An efficient approach to isolate STAT regulated enhancers uncovers STAT92E fundamental role in Drosophila tracheal development

Specification of the endocrine primordia controlling insect moulting and metamorphosis by the JAK/STAT signalling pathway

The corpora allata and the prothoracic glands control moulting and metamorphosis in insects. These endocrine glands are specified in the maxillary and labial segments at positions homologous to those forming the trachea in more posterior segments. Glands and trachea can be homeotically transformed into each other suggesting that all three evolved from a metamerically repeated organ that diverged to form glands in the head and respiratory organs in the trunk. While much is known about tracheal specification, there is limited information about corpora allata and prothorathic gland specification. Here we show that the expression of a key regulator of early gland development, the snail gene, is controlled by the Dfd and Scr Hox genes and by the Hedgehog and Wnt signalling pathways that induce localised transcription of upd, the ligand of the JAK/STAT signalling pathway, which lies at the heart of gland specification. Our results show that the same upstream regulators are required for the early gland and tracheal primordia specification, reinforcing the hypothesis that they originated from a segmentally repeated organ present in an ancient arthropod.

Ribbon boosts ribosomal protein gene expression to coordinate organ form and function

Cell growth is well defined for late (postembryonic) stages of development, but evidence for early (embryonic) cell growth during postmitotic morphogenesis is limited. Here, we report early cell growth as a key characteristic of tubulogenesis in the Drosophila embryonic salivary gland (SG) and trachea. A BTB/POZ domain nuclear factor, Ribbon (Rib), mediates this early cell growth. Rib binds the transcription start site of nearly every SG-expressed ribosomal protein gene (RPG) and is required for full expression of all RPGs tested. Rib binding to RPG promoters in vitro is weak and not sequence specific, suggesting that specificity is achieved through cofactor interactions. Accordingly, we demonstrate Rib’s ability to physically interact with each of the three known regulators of RPG transcription. Surprisingly, Rib-dependent early cell growth in another tubular organ, the embryonic trachea, is not mediated by direct RPG transcription. These findings support a model of early cell growth customized by transcriptional regulatory networks to coordinate organ form and function.

A hemipteran insect reveals new genetic mechanisms and evolutionary insights into tracheal system development

The diversity in the organization of the tracheal system is one of the drivers of insect evolutionary success; however, the genetic mechanisms responsible are yet to be elucidated. Here, we highlight the advantages of utilizing hemimetabolous insects, such as the milkweed bug Oncopeltus fasciatus, in which the final adult tracheal patterning can be directly inferred by examining its blueprint in embryos. By reporting the expression patterns, functions, and Hox gene regulation of trachealess (trh), ventral veinless (vvl), and cut (ct), key genes involved in tracheal development, this study provides important insights. First, Hox genes function as activators, modifiers, and suppressors of trh expression, which in turn results in a difference between the thoracic and abdominal tracheal organization. Second, spiracle morphogenesis requires the input of both trh and ct, where ct is positively regulated by trh As Hox genes regulate trh, we can now mechanistically explain the previous observations of their effects on spiracle formation. Third, the default state of vvl expression in the thorax, in the absence of Hox gene expression, features three lateral cell clusters connected to ducts. Fourth, the exocrine scent glands express vvl and are regulated by Hox genes. These results extend previous findings [Sánchez-Higueras et al., 2014], suggesting that the exocrine glands, similar to the endocrine, develop from the same primordia that give rise to the trachea. The presence of such versatile primordia in the miracrustacean ancestor could account for the similar gene networks found in the glandular and respiratory organs of both insects and crustaceans.

Two-step regulation of trachealess ensures tight coupling of cell fate with morphogenesis in the Drosophila trachea

During organogenesis, inductive signals cause cell differentiation and morphogenesis. However, how these phenomena are coordinated to form functional organs is poorly understood. Here, we show that cell differentiation of the Drosophila trachea is sequentially determined in two steps and that the second step is synchronous with the invagination of the epithelial sheet. The master gene trachealess is dispensable for the initiation of invagination, while it is essential for maintaining the invaginated structure, suggesting that tracheal morphogenesis and differentiation are separately induced. trachealess expression starts in bipotential tracheal/epidermal placode cells. After invagination, its expression is maintained in the invaginated cells but is extinguished in the remaining sheet cells. A trachealess cis-regulatory module that shows both tracheal enhancer activity and silencer activity in the surface epidermal sheet was identified. We propose that the coupling of trachealess expression with the invaginated structure ensures that only invaginated cells canalize robustly into the tracheal fate.

Cells in developing organs have two important decisions to make: where to be and what cell type to become. If cells end up in the wrong places, they can stop an organ from working, so it is vital that one decision depends upon the other. The so-called progenitor cells responsible for forming the trachea, for example, can either become part of a flat sheet or part of a tube. The cells on the sheet need to become epidermal cells, while the cells in the tube need to become tracheal cells. Work on fruit flies found that a gene called 'trachealess' plays an important role in this process. Without it, developing flies cannot make a trachea at all.

At the start of trachea development, some of the cells form thickened structures called placodes. The progenitor cells in the placodes start to divide, and the structures buckle inwards to form pockets. These pockets then lengthen into tubes. The trachealess gene codes for a protein that works as a genetic switch. It turns other genes on or off, helping the progenitor cells inside the pockets to become tracheal cells. But, it is not clear whether trachealess drives the formation of the pockets: the progenitor cells first decide what to be; or whether pocket formation tells the cells to use trachealess: the progenitor cells first decide where to be.

To find out, Kondo and Hayashi imaged developing fly embryos and saw that the trachealess gene does not start pocket formation, but that it is essential to maintain the pockets. Flies without the gene managed to form pockets, but they did not last long. Looking at embryos with defects in other genes involved in pocket formation revealed why. In these flies, some of the progenitor cells using trachealess got left behind when the pockets started to form. But rather than forming pockets of their own (as they might if trachealess were driving pocket formation), they turned their trachealess gene off. Progenitor cells in the fly trachea seem to decide where to be before they decide what cell type to become. This helps to make sure that trachea cells do not form in the wrong places.

A question that still remains is how do the cells know when they are inside a pocket? It is possible that the cells are sensing different mechanical forces or different chemical signals. Further research could help scientists to understand how organs form in living animals, and how they might better recreate that process in the laboratory.

In vivo Hox binding specificity revealed by systematic changes to a single cis regulatory module

Hox proteins belong to a family of transcription factors with similar DNA binding specificities that control animal differentiation along the antero-posterior body axis. Hox proteins are expressed in partially overlapping regions where each one is responsible for the formation of particular organs and structures through the regulation of specific direct downstream targets. Thus, explaining how each Hox protein can selectively control its direct targets from those of another Hox protein is fundamental to understand animal development. Here we analyse a cis regulatory module directly regulated by seven different Drosophila Hox proteins and uncover how different Hox class proteins differentially control its expression. We find that regulation by one or another Hox protein depends on the combination of three modes: Hox-cofactor dependent DNA-binding specificity; Hox-monomer binding sites; and interaction with positive and negative Hox-collaborator proteins. Additionally, we find that similar regulation can be achieved by Amphioxus orthologs, suggesting these three mechanisms are conserved from insects to chordates.

Hox proteins are expressed in partially overlapping regions to inform development along the embryo’s head-tail axis. Here the authors analyse a cis regulatory module directly regulated by seven different Drosophila Hox proteins to uncover how different Hox class proteins differentially control its expression.

JAK/STAT signaling is involved in air sac primordium development of Drosophila melanogaster

The dorsal thoracic air sacs in fruit flies (Drosophila melanogaster) are functionally and developmentally comparable to human lungs. The progenitors of these structures, air sac primordia (ASPs), invasively propagate into wing imaginal disks, employing mechanisms similar to those that promote metastasis in malignant tumors. We investigated whether Janus kinase/signal transducer and activator of transcription JAK/STAT signaling plays a role in the directed morphogenesis of ASPs. We find that JAK/STAT signaling occurs in ASP tip cells and misexpression of core components in the JAK/STAT signaling cascade significantly impedes ASP development. We further identify Upd2 as an activating ligand for JAK/STAT activity in the ASP. Together, these data constitute a considerable step forward in understanding the role of JAK/STAT signaling in ASPs and similar structures in mammalian models.

Development and Function of the Drosophila Tracheal System

The tracheal system of insects is a network of epithelial tubules that functions as a respiratory organ to supply oxygen to various target organs. Target-derived signaling inputs regulate stereotyped modes of cell specification, branching morphogenesis, and collective cell migration in the embryonic stage. In the postembryonic stages, the same set of signaling pathways controls highly plastic regulation of size increase and pattern elaboration during larval stages, and cell proliferation and reprograming during metamorphosis. Tracheal tube morphogenesis is also regulated by physicochemical interaction of the cell and apical extracellular matrix to regulate optimal geometry suitable for air flow. The trachea system senses both the external oxygen level and the metabolic activity of internal organs, and helps organismal adaptation to changes in environmental oxygen level. Cellular and molecular mechanisms underlying the high plasticity of tracheal development and physiology uncovered through research on Drosophila are discussed.

A Switch-like Activation Relay of EGFR-ERK Signaling Regulates a Wave of Cellular Contractility for Epithelial Invagination

The dynamics of extracellular signal-regulated kinase (ERK) signaling underlies its versatile functions in cell differentiation, cell proliferation, and cell motility. Classical studies in Drosophila established that a gradient of epidermal growth factor receptor (EGFR)-ERK signaling is essential for these cellular responses. However, we challenge this view by the real-time monitoring of ERK activation; we show that a switch-like ERK activation is essential for the invagination movement of the Drosophila tracheal placode. This switch-like ERK activation stems from the positive feedback regulation of the EGFR-ERK signaling and a resultant relay of EGFR-ERK signaling among tracheal cells. A key transcription factor Trachealess (Trh) permissively regulates the iteration of the relay, and the ERK activation becomes graded in trh mutant. A mathematical model based on these observations and a molecular link between ERK activation dynamics and myosin shows that the relay mechanism efficiently promotes epithelial invagination while the gradient mechanism does not.

Tumorigenic Properties of Drosophila Epithelial Cells Mutant for lethal giant larvae

BACKGROUND

Mutations in Drosophila tumor suppressor genes (TSGs) lead to the formation of invasive tumors in the brain and imaginal discs.

RESULTS

Here we studied the tumorigenic properties of imaginal discs mutant for the TSG gene lethal giant larvae (lgl). lgl mutant cells display the characteristic features of mammalian tumor cells: they can proliferate indefinitely, induce additional tracheogenesis (an insect counterpart of vasculogenesis) and invade neighboring tissues. Lgl mutant tissues exhibit high apoptotic levels, which lead to the activation of the Jun-N-Terminal Kinase (JNK) pathway. We propose that JNK is a key factor in the acquisition of these tumorigenic properties; it promotes cell proliferation and induces high levels of Mmp1 and confers tumor cells capacity to invade wild-type tissue. Noteworthy, lgl RNAi-mediated down-regulation does not produce similar transformations in the central nervous system (CNS), thereby indicating a fundamental difference between the cells of developing imaginal discs and those of differentiated organs. We discuss these results in the light of the "single big-hit origin" of some human pediatric or developmental cancers.

CONCLUSIONS

Down-regulation of lgl in imaginal discs is sufficient to enhance tracheogenesis and to promote invasion and colonization of other larval structures including the CNS. Developmental Dynamics 245:834-843, 2016. © 2016 Wiley Periodicals, Inc.

Genome-wide identification of Grainy head targets in Drosophila reveals regulatory interactions with the POU domain transcription factor Vvl

Grainy head (Grh) is a conserved transcription factor (TF) controlling epithelial differentiation and regeneration. To elucidate Grh functions we identified embryonic Grh targets by ChIP-seq and gene expression analysis. We show that Grh controls hundreds of target genes. Repression or activation correlates with the distance of Grh-binding sites to the transcription start sites of its targets. Analysis of 54 Grh-responsive enhancers during development and upon wounding suggests cooperation with distinct TFs in different contexts. In the airways, Grh-repressed genes encode key TFs involved in branching and cell differentiation. Reduction of the POU domain TF Ventral veins lacking (Vvl) largely ameliorates the airway morphogenesis defects of grh mutants. Vvl and Grh proteins additionally interact with each other and regulate a set of common enhancers during epithelial morphogenesis. We conclude that Grh and Vvl participate in a regulatory network controlling epithelial maturation.

Multiple strategies of oxygen supply in Drosophila malignancies identify tracheogenesis as a novel cancer hallmark

Angiogenesis is the term used to describe all the alterations in blood vessel growth induced by a tumour mass following hypoxic stress. The occurrence of multiple strategies of vessel recruitment favours drug resistance, greatly complicating the treatment of certain tumours. In Drosophila, oxygen is conveyed to the internal organs by the tracheal system, a closed tubular network whose role in cancer growth is so far unexplored. We found that, as observed in human cancers, Drosophila malignant cells suffer from oxygen shortage, release pro-tracheogenic factors, co-opt nearby vessels and get incorporated into the tracheal walls. We also found that the parallelisms observed in cellular behaviours are supported by genetic and molecular conservation. Finally, we identified a molecular circuitry associated with the differentiation of cancer cells into tracheal cells. In summary, our findings identify tracheogenesis as a novel cancer hallmark in Drosophila, further expanding the power of the fly model in cancer research.

PI3K signaling and Stat92E converge to modulate glial responsiveness to axonal injury

Activation of glial cells following axon injury is mediated by a positive feedback loop downstream of the glial phagocytic receptor Draper, allowing the strength of the response to match the severity of injury.

Glial cells are exquisitely sensitive to neuronal injury but mechanisms by which glia establish competence to respond to injury, continuously gauge neuronal health, and rapidly activate reactive responses remain poorly defined. Here, we show glial PI3K signaling in the uninjured brain regulates baseline levels of Draper, a receptor essential for Drosophila glia to sense and respond to axonal injury. After injury, Draper levels are up-regulated through a Stat92E-modulated, injury-responsive enhancer element within the draper gene. Surprisingly, canonical JAK/STAT signaling does not regulate draper expression. Rather, we find injury-induced draper activation is downstream of the Draper/Src42a/Shark/Rac1 engulfment signaling pathway. Thus, PI3K signaling and Stat92E are critical in vivo regulators of glial responsiveness to axonal injury. We provide evidence for a positive auto-regulatory mechanism whereby signaling through the injury-responsive Draper receptor leads to Stat92E-dependent, transcriptional activation of the draper gene. We propose that Drosophila glia use this auto-regulatory loop as a mechanism to adjust their reactive state following injury.

Acute injuries of the central nervous system (CNS) trigger a robust reaction from glial cells—a non-neuronal population of cells that regulate and support neural development and physiology. Although this process occurs after all types of CNS trauma in mammals, how it is activated and its precise role in recovery remain poorly understood. Using the fruit fly Drosophila melanogaster as a model, we previously identified a cell surface receptor called Draper, which is required for the activation of glia after local axon injury (“axotomy”) and for the removal of degenerating axonal debris by phagocytosis. Here, we show that regulation of Draper protein levels and glial activation through the Draper signaling pathway are mediated by the well-conserved PI3K and signal transducer and activator of transcription (STAT) signaling cascades. We find that STAT transcriptional activity is activated in glia in response to axotomy, and identify an injury-responsive regulatory element within the draper gene that appears to be directly modulated by STAT. Interestingly, the intensity of STAT activity in glial cells after axotomy correlates tightly with the number of local severed axons, indicating that Drosophila glia are able to fine-tune their response to neuronal injury according to its severity. In summary, we propose that the initial phagocytic competence of glia is regulated by setting Draper baseline levels (via PI3K), whereas injury-activated glial phagocytic activity is modulated through a positive feedback loop that requires STAT-dependent activation of draper. We speculate that the level of activation of this cascade is determined by glial cell recognition of Draper ligands present on degenerating axon material, thereby matching the levels of glial reactivity to the amount of injured axonal material.

A comparison of midline and tracheal gene regulation during Drosophila development

Within the Drosophila embryo, two related bHLH-PAS proteins, Single-minded and Trachealess, control development of the central nervous system midline and the trachea, respectively. These two proteins are bHLH-PAS transcription factors and independently form heterodimers with another bHLH-PAS protein, Tango. During early embryogenesis, expression of Single-minded is restricted to the midline and Trachealess to the trachea and salivary glands, whereas Tango is ubiquitously expressed. Both Single-minded/Tango and Trachealess/Tango heterodimers bind to the same DNA sequence, called the CNS midline element (CME) within cis-regulatory sequences of downstream target genes. While Single-minded/Tango and Trachealess/Tango activate some of the same genes in their respective tissues during embryogenesis, they also activate a number of different genes restricted to only certain tissues. The goal of this research is to understand how these two related heterodimers bind different enhancers to activate different genes, thereby regulating the development of functionally diverse tissues. Existing data indicates that Single-minded and Trachealess may bind to different co-factors restricted to various tissues, causing them to interact with the CME only within certain sequence contexts. This would lead to the activation of different target genes in different cell types. To understand how the context surrounding the CME is recognized by different bHLH-PAS heterodimers and their co-factors, we identified and analyzed novel enhancers that drive midline and/or tracheal expression and compared them to previously characterized enhancers. In addition, we tested expression of synthetic reporter genes containing the CME flanked by different sequences. Taken together, these experiments identify elements overrepresented within midline and tracheal enhancers and suggest that sequences immediately surrounding a CME help dictate whether a gene is expressed in the midline or trachea.

Common origin of insect trachea and endocrine organs from a segmentally repeated precursor

Segmented organisms have serially repeated structures [1] that become specialized in some segments [2]. We show here that the Drosophila corpora allata, prothoracic glands, and trachea have a homologous origin and can convert into each other. The tracheal epithelial tubes develop from ten trunk placodes [3, 4], and homologous ectodermal cells in the maxilla and labium form the corpora allata and the prothoracic glands. The early endocrine and trachea gene networks are similar, with STAT and Hox genes inducing their activation. The initial invagination of the trachea and the endocrine primordia is identical, but activation of Snail in the glands induces an epithelial-mesenchymal transition (EMT), after which the corpora allata and prothoracic gland primordia coalesce and migrate dorsally, joining the corpora cardiaca to form the ring gland. We propose that the arthropod ectodermal endocrine glands and respiratory organs arose through an extreme process of divergent evolution from a metameric repeated structure.

The Iroquois complex is required in the dorsal mesoderm to ensure normal heart development in Drosophila

Drosophila heart development is an invaluable system to study the orchestrated action of numerous factors that govern cardiogenesis. Cardiac progenitors arise within specific dorsal mesodermal regions that are under the influence of temporally coordinated actions of multiple signaling pathways. The Drosophila Iroquois complex (Iro-C) consists of the three homeobox transcription factors araucan (ara), caupolican (caup) and mirror (mirr). The Iro-C has been shown to be involved in tissue patterning leading to the differentiation of specific structures, such as the lateral notum and dorsal head structures and in establishing the dorsal-ventral border of the eye. A function for Iro-C in cardiogenesis has not been investigated yet. Our data demonstrate that loss of the whole Iro complex, as well as loss of either ara/caup or mirr only, affect heart development in Drosophila. Furthermore, the data indicate that the GATA factor Pannier requires the presence of Iro-C to function in cardiogenesis. Furthermore, a detailed expression pattern analysis of the members of the Iro-C revealed the presence of a possibly novel subpopulation of Even-skipped expressing pericardial cells and seven pairs of heart-associated cells that have not been described before. Taken together, this work introduces Iro-C as a new set of transcription factors that are required for normal development of the heart. As the members of the Iro-C may function, at least partly, as competence factors in the dorsal mesoderm, our results are fundamental for future studies aiming to decipher the regulatory interactions between factors that determine different cell fates in the dorsal mesoderm.

JAK-STAT pathway in Drosophila morphogenesis: From organ selector to cell behavior regulator

One of the main contributions of Drosophila to the JAK-STAT field is the study of morphogenesis. JAK-STAT signaling controls the formation of many different structures through surprisingly different morphogenetic behaviors that include induction of cell rearrangements, invagination, folding of tissues, modulation of cell shape, and migration. This variability may be explained by the many transcription factors and signaling molecules STAT regulates at early stages of development. But is STAT just acting as an upstream inducer of morphogenesis or does it have a more direct role in controlling cell behaviors? Here we review what is known about how the canonical phosphorylation of STAT contributes to shaping the embryonic and imaginal structures.

Novel mechanisms of tube-size regulation revealed by the Drosophila trachea

The size of various tubes within tubular organs such as the lung, vascular system and kidney must be finely tuned for the optimal delivery of gases, nutrients, waste and cells within the entire organism. Aberrant tube sizes lead to devastating human illnesses, such as polycystic kidney disease, fibrocystic breast disease, pancreatic cystic neoplasm and thyroid nodules. However, the underlying mechanisms that are responsible for tube-size regulation have yet to be fully understood. Therefore, no effective treatments are available for disorders caused by tube-size defects. Recently, the Drosophila tracheal system has emerged as an excellent in vivo model to explore the fundamental mechanisms of tube-size regulation. Here, we discuss the role of the apical luminal matrix, cell polarity and signaling pathways in regulating tube size in Drosophila trachea. Previous studies of the Drosophila tracheal system have provided general insights into epithelial tube morphogenesis. Mechanisms that regulate tube size in Drosophila trachea could be well conserved in mammalian tubular organs. This knowledge should greatly aid our understanding of tubular organogenesis in vertebrates and potentially lead to new avenues for the treatment of human disease caused by tube-size defects.

Cis-regulatory complexity within a large non-coding region in the Drosophila genome

Analysis of cis-regulatory enhancers has revealed that they consist of clustered blocks of highly conserved sequences. Although most characterized enhancers reside near their target genes, a growing number of studies have shown that enhancers located over 50 kb from their minimal promoter(s) are required for appropriate gene expression and many of these ‘long-range’ enhancers are found in genomic regions that are devoid of identified exons. To gain insight into the complexity of Drosophila cis-regulatory sequences within exon-poor regions, we have undertaken an evolutionary analysis of 39 of these regions located throughout the genome. This survey revealed that within these genomic expanses, clusters of conserved sequence blocks (CSBs) are positioned once every 1.1 kb, on average, and that a typical cluster contains multiple (5 to 30 or more) CSBs that have been maintained for at least 190 My of evolutionary divergence. As an initial step toward assessing the cis-regulatory activity of conserved clusters within gene-free genomic expanses, we have tested the in-vivo enhancer activity of 19 consecutive CSB clusters located in the middle of a 115 kb gene-poor region on the 3^rd chromosome. Our studies revealed that each cluster functions independently as a specific spatial/temporal enhancer. In total, the enhancers possess a diversity of regulatory functions, including dynamically activating expression in defined patterns within subsets of cells in discrete regions of the embryo, larvae and/or adult. We also observed that many of the enhancers are multifunctional–that is, they activate expression during multiple developmental stages. By extending these results to the rest of the Drosophila genome, which contains over 70,000 non-coding CSB clusters, we suggest that most function as enhancers.

Mitotic cell rounding accelerates epithelial invagination

Mitotic cells assume a spherical shape by increasing their surface tension and osmotic pressure by extensively reorganizing their interphase actin cytoskeleton into a cortical meshwork and their microtubules into the mitotic spindle. Mitotic entry is known to interfere with tissue morphogenetic events that require cell-shape changes controlled by the interphase cytoskeleton, such as apical constriction. However, here we show that mitosis plays an active role in the epithelial invagination of the Drosophila melanogaster tracheal placode. Invagination begins with a slow phase under the control of epidermal growth factor receptor (EGFR) signalling; in this process, the central apically constricted cells, which are surrounded by intercalating cells, form a shallow pit. This slow phase is followed by a fast phase, in which the pit is rapidly depressed, accompanied by mitotic entry, which leads to the internalization of all the cells in the placode. We found that mitotic cell rounding, but not cell division, of the central cells in the placode is required to accelerate invagination, in conjunction with EGFR-induced myosin II contractility in the surrounding cells. We propose that mitotic cell rounding causes the epithelium to buckle under pressure and acts as a switch for morphogenetic transition at the appropriate time.

Drosophila as a model for epithelial tube formation

Epithelial tubular organs are essential for life in higher organisms and include the pancreas and other secretory organs that function as biological factories for the synthesis and delivery of secreted enzymes, hormones, and nutrients essential for tissue homeostasis and viability. The lungs, which are necessary for gas exchange, vocalization, and maintaining blood pH, are organized as highly branched tubular epithelia. Tubular organs include arteries, veins, and lymphatics, high-speed passageways for delivery and uptake of nutrients, liquids, gases, and immune cells. The kidneys and components of the reproductive system are also epithelial tubes. Both the heart and central nervous system of many vertebrates begin as epithelial tubes. Thus, it is not surprising that defects in tube formation and maintenance underlie many human diseases. Accordingly, a thorough understanding how tubes form and are maintained is essential to developing better diagnostics and therapeutics. Among the best-characterized tubular organs are the Drosophila salivary gland and trachea, organs whose relative simplicity have allowed for in depth analysis of gene function, yielding key mechanistic insight into tube initiation, remodeling and maintenance. Here, we review our current understanding of salivary gland and trachea formation - highlighting recent discoveries into how these organs attain their final form and function.

JAK/Stat signaling regulates heart precursor diversification in Drosophila

Intercellular signal transduction pathways regulate the NK-2 family of transcription factors in a conserved gene regulatory network that directs cardiogenesis in both flies and mammals. The Drosophila NK-2 protein Tinman (Tin) was recently shown to regulate Stat92E, the Janus kinase (JAK) and Signal transducer and activator of transcription (Stat) pathway effector, in the developing mesoderm. To understand whether the JAK/Stat pathway also regulates cardiogenesis, we performed a systematic characterization of JAK/Stat signaling during mesoderm development. Drosophila embryos with mutations in the JAK/Stat ligand upd or in Stat92E have non-functional hearts with luminal defects and inappropriate cell aggregations. Using strong Stat92E loss-of-function alleles, we show that the JAK/Stat pathway regulates tin expression prior to heart precursor cell diversification. tin expression can be subdivided into four phases and, in Stat92E mutant embryos, the broad phase 2 expression pattern in the dorsal mesoderm does not restrict to the constrained phase 3 pattern. These embryos also have an expanded pericardial cell domain. We show the E(spl)-C gene HLHm5 is expressed in a pattern complementary to tin during phase 3 and that this expression is JAK/Stat dependent. In addition, E(spl)-C mutant embryos phenocopy the cardiac defects of Stat92E embryos. Mechanistically, JAK/Stat signals activate E(spl)-C genes to restrict Tin expression and the subsequent expression of the T-box transcription factor H15 to direct heart precursor diversification. This study is the first to characterize a role for the JAK/Stat pathway during cardiogenesis and identifies an autoregulatory circuit in which tin limits its own expression domain.

Trachealess (Trh) regulates all tracheal genes during Drosophila embryogenesis

The Drosophila trachea is a branched tubular epithelia that transports oxygen and other gases. trachealess (trh), which encodes a bHLH-PAS transcription factor, is among the first genes to be expressed in the cells that will form the trachea. In the absence of trh, tracheal cells fail to invaginate to form tubes and remain on the embryo surface. Expression of many tracheal-specific genes depends on trh, but all of the known targets have relatively minor phenotypes compared to loss of trh, suggesting that there are additional targets. To identify uncharacterized transcriptional targets of Trh and to further understand the role of Trh in embryonic tracheal formation, we performed an in situ hybridization screen using a library of ~100 tracheal-expressed genes identified by the Berkeley Drosophila Genome Project (BDGP). Surprisingly, expression of every tracheal gene we tested was dependent on Trh, suggesting a major role for Trh in activation and maintenance of tracheal gene expression. A re-examination of the interdependence of the known early-expressed transcription factors, including trh, ventral veinless (vvl) and knirps/knirps-related (kni/knrl), suggests a new model for how gene expression is controlled in the trachea, with trh regulating expression of vvl and kni, but not vice versa. A pilot screen for the targets of Vvl and Kni/Knrl revealed that Vvl and Kni have only minor roles compared to Trh. Finally, genome-wide microarray experiments identified additional Trh targets and revealed that a variety of biological processes are affected by the loss of trh.

Regulated Crb accumulation controls apical constriction and invagination in Drosophila tracheal cells

Many epithelial tissues undergo extensive remodelling during morphogenesis. How their epithelial features, such as apicobasal polarity or adhesion, are maintained and remodelled and how adhesion and polarity proteins contribute to morphogenesis are two important questions in development. Here, we approach these issues by investigating the role of the apical determinant protein Crumbs (Crb) during the morphogenesis of the embryonic Drosophila tracheal system. Crb accumulates differentially throughout tracheal development and is required for different tracheal events. The earliest requirement for Crb is for tracheal invagination, which is preceded by an enhanced accumulation of Crb in the invagination domain. There, Crb, acting in parallel with the epidermal growth factor receptor (Egfr) pathway, is required for tracheal cell apical constriction and for organising an actomyosin complex, which we propose is mediated by Crb recruitment of moesin (Moe). The ability of a Crb isoform unable to rescue polarity in crb mutants to otherwise rescue their invagination phenotype, and the converse inability of a FERM-binding domain mutant Crb to rescue faulty invagination, support our hypothesis that it is the absence of Crb-dependent Moe enrichment, and not the polarity defect, that mainly underlies the crb invagination phenotype. This hypothesis is supported by the phenotype of lethal giant larvae (lgl); crb double mutants. These results unveil a link between Crb and the organisation of the actin cytoskeleton during morphogenesis.