consequently, dysfunction of omb elicits hyperactivation of Hh signaling that causes an ectopic folding and local overgrowth in the wing columnar epithelium, neither of which are the direct results of reduced Dpp response. In the case of the local overgrowth, it was never seen in mutants for thick veins (tkv) encoding a Dpp receptor, suggesting that the Dpp signaling pathway is divided into two antagonistic branches, one of which contains Omb. Thus defect in feedback between the two morphogens explains both phenotypes, and disruption of a balance between the morphogen targets further accounts for the local overgrowth. These are the mechanisms for generating secondary phenotypes when a single signaling factor Omb fails to function.

Recruitment of cells into the Drosophila wing primordium by a feed-forward circuit of vestigial autoregulation

The Drosophila wing primordium is defined by expression of the selector gene vestigial (vg) in a discrete subpopulation of cells within the wing imaginal disc. Following the early segregation of the disc into dorsal (D) and ventral (V) compartments, vg expression is governed by signals generated along the boundary between the two compartments. Short-range DSL (Delta/Serrate/LAG-2)-Notch signaling between D and V cells drives vg expression in ;border' cells that flank the boundary. It also induces these same cells to secrete the long-range morphogen Wingless (Wg), which drives vg expression in surrounding cells up to 25-30 cell diameters away. Here, we show that Wg signaling is not sufficient to activate vg expression away from the D-V boundary. Instead, Wg must act in combination with a short-range signal produced by cells that already express vg. We present evidence that this vg-dependent, vg-inducing signal feeds forward from one cell to the next to entrain surrounding cells to join the growing wing primordium in response to Wg. We propose that Wg promotes the expansion of the wing primordium following the D-V segregation by fueling this non-autonomous autoregulatory mechanism.

Control of Drosophila wing growth by the vestigial quadrant enhancer

Following segregation of the Drosophila wing imaginal disc into dorsal (D) and ventral (V) compartments, the wing primordium is specified by activity of the selector gene vestigial (vg). In the accompanying paper, we present evidence that vg expression is itself driven by three distinct inputs: (1) short-range DSL (Delta/Serrate/LAG-2)-Notch signaling across the D-V compartment boundary; (2) long-range Wg signaling from cells abutting the D-V compartment boundary; and (3) a short-range signal sent by vg-expressing cells that entrains neighboring cells to upregulate vg in response to Wg. Furthermore, we showed that these inputs define a feed-forward mechanism of vg autoregulation that initiates in D-V border cells and propagates from cell to cell by reiterative cycles of vg upregulation. Here, we provide evidence that this feed-forward mechanism is required for normal wing growth and is mediated by two distinct enhancers in the vg gene. The first is a newly defined ;priming' enhancer (PE), that provides cryptic, low levels of Vg in most or all cells of the wing disc. The second is the previously defined quadrant enhancer (QE), which we show is activated by the combined action of Wg and the short-range vg-dependent entraining signal, but only if the responding cells are already primed by low-level Vg activity. Thus, entrainment and priming constitute distinct signaling and responding events in the Wg-dependent feed-forward circuit of vg autoregulation mediated by the QE. We posit that Wg controls the expansion of the wing primordium following D-V segregation by fueling this autoregulatory mechanism.

Nab controls the activity of the zinc-finger transcription factors Squeeze and Rotund in Drosophila development

Nab proteins form an evolutionarily conserved family of transcriptional co-regulators implicated in multiple developmental events in various organisms. They lack DNA-binding domains and act by associating with other transcription factors, but their precise roles in development are not known. Here we analyze the role of nab in Drosophila development. By employing genetic approaches we found that nab is required for proximodistal patterning of the wing imaginal disc and also for determining specific neuronal fates in the embryonic CNS. We identified two partners of Nab: the zinc-finger transcription factors Rotund and Squeeze. Nab is co-expressed with squeeze in a subset of neurons in the embryonic ventral nerve cord and with rotund in a circular domain of the distal-most area of the wing disc. Our results indicate that Nab is a co-activator of Squeeze and is required to limit the number of neurons that express the LIM-homeodomain gene apterous and to specify Tv neuronal fate. Conversely, Nab is a co-repressor of Rotund in wing development and is required to limit the expression of wingless (wg) in the wing hinge, where wg plays a mitogenic role. We also showed by pull-down assays that Nab binds directly to Rotund and Squeeze via its conserved C-terminal domain. We propose two mechanisms by which the activation of wg expression by Rotund in the wing hinge is repressed in the distal wing.

Antagonizing scalloped with a novel vestigial construct reveals an important role for scalloped in Drosophila melanogaster leg, eye and optic lobe development

Scalloped (SD), a TEA/ATTS-domain-containing protein, is required for the proper development of Drosophila melanogaster. Despite being expressed in a variety of tissues, most of the work on SD has been restricted to understanding its role and function in patterning the adult wing. To gain a better understanding of its role in development, we generated sd(47M) flip-in mitotic clones. The mitotic clones had developmental defects in the leg and eye. Further, by removing the VG domains involved in activation, we created a reagent (VGDeltaACT) that disrupts the ability of SD to form a functional transcription factor complex and produced similar phenotypes to the flip-in mitotic clones. The VGDeltaACT construct also disrupted adult CNS development. Expression of the VGDeltaACT construct in the wing alters the cellular localization of VG and produces a mutant phenotype, indicating that the construct is able to antagonize the normal function of the SD/VG complex. Expression of the protein:protein interaction portion of SD is also able to elicit similar phenotypes, suggesting that SD interacts with other cofactors in the leg, eye, and adult CNS. Furthermore, antagonizing SD in larval tissues results in cell death, indicating that SD may also have a role in cell survival.

Nubbin and Teashirt mark barriers to clonal growth along the proximal-distal axis of the Drosophila wing

The division of the wing imaginal disc into anterior, posterior, dorsal, and ventral compartments is a critical step in Drosophila wing morphogenesis. Here, we investigate the existence of cell lineage restrictions along the proximal-distal (PD) axis of the wing disc. We rule out the existence of classical compartment boundaries in the hinge region, but demonstrate that there are clonal restrictions corresponding to the expression domains of two transcription factors, Nubbin (Nub) and Teashirt (Tsh), present in distal and proximal cells, respectively. Unlike classical compartments, the Nub and Tsh domains do not define absolute lineage restrictions. Instead, due to regulation by Wingless signaling, the Nub and Tsh expression boundaries shift during development. Once established, the Nub and Tsh domains, and the intervening region in which neither factor is expressed, grow independently, because the progeny of cells present in one domain do not freely populate an adjacent domain. We also show that despite shifting position, the Nub and Tsh domain boundaries, like compartment boundaries, impact the expression of secreted signaling molecules. Thus, like the vein/intervein divisions of the wing and mammalian rhombomeres, the Nub and Tsh domains share some of the attributes of classical compartments, but lack their stringent and immobile boundaries.

Genes affecting cell competition in Drosophila

Cell competition is a homeostatic mechanism that regulates the size attained by growing tissues. We performed an unbiased genetic screen for mutations that permit the survival of cells being competed due to haplo-insufficiency for RpL36. Mutations that protect RpL36 heterozygous clones include the tumor suppressors expanded, hippo, salvador, mats, and warts, which are members of the Warts pathway, the tumor suppressor fat, and a novel tumor-suppressor mutation. Other hyperplastic or neoplastic mutations did not rescue RpL36 heterozygous clones. Most mutations that rescue cell competition elevated Dpp-signaling activity, and the Dsmurf mutation that elevates Dpp signaling was also hyperplastic and rescued. Two nonlethal, nonhyperplastic mutations prevent the apoptosis of Minute heterozygous cells and suggest an apoptosis pathway for cell competition . In addition to rescuing RpL36 heterozygous cells, mutations in Warts pathway genes were supercompetitors that could eliminate wild-type cells nearby. The findings show that differences in Warts pathway activity can lead to competition and implicate the Warts pathway, certain other tumor suppressors, and novel cell death components in cell competition, in addition to the Dpp pathway implicated by previous studies. We suggest that cell competition might occur during tumor development in mammals.

Control of growth and positional information by the graded vestigial expression pattern in the wing of Drosophila melanogaster

The size and shape of organs depend on cellular processes such as cell proliferation, cell survival, and spatial arrangement of cells. In turn, all of these processes are a consequence of positional identity of individual cells in whole organs. Links of positional information with organ growth and pattern expression of genes is a little-addressed question. We show that differences in vestigial expression between neighboring cells of the wing blade autonomously and nonautonomously affect cell proliferation along the proximo-distal axis. On the other hand, uniform expression of vestigial inhibits cell proliferation and also perturbs the shape of wing blade altering the preferential orientation of cell divisions. Our observations provide evidence that local cell interactions, triggered by differences in vestigial expression between neighboring cells, confer positional values operating in the control of growth and shape of the wing.

Chameau HAT and DRpd3 HDAC function as antagonistic cofactors of JNK/AP-1-dependent transcription during Drosophila metamorphosis

Gene regulation by AP-1 transcription factors in response to Jun N-terminal kinase (JNK) signaling controls essential cellular processes during development and in pathological situations. Here, we report genetic and molecular evidence that the histone acetyltransferase (HAT) Chameau and the histone deacetylase DRpd3 act as antagonistic cofactors of DJun and DFos to modulate JNK-dependent transcription during thorax metamorphosis and JNK-induced apoptosis in Drosophila. We demonstrate in cultured cells that DFos phosphorylation mediated by JNK signaling plays a central role in coordinating the dynamics of Chameau and DRpd3 recruitment and function at AP-1-responsive promoters. Activating the pathway stimulates the HAT function of Chameau, promoting histone H4 acetylation and target gene transcription. Conversely, in response to JNK signaling inactivation, DRpd3 is recruited and suppresses histone acetylation and transcription. This study establishes a direct link among JNK signaling, DFos phosphorylation, chromatin modification, and AP-1-dependent transcription and its importance in a developing organism.

Wing-to-Leg homeosis by spineless causes apoptosis regulated by Fish-lips, a novel leucine-rich repeat transmembrane protein

Growth, patterning, and apoptosis are mutually interactive during development. For example, cells that select an abnormal fate in a developing field are frequently removed by apoptosis. An important issue in this process that needs to be resolved is the mechanism used by cells to discern their correct fate from an abnormal fate. In order to examine this issue, we developed an animal model that expresses the dioxin receptor homolog Spineless (Ss) ectopically in the Drosophila wing. The presence of mosaic clones ectopically expressing ss results in a local transformation of organ identity, homeosis, from wing into a leg or antenna. The cells with misspecified fates subsequently activate c-Jun N-terminal kinase to undergo apoptosis in an autonomous or nonautonomous manner depending on their position within the wing, suggesting that a cell-cell interaction is, at least in some cases, involved in the detection of misspecified cells. Similar position dependence is commonly observed when various homeotic genes controlling the body segments are ectopically expressed. The autonomous and nonautonomous apoptosis caused by ss is regulated by a novel leucine-rich repeat family transmembrane protein, Fish-lips (Fili) that interacts with surrounding normal cells. These data support a mechanism in which the lack of some membrane proteins helps to recognize the presence of different cell types and direct these cells to an apoptotic fate in order to exclude them from the normal developing field.

Ability of scalloped deletion constructs to rescue sd mutant wing phenotypes in Drosophila melanogaster

Scalloped (SD) and Vestigial (VG) proteins physically interact to form a selector complex that activates genes involved in wing development in Drosophila melanogaster. SD belongs to a conserved family of transcription factors containing the TEA/ATTS DNA-binding motif. VG is also a nuclear protein providing the activator function for the SD VG complex. The TEA DNA-binding domain and the VG interacting domain (VID) of SD have been previously identified and described. However, they, and possibly other functional domains of SD, have not been thoroughly characterized in vivo. Herein, transgenic constructs encoding various truncations of SD were used to assess their respective ability to rescue the mutant wing phenotype of two viable sd recessive mutations (sd(ETX4) and sd(58d)). The transgenic strains produced were also tested for the ability to induce further sd expression, an ability possessed by full length SD. The functional dissection of SD confirms that specific regions are necessary for wing development and provides further information as to how the SD VG complex functions to promote wing fate. Previous experiments have shown that expression of full length SD can cause a dominant negative wing phenotype. We show that expression of constructs that delete the SD DNA-binding domain can also cause a dominant negative phenotype in a background with either of the two tester sd strains. In contrast, SD constructs that delete the VID have no effect on the wing phenotype in either tester background. Finally, a significant portion of SD at the N-terminal end appears to be dispensable with respect to normal wing development, as this construct behaves the same as full length SD in our assays.

Action of fat, four-jointed, dachsous and dachs in distal-to-proximal wing signaling

In the Drosophila wing, distal cells signal to proximal cells to induce the expression of Wingless, but the basis for this distal-to-proximal signaling is unknown. Here, we show that three genes that act together during the establishment of tissue polarity, fat, four-jointed and dachsous, also influence the expression of Wingless in the proximal wing. fat is required cell autonomously by proximal wing cells to repress Wingless expression, and misexpression of Wingless contributes to proximal wing overgrowth in fat mutant discs. Four-jointed and Dachsous can influence Wingless expression and Fat localization non-autonomously, consistent with the suggestion that they influence signaling to Fat-expressing cells. We also identify dachs as a gene that is genetically required downstream of fat, both for its effects on imaginal disc growth and for the expression of Wingless in the proximal wing. Our observations provide important support for the emerging view that Four-jointed, Dachsous and Fat function in an intercellular signaling pathway,identify a normal role for these proteins in signaling interactions that regulate growth and patterning of the proximal wing, and identify Dachs as a candidate downstream effector of a Fat signaling pathway.

The Drosophila wing differentiation factor Vestigial–Scalloped is required for cell proliferation and cell survival at the dorso-ventral boundary of the wing imaginal disc

Links between genes involved in development, proliferation and apoptosis have been difficult to establish. In the Drosophila wing disc, the vestigial (vg) and the scalloped (sd) gene products dimerize to form a functional transcription factor. Ectopic expression of vg in other imaginal discs induces outgrowth and wing tissue specification. We investigated the role of the VG-SD dimer in proliferation and showed that vg antagonizes the effect of dacapo, the cyclin-cdk inhibitor. Moreover, ectopic vg drives cell cycle progression and in HeLa cultured cells, the VG-SD dimer induces cell proliferation per se. In Drosophila, ectopic vg induces expression of dE2F1 and its targets dRNR2 and string. In addition vg, but not dE2F1, interacts with and induces expression of dihydrofolate reductase (DHFR). Moreover, a decrease in VG or addition of aminopterin, a specific DHFR inhibitor, shift the dorso-ventral boundary cells of the disc to a cell death sensitive state that is correlated with reaper induction and DIAP1 downregulation. This indicates that vg in interaction with dE2F1 and DHFR is a critical player for both cell proliferation and cell survival in the presumptive wing margin area.

Defective proventriculus is required for pattern formation along the proximodistal axis, cell proliferation and formation of veins in the Drosophila wing

Many genes have been identified that are required for the establishment of the dorsoventral (DV) and anteroposterior (AP) axes of the Drosophila wing. By contrast, little is known about the genes and mechanisms that pattern the proximodistal (PD) axis. Vestigial (Vg) is instrumental in patterning this axis, but the genes that mediate its effects and the mechanisms that operate during PD patterning are not known. We show that the gene defective proventriculus (dve) is required for a region of the PD axis encompassing the distal region of the proximal wing (PW) and a small part of the adjacent wing pouch. Loss-of-function of dve results in the deletion of this region and, consequently, shortening of the PD axis. dve expression is activated by Vg in a non-autonomous manner, and is repressed at the DV boundary through the combined activity of Nubbin and Wg. Besides its role in the establishment of the distal part of the PW, dve is also required for the formation of the wing veins 2 and 5, and the proliferation of wing pouch cells, especially in regions anterior to wing vein 3 and posterior to wing vein 4. The study of the regulation of dve expression provides information about the strategies employed to subdivide and pattern the PD axis, and reveals the importance of vg during this process.

Further developmental roles of the Vestigial/Scalloped transcription complex during wing development in Drosophila melanogaster

The Drosophila homologue of the human TEF-1 gene, scalloped (sd), is required for wing development. The SD protein forms part of a transcriptional activation complex with the protein encoded by vestigial (vg) that, in turn, activates target genes important for wing formation. One sd function involves a regulatory feedback loop with vg and wingless (wg) that is essential in this process. The dorsal-ventral (D/V) margin-specific expression of wg is lost in sd mutant wing discs while the hinge-specific expression appears normal. In the context of wing development, a VG::sdTEA domain fusion produces a protein that mimics the wild-type SD/VG complex and restores the D/V boundary-specific expression of wg in a sd mutant background. Further, targeted expression of wg at the D/V boundary in the wing disc was able to partially rescue the sd mutant phenotype. This infers that sd could function in either the maintenance or induction of wg at the D/V border. Another functional role for sd is the establishment of sensory organ precursors (SOP) of the peripheral nervous system at the wing margin. Thus, the relationship between sd and senseless (sens) in the development of these cells is also examined, and it appears that sd must be functional for proper sens expression, and ultimately, for sensory organ precursor development.

An in vivo analysis of the vestigial gene in Drosophila melanogaster defines the domains required for Vg function

Considerable evidence indicates an obligate partnership of the Drosophila melanogaster Vestigial (VG) and Scalloped (SD) proteins within the context of wing development. These two proteins interact physically and a 56-amino-acid motif within VG is necessary and sufficient for this binding. While the importance of this SD-binding domain has been clearly demonstrated both in vitro and in vivo, the remaining portions of VG have not been examined for in vivo function. Herein, additional regions within VG were tested for possible in vivo functions. The results identify two additional domains that must be present for optimal VG function as measured by the loss of ability to rescue vg mutants, to induce ectopic sd expression, and to perform other normal VG functions when they are deleted. An in vivo study such as this one is fundamentally important because it identifies domains of VG that are necessary in the cellular context in which wing development actually occurs. The results also indicate that an additional large portion of VG, outside of these two domains and the SD-binding domain, is dispensable in the execution of these normal VG functions.

Regulation of Wingless and Vestigial expression in wing and haltere discs of Drosophila

In the third thoracic segment of Drosophila, wing development is suppressed by the homeotic selector gene Ultrabithorax (Ubx) in order to mediate haltere development. Previously, we have shown that Ubx represses dorsoventral (DV) signaling to specify haltere fate. Here we examine the mechanism of Ubx-mediated downregulation of DV signaling. We show that Wingless (Wg) and Vestigial (Vg) are differentially regulated in wing and haltere discs. In wing discs, although Vg expression in non-DV cells is dependent on DV boundary function of Wg, it maintains its expression by autoregulation. Thus, overexpression of Vg in non-DV cells can bypass the requirement for Wg signaling from the DV boundary. Ubx functions, at least, at two levels to repress Vestigial expression in non-DV cells of haltere discs. At the DV boundary, it functions downstream of Shaggy/GSK3 beta to enhance the degradation of Armadillo (Arm), which causes downregulation of Wg signaling. In non-DV cells, Ubx inhibits event(s) downstream of Arm, but upstream of Vg autoregulation. Repression of Vg at multiple levels appears to be crucial for Ubx-mediated specification of the haltere fate. Overexpression of Vg in haltere discs is enough to override Ubx function and cause haltere-to-wing homeotic transformations.

A re-evaluation of the contributions of Apterous and Notch to the dorsoventral lineage restriction boundary in the Drosophila wing

The Drosophila limb primordia are subdivided into compartments: cell populations that do not mix during development. The wing is subdivided into dorsal (D) and ventral (V) compartments by the activity of the selector gene apterous in D cells. Apterous causes segregation of D and V cell populations by at least two distinct mechanisms. The LRR transmembrane proteins Capricious and Tartan are transiently expressed in D cells and contribute to initial segregation of D and V cells. Signaling between D and V cells mediated by Notch and Fringe contributes to the maintenance of the DV affinity boundary. Given that Notch is activated symmetrically, in D and V cells adjacent to the boundary, its role in boundary formation remains somewhat unclear. We re-examine the roles of Apterous and Fringe activities in DV boundary formation and present evidence that Fringe cannot, by itself, generate an affinity difference between D and V cells. Although not sufficient, Fringe is required via Notch activation for expression of an Apterous-dependent affinity difference. We propose that Apterous controls expression of surface proteins that confer an affinity difference in conjunction with activated Notch. Thus, we view Apterous as instructive and Notch activity as essential, but permissive.

Genetic requirements of vestigial in the regulation of Drosophila wing development

The gene vestigial has been proposed to act as a master gene because of its supposed capacity to initiate and drive wing development. We show that the ectopic expression of vestigial only induces ectopic outgrowths with wing cuticular differentiation and wing blade gene expression patterns in specific developmental and genetic contexts. In the process of transformation, wingless seems to be an essential but insufficient co-factor of vestigial. vestigial ectopic expression alone or vestigial plus wingless co-expression in clones differentiate 'mixed' cuticular patterns (they contain wing blade trichomes and chaetae characteristic of the endogenous surrounding tissue) and express wing blade genes only in patches of cells within the clones. In addition, we have found that these clones, in the wing imaginal disc, may cause autonomous as well as non-autonomous cuticular transformations and wing blade gene expression patterns. These non-autonomous effects in surrounding cells result from recruitment or 'inductive assimilation' of vestigial or wingless-vestigial overexpressing cells.

Interaction between apterous and early expression of vestigial in formation of the dorso-ventral compartments in the Drosophila wing disc

BACKGROUND

Compartment formation is a developmental process that requires the existence of barriers against intermixing between cell groups. In the Drosophila wing disc, the dorso-ventral (D/V) compartment boundary is defined by the expression of the apterous (ap) selector gene in the dorsal compartment. AP activity is under control of dLMO which destabilizes the formation of the AP-CHIP complex.

RESULTS

We report that D/V boundary formation in the wing disc also depends on early expression of vestigial (vg). Our data suggest that vg is already required for wing cell proliferation before D/V compartmentalization. In addition, we show that over-expression of vg can, to some extent, rescue the effect of the absence of ap on D/V boundary formation. Early VG product regulates AP activity by inducing dLMO and thus indirectly regulating ap target genes such as fringe and the PSalpha1 and PSalpha2 integrins.

CONCLUSION

Normal cell proliferation is necessary for ap expression at the level of the D/V boundary. This would be mediated by vg, which interacts in a dose-dependent way with ap.