A molecular link between FGF and Dpp signaling in branch-specific migration of the Drosophila trachea

Dual role of FGF in proliferation and endoreplication of Drosophila tracheal adult progenitor cells

Adult progenitor cells activation is a key event in the formation of adult organs. In Drosophila, formation of abdominal adult trachea depends on the specific activation of tracheal adult progenitors (tracheoblasts) at the Tr4 and Tr5 spiracular branches. Proliferation of these tracheoblasts generates a pool of tracheal cells that migrate toward the posterior part of the trachea by the activation of the branchless/fibroblast growth factor (Bnl/FGF) signaling to form the abdominal adult trachea. Here, we show that, in addition to migration, Bnl/FGF signaling, mediated by the transcription factor Pointed, is also required for tracheoblast proliferation. This tracheoblast activation relies on the expression of the FGF ligand bnl in their nearby branches. Finally, we show that, in the absence of the transcription factor Cut (Ct), Bnl/FGF signaling induces endoreplication of tracheoblasts partially by promoting fizzy-related expression. Altogether, our results suggest a dual role of Bnl/FGF signaling in tracheoblasts, inducing both proliferation and endoreplication, depending on the presence or absence of the transcription factor Ct, respectively.

A Hippo-like Signaling Pathway Controls Tracheal Morphogenesis in Drosophila melanogaster

Hippo-like pathways are ancient signaling modules first identified in yeasts. The best-defined metazoan module forms the core of the Hippo pathway, which regulates organ size and cell fate. Hippo-like kinase modules consist of a Sterile 20-like kinase, an NDR kinase, and non-catalytic protein scaffolds. In the Hippo pathway, the upstream kinase Hippo can be activated by another kinase, Tao-1. Here, we delineate a related Hippo-like signaling module that Tao-1 regulates to control tracheal morphogenesis in Drosophila melanogaster. Tao-1 activates the Sterile 20-like kinase GckIII by phosphorylating its activation loop, a mode of regulation that is conserved in humans. Tao-1 and GckIII act upstream of the NDR kinase Tricornered to ensure proper tube formation in trachea. Our study reveals that Tao-1 activates two related kinase modules to control both growth and morphogenesis. The Hippo-like signaling pathway we have delineated has a potential role in the human vascular disease cerebral cavernous malformation.

TGF-β Family Signaling in Drosophila

The transforming growth factor β (TGF-β) family signaling pathway is conserved and ubiquitous in animals. In Drosophila, fewer representatives of each signaling component are present compared with vertebrates, simplifying mechanistic study of the pathway. Although there are fewer family members, the TGF-β family pathway still regulates multiple and diverse functions in Drosophila. In this review, we focus our attention on several of the classic and best-studied functions for TGF-β family signaling in regulating Drosophila developmental processes such as embryonic and imaginal disc patterning, but we also describe several recently discovered roles in regulating hormonal, physiological, neuronal, innate immunity, and tissue homeostatic processes.

Receptor-Type Guanylyl Cyclase at 76C (Gyc76C) Regulates De Novo Lumen Formation during Drosophila Tracheal Development

Lumen formation and maintenance are important for the development and function of essential organs such as the lung, kidney and vasculature. In the Drosophila embryonic trachea, lumena form de novo to connect the different tracheal branches into an interconnected network of tubes. Here, we identify a novel role for the receptor type guanylyl cyclase at 76C (Gyc76C) in de novo lumen formation in the Drosophila trachea. We show that in embryos mutant for gyc76C or its downsteam effector protein kinase G (PKG) 1, tracheal lumena are disconnected. Dorsal trunk (DT) cells of gyc76C mutant embryos migrate to contact each other and complete the initial steps of lumen formation, such as the accumulation of E-cadherin (E-cad) and formation of an actin track at the site of lumen formation. However, the actin track and E-cad contact site of gyc76C mutant embryos did not mature to become a new lumen and DT lumena did not fuse. We also observed failure of the luminal protein Vermiform to be secreted into the site of new lumen formation in gyc76C mutant trachea. These DT lumen formation defects were accompanied by altered localization of the Arf-like 3 GTPase (Arl3), a known regulator of vesicle-vesicle and vesicle-membrane fusion. In addition to the DT lumen defect, lumena of gyc76C mutant terminal cells were shorter compared to wild-type cells. These studies show that Gyc76C and downstream PKG-dependent signaling regulate de novo lumen formation in the tracheal DT and terminal cells, most likely by affecting Arl3-mediated luminal secretion.

Extracellular Mipp1 Activity Confers Migratory Advantage to Epithelial Cells during Collective Migration

Multiple inositol polyphosphate phosphatase (Mipp), a highly conserved but poorly understood histidine phosphatase, dephosphorylates higher-order IPs (IP4-IP6) to IP3. To gain insight into the biological roles of these enzymes, we have characterized Drosophila mipp1. mipp1 is dynamically expressed in the embryonic trachea, specifically in the leading cells of migrating branches at late stages, where Mipp1 localizes to the plasma membrane and filopodia. FGF signaling activates mipp1 expression in these cells, where extensive filopodia form to drive migration and elongation by cell intercalation. We show that Mipp1 facilitates formation and/or stabilization of filopodia in leading cells through its extracellular activity. mipp1 loss decreases filopodia number, whereas mipp1 overexpression increases filopodia number in a phosphatase-activity-dependent manner. Importantly, expression of Mipp1 gives cells a migratory advantage for the lead position in elongating tracheal branches. Altogether, these findings suggest that extracellular pools of inositol polyphosphates affect cell behavior during development.

As the fat flies: The dynamic lipid droplets of Drosophila embryos

Research into lipid droplets is rapidly expanding, and new cellular and organismal roles for these lipid-storage organelles are continually being discovered. The early Drosophila embryo is particularly well suited for addressing certain questions in lipid-droplet biology and combines technical advantages with unique biological phenomena. This review summarizes key features of this experimental system and the techniques available to study it, in order to make it accessible to researchers outside this field. It then describes the two topics most heavily studied in this system, lipid-droplet motility and protein sequestration on droplets, discusses what is known about the molecular players involved, points to open questions, and compares the results from Drosophila embryo studies to what it is known about lipid droplets in other systems.

The novel Smad protein Expansion regulates the receptor tyrosine kinase pathway to control Drosophila tracheal tube size

Tubes with distinct shapes and sizes are critical for the proper function of many tubular organs. Here we describe a unique phenotype caused by the loss of a novel, evolutionarily-conserved, Drosophila Smad-like protein, Expansion. In expansion mutants, unicellular and intracellular tracheal branches develop bubble-like cysts with enlarged apical membranes. Cysts in unicellular tubes are enlargements of the apical lumen, whereas cysts in intracellular tubes are cytoplasmic vacuole-like compartments. The cyst phenotype in expansion mutants is similar to, but weaker than, that observed in double mutants of Drosophila type III receptor tyrosine phosphatases (RPTPs), Ptp4E and Ptp10D. Ptp4E and Ptp10D negatively regulate the receptor tyrosine kinase (RTK) pathways, especially epithelial growth factor receptor (EGFR) and fibroblast growth factor receptor/breathless (FGFR, Btl) signaling to maintain the proper size of unicellular and intracellular tubes. We show Exp genetically interacts with RTK signaling, the downstream targets of RPTPs. Cyst size and number in expansion mutants is enhanced by increased RTK signaling and suppressed by reduced RTK signaling. Genetic interaction studies strongly suggest that Exp negatively regulates RTK (EGFR, Btl) signaling to ensure proper tube sizes. Smad proteins generally function as intermediate components of the transforming growth factor-β (TGF-β, DPP) signaling pathway. However, no obvious genetic interaction between expansion and TGF-β (DPP) signaling was observed. Therefore, Expansion does not function as a typical Smad protein. The expansion phenotype demonstrates a novel role for Smad-like proteins in epithelial tube formation.

IV. Tools and methods for studying cell migration and cell rearrangement in tissue and organ development

A vast diversity of biological systems, ranging from prokaryotes to multicellular organisms, show cell migration behavior. Many of the basic cellular and molecular concepts in cell migration apply to diverse model organisms. Drosophila, with its vast repertoire of tools for imaging and for manipulation, is one of the favorite organisms to study cell migration. Moreover, distinct Drosophila tissues and organs offer diverse cell migration models that are amenable to live imaging and genetic manipulations. In this review, we will provide an overview of the fruit fly toolbox that is of particular interest for the analysis of cell migration. We provide examples to highlight how those tools were used in diverse migration systems, with an emphasis on tracheal morphogenesis, a process that combines morphogenesis with cell migration.

A comparative study of Pointed and Yan expression reveals new complexity to the transcriptional networks downstream of receptor tyrosine kinase signaling

The biochemical regulatory network downstream of receptor tyrosine kinase (RTK) signaling is controlled by two opposing ETS family members: the transcriptional activator Pointed (Pnt) and the transcriptional repressor Yan. A bistable switch model has been invoked to explain how pathway activation can drive differentiation by shifting the system from a high-Yan/low-Pnt activity state to a low-Yan/high-Pnt activity state. Although the model explains yan and pnt loss-of-function phenotypes in several different cell types, how Yan and Pointed protein expression dynamics contribute to these and other developmental transitions remains poorly understood. Toward this goal we have used a functional GFP-tagged Pnt transgene (Pnt-GFP) to perform a comparative study of Yan and Pnt protein expression throughout Drosophila development. Consistent with the prevailing model of the Pnt-Yan network, we found numerous instances where Pnt-GFP and Yan adopt a mutually exclusive pattern of expression. However we also observed many examples of co-expression. While some co-expression occurred in cells where RTK signaling is presumed low, other co-expression occurred in cells with high RTK signaling. The instances of co-expressed Yan and Pnt-GFP in tissues with high RTK signaling cannot be explained by the current model, and thus they provide important contexts for future investigation of how context-specific differences in RTK signaling, network topology, or responsiveness to other signaling inputs, affect the transcriptional response.

Receptor guanylyl cyclase Gyc76C is required for invagination, collective migration and lumen shape in the Drosophila embryonic salivary gland

The Drosophila embryonic salivary gland is formed by the invagination and collective migration of cells. Here, we report on a novel developmental role for receptor-type guanylyl cyclase at 76C, Gyc76C, in morphogenesis of the salivary gland. We demonstrate that Gyc76C and downstream cGMP-dependent protein kinase 1 (DG1) function in the gland and surrounding mesoderm to control invagination, collective migration and lumen shape. Loss of gyc76C resulted in glands that failed to invaginate, complete posterior migration and had branched lumens. Salivary gland migration defects of gyc76C mutant embryos were rescued by expression of wild-type gyc76C specifically in the gland or surrounding mesoderm, whereas invagination defects were rescued primarily by expression in the gland. In migrating salivary glands of gyc76C mutant embryos, integrin subunits localized normally to gland-mesoderm contact sites but talin localization in the surrounding circular visceral mesoderm and fat body was altered. The extracellular matrix protein, laminin, also failed to accumulate around the migrating salivary gland of gyc76C mutant embryos, and gyc76C and laminin genetically interacted in gland migration. Our studies suggest that gyc76C controls salivary gland invagination, collective migration and lumen shape, in part by regulating the localization of talin and the laminin matrix.

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.

Novel isoforms of the transport regulator klar

Klar is a regulator of microtubule-motor dependent transport processes in Drosophila, including nuclear migration, vesicle motility, and lipid-droplet transport. The single klar locus gives rise to multiple isoforms that presumably have unique functions. Up to now, three Klar isoforms (α, β, γ) were known. Here we describe two novel isoforms, δ and ε, whose expression depends on a previously uncharacterized promoter. Klar δ and/or ε are widely expressed during development, including in the embryonic and larval nervous system as well as in ovaries. When we specifically ablate Klar δ and ε expression genetically, no gross organismal phenotypes are apparent. However, ectopic expression of these isoforms causes nuclear mispositioning in developing photoreceptors and in oocytes, demonstrating their biological activity. Our analysis identifies novel forms of the Klar protein and provides new tools for functionally dissecting the complex klar locus.

Comparative molecular developmental aspects of the mammalian- and the avian lungs, and the insectan tracheal system by branching morphogenesis: recent advances and future directions

Gas exchangers fundamentally form by branching morphogenesis (BM), a mechanistically profoundly complex process which derives from coherent expression and regulation of multiple genes that direct cell-to-cell interactions, differentiation, and movements by signaling of various molecular morphogenetic cues at specific times and particular places in the developing organ. Coordinated expression of growth-instructing factors determines sizes and sites where bifurcation occurs, by how much a part elongates before it divides, and the angle at which branching occurs. BM is essentially induced by dualities of factors where through feedback- or feed forward loops agonists/antagonists are activated or repressed. The intricate transactions between the development orchestrating molecular factors determine the ultimate phenotype. From the primeval time when the transformation of unicellular organisms to multicellular ones occurred by systematic accretion of cells, BM has been perpetually conserved. Canonical signalling, transcriptional pathways, and other instructive molecular factors are commonly employed within and across species, tissues, and stages of development. While much still remain to be elucidated and some of what has been reported corroborated and reconciled with rest of existing data, notable progress has in recent times been made in understanding the mechanism of BM. By identifying and characterizing the morphogenetic drivers, and markers and their regulatory dynamics, the elemental underpinnings of BM have been more precisely explained. Broadening these insights will allow more effective diagnostic and therapeutic interventions of developmental abnormalities and pathologies in pre- and postnatal lungs. Conservation of the molecular factors which are involved in the development of the lung (and other branched organs) is a classic example of nature's astuteness in economically utilizing finite resources. Once purposefully formed, well-tested and tried ways and means are adopted, preserved, and widely used to engineer the most optimal phenotypes. The material and time costs of developing utterly new instruments and routines with every drastic biological change (e.g. adaptation and speciation) are circumvented. This should assure the best possible structures and therefore functions, ensuring survival and evolutionary success.

Receptor-type guanylyl cyclase Gyc76C is required for development of the Drosophila embryonic somatic muscle

Guanylyl cyclases mediate a number of physiological processes, including smooth muscle function and axonal guidance. Here, we report a novel role for Drosophila receptor-type guanylyl cyclase at 76C, Gyc76C, in development of the embryonic somatic muscle. In embryos lacking function of Gyc76C or the downstream cGMP-dependent protein kinase (cGK), DG1, patterning of the somatic body wall muscles was abnormal with ventral and lateral muscle groups showing the most severe defects. In contrast, specification and elongation of the dorsal oblique and dorsal acute muscles of gyc76C mutant embryos was normal, and instead, these muscles showed defects in proper formation of the myotendinous junctions (MTJs). During MTJ formation in gyc76C and pkg21D mutant embryos, the βPS integrin subunit failed to localize to the MTJs and instead was found in discrete puncta within the myotubes. Tissue-specific rescue experiments showed that gyc76C function is required in the muscle for proper patterning and βPS integrin localization at the MTJ. These studies provide the first evidence for a requirement for Gyc76C and DG1 in Drosophila somatic muscle development, and suggest a role in transport and/or retention of integrin receptor subunits at the developing MTJs.

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.

A systematic screen for tube morphogenesis and branching genes in the Drosophila tracheal system

Many signaling proteins and transcription factors that induce and pattern organs have been identified, but relatively few of the downstream effectors that execute morphogenesis programs. Because such morphogenesis genes may function in many organs and developmental processes, mutations in them are expected to be pleiotropic and hence ignored or discarded in most standard genetic screens. Here we describe a systematic screen designed to identify all Drosophila third chromosome genes (∼40% of the genome) that function in development of the tracheal system, a tubular respiratory organ that provides a paradigm for branching morphogenesis. To identify potentially pleiotropic morphogenesis genes, the screen included analysis of marked clones of homozygous mutant tracheal cells in heterozygous animals, plus a secondary screen to exclude mutations in general “house-keeping” genes. From a collection including more than 5,000 lethal mutations, we identified 133 mutations representing ∼70 or more genes that subdivide the tracheal terminal branching program into six genetically separable steps, a previously established cell specification step plus five major morphogenesis and maturation steps: branching, growth, tubulogenesis, gas-filling, and maintenance. Molecular identification of 14 of the 70 genes demonstrates that they include six previously known tracheal genes, each with a novel function revealed by clonal analysis, and two well-known growth suppressors that establish an integral role for cell growth control in branching morphogenesis. The rest are new tracheal genes that function in morphogenesis and maturation, many through cytoskeletal and secretory pathways. The results suggest systematic genetic screens that include clonal analysis can elucidate the full organogenesis program and that over 200 patterning and morphogenesis genes are required to build even a relatively simple organ such as the Drosophila tracheal system.

Elucidating the genetic programs that control formation and maintenance of body organs is a central goal of developmental biology, and understanding how these programs go awry in disease has important implications for medicine. Many such organogenesis genes have been identified, but most are early-acting “patterning genes” encoding signaling proteins and gene regulators that control expression of a poorly characterized set of downstream “morphogenesis genes,” which encode proteins that generate the remarkable organ forms and structures of the constituent cells. We screened ∼40% of the fruit fly Drosophila genome for mutations that affect tracheal (respiratory) system development. We included steps to bypass complexities from mutant effects on other tissues and steps to exclude mutations in general cell “housekeeping genes.” We isolated mutations in ∼70 genes that identify major steps in the organogenesis program including an integral cell growth control step. Many of the new tracheal genes are “morphogenesis genes” that encode proteins involved in cell structure or intracellular transport. The results suggest that genetic screens can elucidate a full organogenesis program and that over 200 patterning and morphogenesis genes are required to build even a relatively simple organ.

Targeting the motor regulator Klar to lipid droplets

Background

In Drosophila, the transport regulator Klar displays tissue-specific localization: In photoreceptors, it is abundant on the nuclear envelope; in early embryos, it is absent from nuclei, but instead present on lipid droplets. Differential targeting of Klar appears to be due to isoform variation. Droplet targeting, in particular, has been suggested to occur via a variant C-terminal region, the LD domain. Although the LD domain is necessary and sufficient for droplet targeting in cultured cells, lack of specific reagents had made it previously impossible to analyze its role in vivo.

Results

Here we describe a new mutant allele of klar with a lesion specifically in the LD domain; this lesion abolishes both droplet localization of Klar and the ability of Klar to regulate droplet motion. It does not disrupt Klar's function for nuclear migration in photoreceptors. Using a GFP-LD fusion, we show that the LD domain is not only necessary but also sufficient for droplet targeting in vivo; it mediates droplet targeting in embryos, in ovaries, and in a number of somatic tissues.

Conclusions

Our analysis demonstrates that droplet targeting of Klar occurs via a cis-acting sequence and generates a new tool for monitoring lipid droplets in living tissues of Drosophila.

Drosophila Jing is part of the breathless fibroblast growth factor receptor positive feedback loop

In the developing Drosophila trachea, extensive cell migration lays the foundation for an elaborate network of tubules to form. This process is controlled by the Drosophila fibroblast growth factor receptor, known as Breathless (Btl), whose expression is activated by the Trachealess (Trh) and Tango (Tgo) basic helix-loop-helix (bHLH)-PAS transcription factors. We previously identified the jing zinc finger transcription factor as a gene sensitive to the dosage of bHLH-PAS transcriptional activity and showed that its mutations interact genetically with those of trh and btl. Here, we demonstrate that jing is required for btl expression in the branching trachea and dominantly interacts with known regulators of btl expression, including the ETS and POU transcription factors, pointed, and drifter/ventral veinless, respectively. Furthermore, the zinc finger-containing C-terminus of Jing associates with a btl tracheal enhancer in a Trh/Tgo-dependent manner in chromatin immunoprecipitation assays in vitro and interferes with btl in vitro and in vivo. Together, our results support a model by which Jing/Trh/Tgo complexes regulate btl transcript levels during primary tracheal branching.

Staying alive: dalmation mediated blocking of apoptosis is essential for tissue maintenance

In an EMS screen for mutations disrupting tracheal development, we identified new alleles of the dalmation (dmt) gene, which had previously been shown to affect peripheral nervous system (PNS) development. Here, we demonstrate that dmt loss results in programmed cell death, disrupting PNS patterning and leading to large gaps in the salivary duct and trachea. Dmt loss results in increased expression of the proapoptotic regulator genes head involution defective (hid) and reaper (rpr), and deletion of these genes or tissue-specific expression of the baculoviral apoptotic inhibitor P35 rescues the dmt defects. dmt is also required to protect cells from irradiation induced expression of hid and rpr during the irradiation resistant stage, which begins as cells become irreversibly committed to their final fates. Thus, we propose that Dmt keeps cells alive by blocking activation of hid and rpr as cells become irreversibly committed.

Diverse modes of Drosophila tracheal fusion cell transcriptional regulation

Drosophila tracheal fusion cells play multiple important roles in guiding and facilitating tracheal branch fusion. Mechanistic understanding of how fusion cells function during development requires deciphering their transcriptional circuitry. In this paper, three genes with distinct patterns of fusion cell expression were dissected by transgenic analysis to identify the cis-regulatory modules that mediate their transcription. Bioinformatic analysis involving phylogenetic comparisons coupled with mutational experiments were employed. The dysfusion bHLH-PAS gene was shown to have two fusion cell cis-regulatory modules; one driving initial expression and another autoregulatory module to enhance later transcription. Mutational dissection of the early module identified at least four distinct inputs, and included putative binding sites for ETS and POU-homeodomain proteins. The ETS transcription factor Pointed mediates the transcriptional output of the branchless/breathless signaling pathway, suggesting that this pathway directly controls dysfusion expression. Fusion cell cis-regulatory modules of CG13196 and CG15252 require two Dysfusion:Tango binding sites, but additional sequences modulate the breadth of activation in different fusion cell classes. These results begin to decode the regulatory circuitry that guides transcriptional activation of genes required for fusion cell morphogenesis.

Morphogenesis of epithelial tubes: Insights into tube formation, elongation, and elaboration

Epithelial tubes are a fundamental tissue across the metazoan phyla and provide an essential functional component of many of the major organs. Recent work in flies and mammals has begun to elucidate the cellular mechanisms driving the formation, elongation, and branching morphogenesis of epithelial tubes during development. Both forward and reverse genetic techniques have begun to identify critical molecular regulators for these processes and have revealed the conserved role of key pathways in regulating the growth and elaboration of tubular networks. In this review, we discuss the developmental programs driving the formation of branched epithelial networks, with specific emphasis on the trachea and salivary gland of Drosophila melanogaster and the mammalian lung, mammary gland, kidney, and salivary gland. We both highlight similarities in the development of these organs and attempt to identify tissue and organism specific strategies. Finally, we briefly consider how our understanding of the regulation of proliferation, apicobasal polarity, and epithelial motility during branching morphogenesis can be applied to understand the pathologic dysregulation of these same processes during metastatic cancer progression.

The Drosophila F-box protein Archipelago controls levels of the Trachealess transcription factor in the embryonic tracheal system

The archipelago gene (ago) encodes the F-box specificity subunit of an SCF(skp-cullin-f box) ubiquitin ligase that inhibits cell proliferation in Drosophila melanogaster and suppresses tumorigenesis in mammals. ago limits mitotic activity by targeting cell cycle and cell growth proteins for ubiquitin-dependent degradation, but the diverse developmental roles of other F-box proteins suggests that it is likely to have additional protein targets. Here we show that ago is required for the post-mitotic shaping of the Drosophila embryonic tracheal system, and that it acts in this tissue by targeting the Trachealess (Trh) protein, a conserved bHLH-PAS transcription factor. ago restricts Trh levels in vivo and antagonizes transcription of the breathless FGF receptor, a known target of Trh in the tracheal system. At a molecular level, the Ago protein binds Trh and is required for proteasome-dependent elimination of Trh in response to expression of the Dysfusion protein. ago mutations that elevate Trh levels in vivo are defective in binding forms of Trh found in Dysfusion-positive cells. These data identify a novel function for the ago ubiquitin-ligase in tracheal morphogenesis via Trh and its target breathless, and suggest that ago has distinct functions in mitotic and post-mitotic cells that influence its role in development and disease.

Drosophila Varicose, a member of a new subgroup of basolateral MAGUKs, is required for septate junctions and tracheal morphogenesis

Epithelial tubes are the functional units of many organs, but little is known about how tube sizes are established. Using the Drosophila tracheal system as a model, we previously showed that mutations in varicose (vari) cause tubes to become elongated without increasing cell number. Here we show vari is required for accumulation of the tracheal size-control proteins Vermiform and Serpentine in the tracheal lumen. We also show that vari is an essential septate junction (SJ) gene encoding a membrane associated guanylate kinase (MAGUK). In vivo analyses of domains important for MAGUK scaffolding functions demonstrate that while the Vari HOOK domain is essential, the L27 domain is dispensable. Phylogenetic analyses reveal that Vari helps define a new MAGUK subgroup that includes mammalian PALS2. Importantly, both Vari and PALS2 are basolateral, and the interaction of Vari with the cell-adhesion protein Neurexin IV parallels the interaction of PALS2 and another cell-adhesion protein, Necl-2. Vari therefore bolsters the similarity between Drosophila and vertebrate epithelial basolateral regions, which had previously been limited to the common basolateral localization of Scrib, Dlg and Lgl, proteins required for epithelial polarization at the beginning of embryogenesis. However, by contrast to Scrib, Dlg and Lgl, Vari is not required for cell polarity but rather is part of a cell-adhesion complex. Thus, Vari fundamentally extends the similarity of Drosophila and vertebrate basolateral regions from sharing only polarity complexes to sharing both polarity and cell-adhesion complexes.

From fate to function: the Drosophila trachea and salivary gland as models for tubulogenesis

Tube formation is a ubiquitous process required to sustain life in multicellular organisms. The tubular organs of adult mammals include the lungs, vasculature, digestive and excretory systems, as well as secretory organs such as the pancreas, salivary, prostate, and mammary glands. Other tissues, including the embryonic heart and neural tube, have requisite stages of tubular organization early in development. To learn the molecular and cellular basis of how epithelial cells are organized into tubular organs of various shapes and sizes, investigators have focused on the Drosophila trachea and salivary gland as model genetic systems for branched and unbranched tubes, respectively. Both organs begin as polarized epithelial placodes, which through coordinated cell shape changes, cell rearrangement, and cell migration form elongated tubes. Here, we discuss what has been discovered regarding the details of cell fate specification and tube formation in the two organs; these discoveries reveal significant conservation in the cellular and molecular events of tubulogenesis.

Dysfusion transcriptional control of Drosophila tracheal migration, adhesion, and fusion

The Drosophila dysfusion basic-helix-loop-helix-PAS transcription factor gene is expressed in specialized fusion cells that reside at the tips of migrating tracheal branches. dysfusion mutants were isolated, and genetic analysis of live embryos revealed that mutant tracheal branches migrate to close proximity but fail to recognize and adhere to each other. Misexpression of dysfusion throughout the trachea further indicated that dysfusion has the ability to both inhibit cell migration and promote ectopic tracheal fusion. Nineteen genes whose expression either increases or decreases in fusion cells during development were analyzed in dysfusion mutant embryos. dysfusion upregulates the levels of four genes, including the shotgun cell adhesion protein gene and the zona pellucida family transmembrane protein gene, CG13196. Misexpression experiments with CG13196 result in ectopic tracheal fusion events, suggesting that it also encodes a cell adhesion protein. Another target gene of dysfusion is members only, which inhibits protein nuclear export and influences tracheal fusion. dysfusion also indirectly downregulates protein levels of Trachealess, an important regulator of tracheal development. These results indicate that fusion cells undergo dynamic changes in gene expression as they switch from migratory to fusion modes and that dysfusion regulates a discrete, but important, set of these genes.

Egfr is essential for maintaining epithelial integrity during tracheal remodelling in Drosophila

A fundamental requirement during organogenesis is to preserve tissue integrity to render a mature and functional structure. Many epithelial organs, such as the branched tubular structures, undergo a tremendous process of tissue remodelling to attain their final pattern. The cohesive properties of these tissues need to be finely regulated to promote adhesion yet allow flexibility during extensive tissue remodelling. Here, we report a new role for the Egfr pathway in maintaining epithelial integrity during tracheal development in Drosophila. We show that the integrity-promoting Egfr function is transduced by the ERK-type MAPK pathway, but does not require the downstream transcription factor Pointed. Compromising Egfr signalling, by downregulating different elements of the pathway or by overexpressing the Mkp3 negative regulator, leads to loss of tube integrity, whereas upregulation of the pathway results in increased tissue stiffness. We find that regulation of MAPK pathway activity by Breathless signalling does not impinge on tissue integrity. Egfr effects on tissue integrity correlate with differences in the accumulation of markers for cadherin-based cell-cell adhesion. Accordingly, downregulation of cadherin-based cell-cell adhesion gives rise to tracheal integrity defects. Our results suggest that the Egfr pathway regulates maintenance of tissue integrity, at least in part, through the modulation of cell adhesion. This finding establishes a link between a developmental pathway governing tracheal formation and cell adhesiveness.

Organ positioning in Drosophila requires complex tissue-tissue interactions

Positioning an organ with respect to other tissues is a complex process necessary for proper anatomical development and organ function. The local environment surrounding an organ can serve both as a substrate for migration and as a source of guidance cues that direct migration. Little is known about the factors guiding Drosophila salivary gland movement or about the contacts the glands establish along their migratory path. Here, we provide a detailed description of the spatial and temporal interactions between the salivary glands and surrounding tissues during embryogenesis. The glands directly contact five other tissues: the visceral mesoderm, gastric caecae, somatic mesoderm, fat body, and central nervous system. Mutational analysis reveals that all of the tissues tested in this study are important for normal salivary gland positioning; proper differentiation of the visceral and somatic mesoderm is necessary for the glands to attain their final correct position. We also provide evidence that the segment-polarity gene, gooseberry (gsb), controls expression of signals from the developing fat body that direct posterior migration of the glands. These data further the understanding of how organ morphology and position are determined by three-dimensional constraints and guidance cues provided by neighboring tissues.