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J Appl Physiol 97: 2347–2353, 2004; doi:10.1152/japplphysiol.00435.2004. HIGHLIGHTED TOPIC Invited Review Lung Growth and Repair Cellular and molecular mechanisms involved in branching morphogenesis of the Drosophila tracheal system Clemens Cabernard, Marc Neumann, and Markus Affolter Abteilung Zellbiologie, Biozentrum der Universität Basel, Basel, Switzerland tubulogenesis; cell migration; fibroblast growth factor signaling; air sacs; zona pellucida domain-containing proteins; adherens junction remodeling THE QUESTION OF HOW PARTICULAR ORGANS form during development and how these organs ultimately function during adult life has moved to center stage in developmental genetics. The conservation of genetic regulatory cascades and networks during evolution argues that simple model organisms might contribute important information concerning organ formation, although the architecture of these organs is simpler and their function more limited compared with higher organisms, such as mice and humans. The fruit fly Drosophila melanogaster is a particularly wellcharacterized model organism. Genetic analysis over the past century have led to the characterization of approximately onefourth of the estimated 13,500 genes present in the genome (2). All major signaling pathways involved in cell-cell interactions during development of higher animals are conserved in the fruit fly, and many of the organ-specific transcriptional regulators play similar roles in the fly compared with mice or humans (37, 57). Several organ systems have been dissected in Drosophila using genetic and molecular approaches (1, 5, 15, 17, 21, 47). Here, we briefly summarize the current state of knowledge concerning the development of the respiratory organ of the fly (the trachea) and mention parallels between tracheal development and the development of the lung in mammals. The larval tracheal system consists of hundreds of interconnected tubes that transport oxygen and other gases throughout the body (3, 18, 51). Tracheal branches are simple tubes consisting of an epithelial monolayer wrapped into a tube around a central lumen. The development of the tracheal system is initiated in the early embryo on the determination of 10 bilaterally symmetrical clusters of ⬃20 tracheal precursor cells; on invagination and two additional cell divisions, each of the tracheal sacs undergoes a similar sequence of developmental events to generate one segment of the network in the absence of further cell divisions (Fig. 1, A–H). The transformation, in a few hours time, of a simple epithelial sac into a highly ramified, yet stereotyped, tubular network is a fascinating phenomenon and has attracted the attention of a number of research groups sharing the aim of better understanding the cellular and molecular principles underlying the control and regulation of branching morphogenesis. As it turns out, three key cellular processes are involved in branch formation: directional cell migration, cell rearrangements, and cell shape changes. To interconnect the individual metameres, certain tubes ultimately fuse with corresponding tubes in neighbor segments (38, 48). The most insightful finding with regard to the spatial control of tracheal branch formation in the Drosophila embryo came from the identification and characterization of two genes, breathless/FGFR (btl/FGFR) and branchless/FGF (bnl/FGF). The absence of either of these two gene products leads to a complete failure of branch outgrowth despite proper determination of tracheal cells (29, 45). btl/FGFR encodes a fibroblast growth factor (FGF) receptor expressed in tracheal cells but not Address for reprint requests and other correspondence: M. Affolter, Abteilung Zellbiologie, Biozentrum der Universität Basel, CH-4056 Basel, Switzerland (E-mail: [email protected]). The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. CELL MIGRATION CONTROLS BRANCH FORMATION IN THE EMBRYO http://www. jap.org 8750-7587/04 $5.00 Copyright © 2004 the American Physiological Society 2347 Downloaded from http://jap.physiology.org/ by 10.220.33.5 on June 18, 2017 Cabernard, Clemens, Marc Neumann, and Markus Affolter. Cellular and molecular mechanisms involved in branching morphogenesis of the Drosophila tracheal system. J Appl Physiol 97: 2347–2353, 2004; doi:10.1152/japplphysiol. 00435.2004.—Recent comparative studies have shown that, in many instances, the genetic network underlying the development of distinct organ systems is similar in invertebrate and vertebrate organisms. Genetically well-characterized, simple invertebrate model systems, such as Caenorhabditis elegans and Drosophila melanogaster, can thus provide useful insight for understanding more complex organ systems in vertebrates. Here, we summarize recent progress in the genetic analysis of tracheal development in Drosophila and compare the results to studies aimed at a better understanding of lung development in mouse and man. Clearly, both striking similarities and important differences are apparent, but it might still be too early to conclude whether the former or the latter prevail. Invited Review 2348 DROSOPHILA TRACHEAL SYSTEM MORPHOGENESIS in surrounding epithelial cells (29), whereas bnl/FGF encodes the FGF ligand of Btl/FGFR and is expressed in individual cells or cell clusters surrounding the invaginating tracheal placode (50). Gain-of-function and loss-of-function studies (50) combined with live imaging (46) revealed that the Bnl/ FGF ligand induces a migratory behavior by promoting cytoskeletal dynamics in a few cells at the tip of the tracheal branches (see Fig.1, K–N). The stalk cells appear to follow these tip cells passively; during branch formation, all tracheal cells maintain their epithelial character and remain attached to each other via their subapical adherens junctions (AJ). It is not clear yet whether Bnl/FGF acts as a true chemoattractant or as a motogen. A function as a chemoattractant is supported by the finding that bnl/FGF expression in ectopic positions induces branch outgrowth toward such sites (49, 50). However, it was shown that the direction of migration is influenced by other signaling systems, such as Dpp/BMP (46, 52). In the absence of Dpp/BMP signaling, prospective dorsal branch cells start to migrate dorsally but ultimately reintegrate into the dorsal trunk and thus never form a definitive dorsal branch (for nomenclature, see Fig. 1I) (46). Conversely, ectopic Dpp/BMP signaling is able to direct cells from migration along the anterior-posterior axis toward migration along the dorsal-ventral axis (52). How Dpp/BMP influences the direction of migration is not known and remains to be investigated. What are the molecular consequences of FGF signaling in tracheal cells, or, in other words, how are cells instructed to become motile and follow directions under the control of Bnl/FGF? Genetic studies have led to the identification of a mutation in a gene called downstream-of-FGFR (dof), which displays a tracheal phenotype identical to bnl/FGF and btl/ FGFR (25, 40, 53). Dof turns out to be a cytoplasmic signaling J Appl Physiol • VOL adaptor protein, present in only those cells of the embryo that express FGF receptors. Dof interacts constitutively with the Drosophila FGF receptors and becomes phosphorylated by the latter on activation, leading to the recruitment of the Corkscrew phosphatase (44, 59). Corkscrew recruitment represents an essential step in Bnl/FGF-induced cell migration and in the activation of the Ras/MAPK pathway. However, it appears that Ras activation is not sufficient to activate the migration machinery in tracheal cells (44). Additional proteins binding either to the FGFRs, to Dof, or to Corkscrew appear to be crucial for a migratory response, and it will be important to identify these missing links. Somewhat surprisingly, tracheal cells also migrate properly when the spatial information of Bnl/FGF ligand expression is transmitted to the cell interior by the kinase domain of a different receptor tyrosine kinase; this was shown via the expression of chimeric receptor molecules carrying the extracellular domain of Btl/FGFR and the intracellular kinase domain of unrelated receptor tyrosine kinases, such as epidermal growth factor receptor or Torso (16). These results demonstrate that tracheal cells respond with directed migration to receptor tyrosine kinase signaling (whereas other cells in the developing embryos respond differently). Therefore, it will be important to decipher the regulatory program that leads to the determination of epithelial cells toward the tracheal fate; identification and characterization of genes that are specifically activated or repressed in tracheal cells might shed light on why and how tracheal cells respond to FGF signaling with migration. These studies on the formation of the tracheal tree provide the first insights into molecular events of guided cell migration controlled by FGF signaling. However, the developing embryo of Drosophila is not an ideal model system to investigate these 97 • DECEMBER 2004 • www.jap.org Downloaded from http://jap.physiology.org/ by 10.220.33.5 on June 18, 2017 Fig. 1. Tracheal development in the Drosophila embryo. A–D: development of the embryonic tracheal system. The tracheal system arises from 10 placodes on each side of the embryo that bud in from the epidermis. On invagination of the tracheal cells and after 2 rounds of cell division, the individual tracheal metameres consist of ⬃80 cells (A). From this placode, 6 branches grow out into different directions (B–D). Subsequently, some of these branches fuse to form a continuous network that will be used for oxygen supply in the hatching larva. E–H: schematic drawing of the development of an individual (boxed) tracheal metamere. Lateral views show the stereotyped branching pattern of the tracheal epithelium (TR; red) as well as the expression of the fibroblast growth factor (FGF) ligand bnl (blue). Light blue indicates fading bnl/FGF expression. Top views (e– h) indicate the connection to the epidermis (EP) and the flattening of the tracheal sac on progression of development. Note that embryonic branching morphogenesis does not rely on cell division but on cell migration, cell intercalation, and cell shape changes. I: nomenclature of the tracheal branches. DB, dorsal branch; DTa/p, dorsal trunk anterior/posterior; VB, visceral branch; SB, spiracular branch; LTa/p, lateral trunk anterior/posterior. K–N: live images of a dorsal branch expressing actin tagged with green fluorescent protein. Branch outgrowth is driven by Bnl/FGF, which induces filopodia in tracheal tip cells (arrows). During branch outgrowth, the cells are rearranged from a side-by-side (K) to an end-to-end (N) arrangement, visualized here by the rearrangement of the nuclei (*). Invited Review DROSOPHILA TRACHEAL SYSTEM MORPHOGENESIS CELL REARRANGEMENTS AND CELL SHAPE CHANGES LEAD TO BRANCH ELONGATION IN THE EMBRYO Quite obviously, the active migration of a small number of tracheal tip cells away from the saclike structure of the invaginated tracheal placode has to be accompanied by cell rearrangements and cell shape changes to generate elongated tracheal branches. Because tracheal cells are at all time part of a tightly sealed epithelium, epithelial AJs must be remodeled to a significant extent during branch formation. Indeed, extensive AJ remodeling is required for the generation of most tracheal branches of a late Drosophila embryo, and individual steps in the remodeling process have been described in dorsal branch formation (27, 48). During the early stages of branch formation, tracheal cells are initially arranged in a side-by-side-side fashion (see Fig. 1K). Through cell intercalation (Fig. 1, L–M), this conformation is transformed into an end-to-end arrangement (Fig. 1N). The dorsal branch (as well as most other branches) finally consists of single cells wrapped around the lumen and closed up by autocellular AJs (27, 48). The molecular events underlying AJ remodeling have not yet been elucidated, but it appears that the coordination of cell rearrangements with cell migration requires additional developmental signaling systems. For example, the outgrowth of a particular branch, the dorsal branch, required Dpp/BMP signaling in addition to Bnl/FGF signaling (46). The lack of Dpp/BMP leads to the absence of dorsal branches, despite the presence of filopodia formation and initial dorsal branch outgrowth under the control of Bnl/FGF signaling; in the absence of Dpp/BMP signaling, cells in the initial dorsal bud reintegrate into the major tracheal branch, the dorsal trunk (46). Thus Dpp/BMP signaling appears to be required for proper cell rearrangements to take place, and in the absence of concomitant cell rearrangements cell migration is not sufficient to bring about the formation of distinct tracheal branches. It also appears that the activity level of the small GTPase Rac has a strong impact on cell rearrangements; reduced Rac activity inhibits tracheal cell rearrangements, whereas hyperactivation leads to the loss of tracheal cell adhesion (12). These effects J Appl Physiol • VOL might be due to a regulation of the levels of E-cadherin in tracheal cells, but how exactly Rac activity levels and Ecadherin protein levels are coordinated remains to be determined. Interestingly, a recent study has made a link between tracheal cell rearrangements and the presence of proteins secreted apically into the luminal space (27). Piopio and Dumpy, two proteins containing a zona pellucida domain (28), have been shown to contribute to the apical extracellular matrix and to play an important role during the intercalation process (see Fig. 1, K–N) and autocellular AJ formation in tracheal development (27). In the absence of these two proteins, all fine tracheal branches form cysts instead of tubes, leading to a disruption of the tracheal network. Although the precise role of the two proteins remains to be firmly established, this study demonstrates for the first time an important requirement for luminal matrix proteins in tubulogenesis. DEVELOPMENTAL CONTROL OF TUBE SIZE, TUBE DIAMETER, AND LIQUID CLEARANCE The size and shape of the branches making up a tubular organ are fundamental parameters in the control of transport demands. Like in other tubular organs, tracheal tubes increase in size and diameter as the animal grows during development. At the end of larval life, tracheal tubes have expanded up to 40 times their initial size. This increase in tube size does not depend on cell division or on the number of cells in the branches but is controlled mostly at the apical surface of tracheal cells; although there is a dramatic increase in the inner tube diameter, little change is observed in the outer tube diameter (7). Tube size is controlled genetically and is not modified under high- or low-oxygen conditions. A number of mutants, in which the length as well as the diameter of tracheal branches is altered, have been described (7). The two Drosophila Claudin family members Megatrachea (6, 7) and Sinuous (60) as well as the cell surface protein Lachesin (36) are components of septate junctions, and mutations in these genes show overgrown tubes. Other components of septate junctions, such as Coracle and Neurexin, also show similar phenotypes in the tracheal system when absent (6, 43). However, whether the developmental control of tube shape, e.g., the increase in tube diameter in larval stages, is exerted via these genes remains to be investigated. Apart from septate junction components, the control of the growth of the apical membrane seems to be equally important for maintaining a balanced tube size and length. The transcription factor Grainy head (Grh) limits apical membrane growth; in grh mutants, several branches are convoluted, and grh tracheal cells show an expansion of the apical membrane in late embryonic stages, resulting in an enlarged and anomalous luminal surface (22). Neither Grh target genes nor other effector proteins involved in the control of the apical surface of tracheal cells have been identified so far. Liquid clearance of the tracheal system provides a developmentally controlled physiological aspect of this tubular organ. At the end of embryogenesis, 2 h before larvae emerge, the liquid in the tracheal system is cleared and replaced by air (38). Clearance occurs by active epithelial absorption, and epithelial Na⫹ channels appear to be involved in this process. Several 97 • DECEMBER 2004 • www.jap.org Downloaded from http://jap.physiology.org/ by 10.220.33.5 on June 18, 2017 molecular events at a much more detailed level using a genetic approach. This is due to the fact that many proteins essential for either cell signaling (i.e., Ras) and/or cytoskeletal rearrangements (i.e., Rac) are maternally provided (large amounts of RNA and/or protein products of numerous genes are transferred from the polyploid nurse cells of the mother to the oocyte and are sufficient for the early development of the embryo) and thus often fail to be detected in standard genetic screens for zygotic effects. In addition, the lack of proteins involved in important, basic cellular events might lead to dramatic defects in embryonic stages before tracheal branching, making analysis of their contribution to branching morphogenesis more difficult. However, and as we will outline below, FGF signaling via Bnl/FGF and Btl/FGFR is again used during larval and pupal stages to induce branching morphogenesis in a growing tracheal epithelium, the developing air sac (49). This system allows the identification of genes involved in branching morphogenesis in a genetically more amenable context, and major progress toward a better molecular understanding of FGF signaling in branching morphogenesis is expected to be obtained by studying this system. 2349 Invited Review 2350 DROSOPHILA TRACHEAL SYSTEM MORPHOGENESIS Degenerin/epithelial Na⫹ channel family members are expressed quite specifically in the tracheal system. Inhibitor as well as RNAi studies targeting the products of two of these genes (PPK4, PPK11) inhibited liquid clearance (34), suggesting that the regulated activity of these channels contributes to the proper differentiation of tracheal cells. PHYSIOLOGICAL ASPECTS OF TRACHEAL DEVELOPMENT AND FUNCTION COORDINATION BETWEEN CELL MIGRATION AND CELL DIVISION DURING AIR SAC FORMATION IN THE LARVA A striking difference between the development of branched organs in higher vertebrates and the formation of the tracheal system in the Drosophila embryo lies in the fact that cells do not divide during the branching process in the embryonic tracheal system of the fly; the increase in the three-dimensional complexity of the network is due to cell migration, cell rearrangements, and cell shape changes. During larval development, however, the tracheal system is remodelled tremendously to satisfy the needs in the adult fly, and the formation of new structures includes, and presumably relies on, controlled cell proliferation (38). Recently, the formation of the thoracic air sacs of the adult fly has been investigated (49). The thoracic air sac is an offshoot of an existing tracheal branch, the transverse connective, which attaches to the wing imaginal disc (see Fig. 2). In the early-third instar larva, some 50 h before pupation, a few tracheal cells of the transverse connective branch start to proliferate and migrate out in a stereotypical manner. Both btl/FGFR and dof are expressed in the tracheal cells of this growing air sac, suggesting that the FGF signaling system, Fig. 2. Air sac development in the third instar larva of Drosophila. A: cross section through a wing imaginal disc of a Drosophila third instar larva. The transverse connective branch (TC) attaches to the columnar epithelial (CE) of the wing imaginal disc. A group of CE cells, lying underneath the outgrowing air sac, secrete FGF/Bnl (depicted in blue). PM, peripodial membrane. B: schematic drawings of three stages of air sac development corresponding to early, mid, and late third instar larval stages. The tracheal cells of the TC as well as the air sac tracheoblast cells express the Bnl/FGF receptor Btl/ FGFR (red), whereas cells from CE express Bnl/FGF (blue). Air sac tracheoblast cells follow the Bnl/FGF-secreting CE cells via cell migration, and the air sac increases in size through cell proliferation. C: projection of a wing imaginal disc from a late third instar larva. D: close up of the boxed areas in B. A membrane-targeted GFP protein outlines all wing imaginal disc cells, whereas the tracheal cells express Moesin tagged with real fluorescent protein, under the control of the trachea-specific enhancer of btl/FGFR. J Appl Physiol • VOL 97 • DECEMBER 2004 • www.jap.org Downloaded from http://jap.physiology.org/ by 10.220.33.5 on June 18, 2017 Quite in contrast to the stereotyped, developmentally controlled organization of the major branches of the larval tracheal system, the organization and ramification of the so-called terminal branches are under physiological control. Terminal branch formation is regulated by the gene DSRF/blistered, which encodes the Drosophila homolog of serum response factor (4, 20, 42). DSRF/blistered is specifically expressed in terminal tracheal cells and required for the formation of tens to hundreds of long cytoplasmic processes by each of these cells during the larval stages. These cytoplasmic processes form an intracellular lumen, which connects up to the lumen of the main tracheal network. Larvae grown under nonphysiological, low-oxygen conditions show a dramatic increase in terminal branching; conversely, larvae grown under high-oxygen tension show the opposite phenotype, having relatively few terminal branches (26). A link between terminal branching and oxygen conditions has been established through the analysis of bnl/FGF, which is expressed broadly in the larva and in all tissues that become heavily tracheated. Strikingly, bnl/FGF transcription is enhanced in many cells under low hypoxic conditions. Oxygen deprivation stimulates expression of bnl/FGF, and the secreted growth factor functions as a chemoattractant that guides new terminal branches to the expressing cells. This implies that, in Drosophila, Bnl/FGF is either a or possibly the key mediator of the hypoxic response (26). These experiments suggested that an oxygen-sensing pathway exists in Drosophila. Indeed, it has been shown more recently that the hypoxia-inducing factor-␣, encoded by the gene similar (sima), as well as the hypoxia-inducing factor- subunit encoded by tango, are required for the hypoxic response. Sima and tango code for basic helix-loop-helix PAS transcription factors that form heterodimers. It was also proposed that Sima protein stability is controlled by oxygen through the Prolyl 4-hydroxylase homolog encoded by CG1114; a mutant in which CG1114 protein expression is reduced shows an upregulation of Sima under normal physiological oxygen conditions (31). Thus fine terminal branches are formed from terminal tracheal cells as a result of oxygen needs in target tissues, but how the outgrowth of terminal branches is controlled and coordinated by FGF signaling remains to be established. Invited Review DROSOPHILA TRACHEAL SYSTEM MORPHOGENESIS SIMILARITIES BETWEEN TRACHEAL BRANCHING IN DROSOPHILA AND LUNG BRANCHING IN MAMMALS Clearly, the most striking similarity between tracheal development in Drosophila and lung-branching morphogenesis in mammals is the involvement of the FGF signaling pathway as a key regulator of branching (3, 14, 23, 54, 55). In mice, FGF10 is clearly associated with early branching, because in FGF10 mutant mice branching morphogenesis is completely absent (41). In addition, FGF10 expression in the mesenchyme surrounding the growing lung epithelium correlates well with local branch outgrowth (8). The precise function of FGF10 in lung development in mice is less well understood than the role of Bnl/FGF in tracheal branching, but organ culture experiments favor a role of FGF10 as both a chemoattractant/ motogen and a mitogen (58), similar to the role of Bnl/FGF in J Appl Physiol • VOL larval air sac branching in the fly (49). Studying the molecular consequences of FGF signaling in the larval tracheal system in Drosophila will provide further insight into the molecular events downstream of FGF signaling during branch formation. In addition, these studies might shed light on the dual role of FGF in mammals, the control and coordination of cell proliferation, and cell migration. Like in Drosophila, other signaling pathways, including the Hh and the Dpp/BMP pathway, play important roles in lung branching in mice. It appears that, in the mouse, these signaling pathways act in part by limiting the extent of FGF10 signaling in the lung epithelium (13). In Drosophila, both the Wnt (11, 35) and the Dpp/BMP pathway are essential for successful branching in the embryo but, as outlined above, Dpp/BMP plays a major role in controlling cell rearrangements during branch outgrowth. To our knowledge, it is not clear whether AJ remodeling, in particular the formation of tubes with autocellular AJs, is important for the formation of fine terminal branches in the lung. Thus it remains to be seen to what extent similarities between Drosophila and mice or humans exist with regard to cell rearrangements. Recent studies have demonstrated a requirement of luminal zona pellucida-domain proteins during the branching process in Drosophila (27). Zona pellucida-domain proteins are also major components of lumen-containing epithelial organs in vertebrates (10, 24, 30, 32, 33, 39, 56). It remains to be seen whether they contribute to the proper formation of these organs during development or whether they play physiological roles. Without doubt, the number of molecules linked to the process of branching morphogenesis of specific organs will dramatically increase in the future, and it is hoped that this will lead not only to an increase in the molecular complexity underlying the branching process but also to an enhancement of clarity with regard to developmental logic. ACKNOWLEDGMENTS We thank all members of the Affolter laboratory for discussions. GRANTS The authors are supported by funds from the Swiss National Science Foundation and the Kantons Basel-Stadt and Basel-Land. REFERENCES 1. Abrams EW, Vining MS, and Andrew DJ. Constructing an organ: the Drosophila salivary gland as a model for tube formation. Trends Cell Biol 13: 247–254, 2003. 2. Adams MD, Celniker SE, Holt RA, Evans CA, Gocayne JD, Amanatides PG, Scherer SE, Li PW, Hoskins RA, Galle RF, George RA, Lewis SE, Richards S, Ashburner M, Henderson SN, Sutton GG, Wortman JR, Yandell MD, Zhang Q, Chen LX, Brandon RC, Rogers YH, Blazej RG, Champe M, Pfeiffer BD, Wan KH, Doyle C, Baxter EG, Helt G, Nelson CR, Gabor GL, Abril JF, Agbayani A, An HJ, AndrewsPfannkoch C, Baldwin D, Ballew RM, Basu A, Baxendale J, Bayraktaroglu L, Beasley EM, Beeson KY, Benos PV, Berman BP, Bhandari D, Bolshakov S, Borkova D, Botchan MR, Bouck J, Brokstein P, Brottier P, Burtis KC, Busam DA, Butler H, Cadieu E, Center A, Chandra I, Cherry JM, Cawley S, Dahlke C, Davenport LB, Davies P, de Pablos B, Delcher A, Deng Z, Mays AD, Dew I, Dietz SM, Dodson K, Doup LE, Downes M, Dugan-Rocha S, Dunkov BC, Dunn P, Durbin KJ, Evangelista CC, Ferraz C, Ferriera S, Fleischmann W, Fosler C, Gabrielian AE, Garg NS, Gelbart WM, Glasser K, Glodek A, Gong F, Gorrell JH, Gu Z, Guan P, Harris M, Harris NL, Harvey D, Heiman TJ, Hernandez JR, Houck J, Hostin D, Houston KA, Howland TJ, Wei MH, Ibegwam C, Jalali M, Kalush F, Karpen GH, 97 • DECEMBER 2004 • www.jap.org Downloaded from http://jap.physiology.org/ by 10.220.33.5 on June 18, 2017 which shapes the embryonic branches, might also be involved in late branching. Indeed, both the proliferation and migration of tracheal cells in the developing air sac are dependent on a functional FGF signaling pathway (49). The ligand Bnl/FGF itself is produced and secreted from a small number of columnar epithelial cells in close proximity to the tracheal cells of the transverse connective (Fig. 2, A and B). As air sac tracheoblasts undergo intensive proliferation and migrate toward the Bnl/ FGF-expressing columnar epithelial cells (Fig. 2, A and B), cells at the tip of the growing sac extend filopodia, probably as a result of Bnl/FGF signal reception. Bnl/FGF appears to guide the migration of air sac tracheal cells, an interpretation that is supported by the finding that filopodia extend toward ectopic Bnl/FGF sources in genetic mosaic experiments. During pupal stages, the air sac does not cease to grow and eventually forms three distinct branches, the medioscutal, the lateroscutal, and the scutellar sac, in the thorax of the adult fly (49). Intriguingly, Bnl/FGF fulfils a dual role during air sac formation, serving both as a chemoattractant/motogen as well as a mitogen. The molecular basis for this dual role has not been addressed so far, and many questions remain open concerning the development of air sacs. As mentioned above, there are a number of serious limitations to the genetic analysis of cell migration in the developing embryo. The proliferative capacity of the air sac tracheoblasts in late larval stages opens the door for the application of genetic mosaic analysis, namely the generation of genetically altered (for example mutant) cells in an otherwise heterozygous organ via mitotic recombination (9, 19, 61). Because the migration of air sac cells occurs some 70 h after egg laying, the maternal store of RNA and/or protein of most genes has been used up, and the effect of the removal of these genes on tracheal cell migration can be studied in genetic mosaic analysis in these late developmental stages. Generation of mosaic air sacs, including small clusters of mutated cells, will allow addressing a range of open questions, such as the role of different molecules in cell migration and cell proliferation, in particular with regard to components of the FGFR signaling pathway, such as Ras, Raf, or Erk (all of which are maternally provided). These studies will allow a better understanding of the molecular pathways that link FGF signaling to cytoskeletal dynamics and eventually provide a more precise molecular view of how FGF signaling controls branching morphogenesis. 2351 Invited Review 2352 3. 4. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. Ke Z, Kennison JA, Ketchum KA, Kimmel BE, Kodira CD, Kraft C, Kravitz S, Kulp D, Lai Z, Lasko P, Lei Y, Levitsky AA, Li J, Li Z, Liang Y, Lin X, Liu X, Mattei B, McIntosh TC, McLeod MP, McPherson D, Merkulov G, Milshina NV, Mobarry C, Morris J, Moshrefi A, Mount SM, Moy M, Murphy B, Murphy L, Muzny DM, Nelson DL, Nelson DR, Nelson KA, Nixon K, Nusskern DR, Pacleb JM, Palazzolo M, Pittman GS, Pan S, Pollard J, Puri V, Reese MG, Reinert K, Remington K, Saunders RD, Scheeler F, Shen H, Shue BC, Siden-Kiamos I, Simpson M, Skupski MP, Smith T, Spier E, Spradling AC, Stapleton M, Strong R, Sun E, Svirskas R, Tector C, Turner R, Venter E, Wang AH, Wang X, Wang ZY, Wassarman DA, Weinstock GM, Weissenbach J, Williams SM, WoodageT, Worley KC, Wu D, Yang S, Yao QA, Ye J, Yeh RF, Zaveri JS, Zhan M, Zhang G, Zhao Q, Zheng L, Zheng XH, Zhong FN, Zhong W, Zhou X, Zhu S, Zhu X, Smith HO, Gibbs RA, Myers EW, Rubin GM, and Venter JC. The genome sequence of Drosophila melanogaster. Science 287: 2185–2195, 2000. Affolter M, Bellusci S, Itoh N, Shilo B, Thiery JP, and Werb Z. Tube or not tube: remodeling epithelial tissues by branching morphogenesis. Dev Cell 4: 11–18, 2003. Affolter M, Montagne J, Walldorf U, Groppe J, Kloter U, LaRosa M, and Gehring WJ. The Drosophila SRF homolog is expressed in a subset of tracheal cells and maps within a genomic region required for tracheal development. Development 120: 743–753, 1994. Baylies MK and Michelson AM. Invertebrate myogenesis: looking back to the future of muscle development. Curr Opin Genet Dev 11: 431– 439, 2001. Behr M, Riedel D, and Schuh R. The claudin-like megatrachea is essential in septate junctions for the epithelial barrier function in Drosophila. Dev Cell 5: 611– 620, 2003. Beitel GJ and Krasnow MA. Genetic control of epithelial tube size in the Drosophila tracheal system. Development 127: 3271–3282, 2000. Bellusci S, Grindley J, Emoto H, Itoh N, and Hogan BL. Fibroblast growth factor 10 (FGF10) and branching morphogenesis in the embryonic mouse lung. Development 124: 4867– 4878, 1997. Blair SS. Genetic mosaic techniques for studying Drosophila development. Development 130: 5065–5072, 2003. Cheng H, Bjerknes M, and Chen H. CRP-ductin: a gene expressed in intestinal crypts and in pancreatic and hepatic ducts. Anat Rec 244: 327–343, 1996. Chihara T and Hayashi S. Control of tracheal tubulogenesis by Wingless signaling. Development 127: 4433– 4442, 2000. Chihara T, Kato K, Taniguchi M, Ng J, and Hayashi S. Rac promotes epithelial cell rearrangement during tracheal tubulogenesis in Drosophila. Development 130: 1419 –1428, 2003. Chuang PT, Kawcak T, and McMahon AP. Feedback control of mammalian Hedgehog signaling by the Hedgehog-binding protein, Hip1, modulates Fgf signaling during branching morphogenesis of the lung. Genes Dev 17: 342–347, 2003. Chuang PT and McMahon AP. Branching morphogenesis of the lung: new molecular insights into an old problem. Trends Cell Biol 13: 86 –91, 2003. Dickson BJ. Molecular mechanisms of axon guidance. Science 298: 1959 –1964, 2002. Dossenbach C, Rock S, and Affolter M. Specificity of FGF signaling in cell migration in Drosophila. Development 128: 4563– 4572, 2001. Gehring WJ. The genetic control of eye development and its implications for the evolution of the various eye-types. Int J Dev Biol 46: 65–73, 2002. Ghabrial A, Luschnig S, Metzstein MM, and Krasnow MA. Branching morphogenesis of the Drosophila tracheal system. Annu Rev Cell Dev Biol 19: 623– 647, 2003. Golic KG. Site-specific recombination between homologous chromosomes in Drosophila. Science 252: 958 –961, 1991. Guillemin K, Groppe J, Ducker K, Treisman R, Hafen E, Affolter M, and Krasnow MA. The pruned gene encodes the Drosophila serum response factor and regulates cytoplasmic outgrowth during terminal branching of the tracheal system. Development 122: 1353–1362, 1996. Hartmann B and Reichert H. The genetics of embryonic brain development in Drosophila. Mol Cell Neurosci 12: 194 –205, 1998. Hemphala J, Uv A, Cantera R, Bray S, and Samakovlis C. Grainy head controls apical membrane growth and tube elongation in response to Branchless/FGF signalling. Development 130: 249 –258, 2003. J Appl Physiol • VOL 23. Hogan BL and Kolodziej PA. Organogenesis: molecular mechanisms of tubulogenesis. Nat Rev Genet 3: 513–523, 2002. 24. Hoops TC and Rindler MJ. Isolation of the cDNA encoding glycoprotein-2 (GP-2), the major zymogen granule membrane protein. Homology to uromodulin/Tamm-Horsfall protein. J Biol Chem 266: 4257– 4263, 1991. 25. Imam F, Sutherland D, Huang W, and Krasnow MA. Stumps, a Drosophila gene required for fibroblast growth factor (FGF)-directed migrations of tracheal and mesodermal cells. Genetics 152: 307–318, 1999. 26. Jarecki J and Keshishian H. Role of neural activity during synaptogenesis in Drosophila. J Neurosci 15: 8177– 8190, 1995. 27. Jazwinska A, Ribeiro C, and Affolter M. Epithelial tube morphogenesis during Drosophila tracheal development requires Piopio, a luminal ZP protein. Nat Cell Biol 5: 895–901, 2003. 28. Jovine L, Qi H, Williams Z, Litscher E, and Wassarman PM. The ZP domain is a conserved module for polymerization of extracellular proteins. Nat Cell Biol 4: 457– 461, 2002. 29. Klambt C, Glazer L, and Shilo BZ. Breathless, a Drosophila FGF receptor homolog, is essential for migration of tracheal and specific midline glial cells. Genes Dev 6: 1668 –1678, 1992. 30. Kokot F and Dulawa J. Tamm-Horsfall protein updated. Nephron 85: 97–102, 2000. 31. Lavista-Llanos S, Centanin L, Irisarri M, Russo DM, Gleadle JM, Bocca SN, Muzzopappa M, Ratcliffe PJ, and Wappner P. Control of the hypoxic response in Drosophila melanogaster by the basic helix-loophelix PAS protein similar. Mol Cell Biol 22: 6842– 6853, 2002. 32. Li DY, Sorensen LK, Brooke BS, Urness LD, Davis EC, Taylor DG, Boak BB, and Wendel DP. Defective angiogenesis in mice lacking endoglin. Science 284: 1534 –1537, 1999. 33. Li XJ and Snyder SH. Molecular cloning of Ebnerin, a von Ebner’s gland protein associated with taste buds. J Biol Chem 270: 17674 –17679, 1995. 34. Liu L, Johnson WA, and Welsh MJ. Drosophila DEG/ENaC pickpocket genes are expressed in the tracheal system, where they may be involved in liquid clearance. Proc Natl Acad Sci USA 100: 2128 –2133, 2003. 35. Llimargas M. Wingless and its signalling pathway have common and separable functions during tracheal development. Development 127: 4407– 4417, 2000. 36. Llimargas M, Strigini M, Katidou M, Karagogeos D, and Casanova J. Lachesin is a component of a septate junction-based mechanism that controls tube size and epithelial integrity in the Drosophila tracheal system. Development 131: 181–190, 2004. 37. Mann RS and Carroll SB. Molecular mechanisms of selector gene function and evolution. Curr Opin Genet Dev 12: 592– 600, 2002. 38. Manning G and Krasnow MA. Development of the Drosophila tracheal system. In: The Development of Drosophila Melanogaster, edited by Martinez Arias A. Plainview, NY: Cold Spring Harbor Laboratory Press, 1993, p. 609 – 685. 39. McAllister KA, Grogg KM, Johnson DW, Gallione CJ, Baldwin MA, Jackson CE, Helmbold EA, Markel DS, McKinnon WC, Murrell J, et al. Endoglin, a TGF- binding protein of endothelial cells, is the gene for hereditary haemorrhagic telangiectasia type 1. Nat Genet 8: 345–351, 1994. 40. Michelson AM, Gisselbrecht S, Buff E, and Skeath JB. Heartbroken is a specific downstream mediator of FGF receptor signalling in Drosophila. Development 125: 4379 – 4389, 1998. 41. Min H, Danilenko DM, Scully SA, Bolon B, Ring BD, Tarpley JE, DeRose M, and Simonet WS. Fgf-10 is required for both limb and lung development and exhibits striking functional similarity to Drosophila branchless. Genes Dev 12: 3156 –3161, 1998. 42. Montagne J, Groppe J, Guillemin K, Krasnow MA, Gehring WJ, and Affolter M. The Drosophila serum response factor gene is required for the formation of intervein tissue of the wing and is allelic to blistered. Development 122: 2589 –2597, 1996. 43. Paul SM, Ternet M, Salvaterra PM, and Beitel GJ. The Na⫹/K⫹ ATPase is required for septate junction function and epithelial tube-size control in the Drosophila tracheal system. Development 130: 4963– 4974, 2003. 44. Petit V, Nussbaumer U, Dossenbach C, and Affolter M. Downstreamof-FGFR is a fibroblast growth factor-specific scaffolding protein and recruits Corkscrew upon receptor activation. Mol Cell Biol 24: 3769 – 3781, 2004. 97 • DECEMBER 2004 • www.jap.org Downloaded from http://jap.physiology.org/ by 10.220.33.5 on June 18, 2017 5. DROSOPHILA TRACHEAL SYSTEM MORPHOGENESIS Invited Review DROSOPHILA TRACHEAL SYSTEM MORPHOGENESIS J Appl Physiol • VOL 53. Vincent S, Wilson R, Coelho C, Affolter M, and Leptin M. The Drosophila protein Dof is specifically required for FGF signaling. Mol Cell 2: 515–525, 1998. 54. Warburton D, Bellusci S, Del Moral PM, Kaartinen V, Lee M, Tefft D, and Shi W. Growth factor signaling in lung morphogenetic centers: automaticity, stereotypy and symmetry. Respir Res 4: 5, 2003. 55. Warburton D, Schwarz M, Tefft D, Flores-Delgado G, Anderson KD, and Cardoso WV. The molecular basis of lung morphogenesis. Mech Dev 92: 55– 81, 2000. 56. Wassarman PM, Jovine L, and Litscher ES. A profile of fertilization in mammals. Nat Cell Biol 3: 59 – 64, 2001. 57. Weatherbee SD and Carroll SB. Selector genes and limb identity in arthropods and vertebrates. Cell 97: 283–286, 1999. 58. Weaver M, Dunn NR, and Hogan BL. Bmp4 and Fgf10 play opposing roles during lung bud morphogenesis. Development 127: 2695–2704, 2000. 59. Wilson R, Battersby A, Csiszar A, Vogelsang E, and Leptin M. A functional domain of Dof that is required for fibroblast growth factor signaling. Mol Cell Biol 24: 2263–2276, 2004. 60. Wu VM, Schulte J, Hirschi A, Tepass U, and Beitel GJ. Sinuous is a Drosophila claudin required for septate junction organization and epithelial tube size control. J Cell Biol 164: 313–323, 2004. 61. Xu T and Rubin GM. Analysis of genetic mosaics in developing and adult Drosophila tissues. Development 117: 1223–1237, 1993. 97 • DECEMBER 2004 • www.jap.org Downloaded from http://jap.physiology.org/ by 10.220.33.5 on June 18, 2017 45. Reichman-Fried M, Dickson B, Hafen E, and Shilo BZ. Elucidation of the role of breathless, a Drosophila FGF receptor homolog, in tracheal cell migration. Genes Dev 8: 428 – 439, 1994. 46. Ribeiro C, Ebner A, and Affolter M. In vivo imaging reveals different cellular functions for FGF and Dpp signaling in tracheal branching morphogenesis. Dev Cell 2: 677– 683, 2002. 47. Ribeiro C, Petit V, and Affolter M. Signaling systems, guided cell migration, and organogenesis: insights from genetic studies in Drosophila. Dev Biol 260: 1– 8, 2003. 48. Samakovlis C, Hacohen N, Manning G, Sutherland DC, Guillemin K, and Krasnow MA. Development of the Drosophila tracheal system occurs by a series of morphologically distinct but genetically coupled branching events. Development 122: 1395–1407, 1996. 49. Sato M and Kornberg TB. FGF is an essential mitogen and chemoattractant for the air sacs of the Drosophila tracheal system. Dev Cell 3: 195–207, 2002. 50. Sutherland D, Samakovlis C, and Krasnow MA. Branchless encodes a Drosophila FGF homolog that controls tracheal cell migration and the pattern of branching. Cell 87: 1091–1101, 1996. 51. Uv A, Cantera R, and Samakovlis C. Drosophila tracheal morphogenesis: intricate cellular solutions to basic plumbing problems. Trends Cell Biol 13: 301–309, 2003. 52. Vincent S, Ruberte E, Grieder NC, Chen CK, Haerry T, Schuh R, and Affolter M. DPP controls tracheal cell migration along the dorsoventral body axis of the Drosophila embryo. Development 124: 2741–2750, 1997. 2353