The Drosophila melanogaster lymph gland is a hematopoietic organ1 and, together with prospective vascular cells (cardioblasts) and excretory cells (pericardial nephrocytes), arises from the cardiogenic mesoderm. Clonal analysis provided evidence for a hemangioblast2,3 that can give rise to two daughter cells: one that differentiates into heart or aorta and another that differentiates into blood. In addition, the GATA factor gene pannier (pnr) and the homeobox gene tinman (tin), which are controlled by the convergence of Decapentaplegic (Dpp), fibroblast growth factor (FGF), Wingless (Wg) and Notch signaling, are required for the development of all cardiogenic mesoderm, including the lymph gland. Here we show that an essential genetic switch that differentiates between the blood or nephrocyte and vascular lineages involves the Notch pathway. Further specification occurs through specific expression of the GATA factor Serpent (Srp) in the lymph-gland primordium. Our findings suggest that there is a close parallel between the molecular mechanisms functioning in the D. melanogaster cardiogenic mesoderm and those functioning in the mammalian aorta-gonadal-mesonephros mesoderm4.
Blood and vascular cells in the vertebrate embryo are thought to derive from oligopotent progenitor cells, called hemangioblasts5, that arise in the yolk sac and in the aorta-gonadal-mesonephros (AGM) mesenchyme4. A close relationship between blood and vascular progenitors is well established, but in vivo evidence that a single cell can divide to produce a blood cell and an endothelial cell is lacking in vertebrate systems. Similarly, the molecular mechanism that distinguishes between the two lineages is not well understood. To address these issues in a simple, genetically amenable system, we analyzed the genetic control of hematopoiesis in D. melanogaster. Our results show that there are close lineage relationships between hematopoietic and vascular cells, similar to those present in the AGM of mammalian systems. We also provide evidence for conserved cassettes of transcription factors and signaling cascades that limit the pool of hemangioblastic cells and promote the blood versus vascular fate.
In the mature D. melanogaster embryo, the lymph gland is formed by a paired cluster of ∼20 cells flanking the aorta (Fig. 1a). The aorta and heart represent a contractile tube lined by a layer of myoepithelial vascular cells called cardioblasts. The cells flanking the aorta and heart posterior to the lymph gland are the pericardial cells, which function as excretory cells (nephrocytes)6. We found that lymph gland progenitors express the prohemocyte marker Srp (Fig. 1b,i) and ultrastructurally resemble prohemocytes that develop at an earlier stage from the head mesoderm (Fig. 1c). Monitoring expression of the zinc-finger protein Odd-skipped (Odd) showed that the lymph gland originates from the dorsal thoracic mesoderm (Fig. 1d–h). Odd is expressed in segmental clusters in the dorsal mesoderm of segments T1–A6 (ref. 7). The three thoracic Odd-positive clusters coalesced to form the lymph gland, whereas the abdominal clusters formed the pericardial nephrocytes (Fig. 1g,h).
Lymph-gland progenitors, cardioblasts and pericardial cells are closely related by lineage. We induced labeled 'flipout' (FLP/FRT) clones in embryos aged 3–4 h such that the clones contained only 2–4 cells (Fig. 2a). Of the two-cell clones, ∼50% contained cardioblast and lymph-gland cells (Fig. 2b–e); the other clones comprised either cardioblasts7,8 or lymph-gland cells alone (Fig. 2c). We also recovered mixed clones at the late third larval stage (Fig. 2f,g). The finding of mixed clones indicates that the cardiogenic mesoderm of D. melanogaster contains oligopotent progenitors that, up to the final division, can give rise both to Srp-positive blood-cell progenitors that form the lymph gland and to vascular cells (Fig. 2h).
The cardiogenic mesoderm forms part of the dorsal mesoderm, which requires the homeobox protein Tin and the GATA factor Pnr9. In embryos with mutations in tin or pnr, the lymph gland was absent (Fig. 3a,b,d). Maintenance of Tin expression in the dorsal mesoderm requires the activity of at least two signaling pathways regulated by Dpp (the D. melanogaster homolog of transforming growth factor-β) and Heartless (Htl; one of the D. melanogaster homologs of the FGF receptor); the dependence of cardioblast and pericardial nephrocyte development on these signaling pathways is documented10,11. We found that lymph-gland progenitors did not develop in loss-of-function dpp and htl mutants (Fig. 3c,e).
Between 6 h and 8 h of development, the dorsal mesoderm splits into the cardiogenic mesoderm and the visceral mesoderm. The cardiogenic mesoderm is regulated positively by Wg and negatively by Notch. Lack of Wg signaling results in the absence of all cardiogenic lineages12 including lymph gland (Fig. 3f). Notch signaling has the opposite effect and restricts cardiogenic mesodermal fate. Notch is active in the dorsal mesoderm from 6 h to 10 h of development. We found that eliminating Notch during the first half of this interval by raising embryos homozygous with respect to the temperature-sensitive allele Nts1 at the restrictive temperature resulted in substantially more cardioblasts13, pericardial cells and lymph-gland progenitors (Fig. 3g–l).
Lymph-gland progenitors, cardioblasts and pericardial nephrocytes are specified in the cardiogenic mesoderm around the phase of germband retraction 8–10 h after fertilization. At this stage, we found that Tin, which was initially expressed in the whole cardiogenic mesoderm (Fig. 4a), became restricted to a narrow medial compartment containing the cardioblasts (Fig. 4b). Pnr followed the same restriction (Fig. 4c). Cells located at a more lateral level in the cardiogenic mesoderm gave rise to lymph-gland progenitors (in the thoracic domain) and pericardial nephrocytes (in the abdominal domain) and activated the gene odd (Fig. 4a,b). Slightly later, Srp was expressed in lymph-gland progenitors (Fig. 4d). As reported for the early hemocytes derived from the embryonic head14,15, srp is centrally involved in lymph-gland specification. In srp-null embryos, Odd-expressing cells still formed a lymph gland–shaped cluster flanking the aorta, but these cells also expressed the pericardial marker pericardin (Prc)16 (Fig. 4e), suggesting that they had lost some aspects of hemocyte precursor identity or gained properties of nephrocytes. As a countercorrelate, ectopic expression of Srp in the whole cardiogenic mesoderm directed by mef2-Gal4 induced pericardial cells to adopt lymph-gland fate (Fig. 4f).
Downregulation of tin and pnr in cells in the lateral domain of the cardiogenic mesoderm is essential for lymph-gland specification. Ectopic expression of tin or pnr by twist-Gal4 (or mef2-Gal4) caused a marked reduction in the number of lymph-gland and pericardial cells (Fig. 4g,h). The antagonistic effect of tin on lymph-gland progenitors resembled its earlier role in the head mesoderm that gives rise to the larval blood cells17; here too, ectopic expression of tin causes a reduction in the number of hemocytes.
Inhibiting tin and upregulating odd and srp requires input from the Notch signaling pathway. We described a function of Notch at 6–8 h in specification of the cardiogenic mesoderm (Fig. 3). Reducing Notch function between 8 h and 10 h caused an increase in the number of cardioblasts (Fig. 5d) and a concomitant loss of pericardial and lymph-gland cells (Fig. 5a,b). Overexpressing an activated Notch construct caused a marked increase in lymph-gland size (Fig. 5c). This late requirement for Notch signaling was separable from the earlier role of Notch in restricting the overall size of the cardiogenic mesoderm. Thus, the sum total of cardioblasts and pericardial or lymph-gland cells in Nts1 embryos shifted between 8 h and 10 h did not differ substantially from that in wild type, whereas a combined effect on cell number and cell fate was seen in embryos with a Notch deletion (Fig. 5e). In these embryos, the cardiogenic mesoderm was hyperplasic and developed as cardioblasts at the expense of lymph-gland progenitors and pericardial nephrocytes. The dual role of Notch in restricting the numbers of a pluripotent progenitor pool and in distinguishing between the progeny of these progenitors is reminiscent of the function of Notch in sense-organ development18.
Lymph-gland formation is restricted to the thoracic region by positional cues that are provided by expression of the homeobox proteins of the Antennapedia and Bithorax complex. Specifically, Ultrabithorax (Ubx), which is expressed in segments A2–A5 of the cardiogenic mesoderm19, inhibits lymph-gland formation. We found that loss of Ubx resulted in the expansion of the lymph-gland fate into the abdominal segments (Fig. 5f). Conversely, overexpression of Ubx driven by mef2-Gal4 caused the transformation of lymph-gland progenitors into pericardial nephrocytes (Fig. 5g).
Our findings are suggestive of a model of lymph-gland development in D. melanogaster that is similar to mammalian hematopoiesis (Fig. 6). Lymph-gland progenitors develop as part of the cardiogenic mesoderm that also gives rise to the vascular cells (aorta and heart) and to excretory cells. Similarly, progenitor cells of the blood, aorta and excretory system are closely related both molecularly and developmentally in mammals, where they form part of the AGM. Specification of the cardiogenic mesoderm requires the input of FGF10, Dpp11 and Wg12 signaling, as in vertebrate hematopoiesis, where the AGM region is induced in response to several converging signaling pathways including FGF20, BMP21 and Wnt22.
Our findings show that in D. melanogaster, the cardiovascular and blood-cell lineages are differentiated by an antagonistic relationship between Tin or Pnr expression in the cardioblasts and Srp expression in the lymph-gland progenitors. In vertebrates, GATA factors also have a pivotal role in specifying different lineages among blood-cell progenitors23, although not much is known about what differentiates between blood progenitors as a group and endothelial progenitors. Our results indicate that this step is driven by input from the Notch signaling pathway. In the thoracic cardiogenic mesoderm, Notch antagonizes tin and pnr expression and aortic cardioblast formation, and promotes srp expression and the development of lymph-gland progenitors. In vertebrates, Notch signaling is also involved in both blood and vascular development24,25,26. The role of Notch during AGM morphogenesis remains to be investigated.
Cardioblasts and lymph-gland cells can arise from the division of a single cardiogenic mesodermal cell, which should be called a hemangioblast. A previous study induced clones in the cardiogenic mesoderm but used only Tin as a marker8. This study also yielded mixed two-cell clones comprising a cardioblast and a nonlabeled cell, which, in light of our findings, must be interpreted as a lymph-gland cell. Hemangioblasts have been proposed in vertebrates, although the definitive experiment in which a precursor is marked and its lineage is tracked has not been done. Blast colony-forming cells that give rise to both lineages in vitro3,6 and common markers that belong to both cell types in vivo27 have been identified, but direct evidence for the existence of a common precursor has not yet been found. Our study, using genetic analysis of two-cell clones, establishes the existence of such a population in D. melanogaster. On the basis of these results, and given the conservation of the signaling and transcriptional components described here, our prediction is that many cells of the AGM in vertebrates may give rise to only blood or only vascular cells, but a number of intermixed hemangioblasts may give rise to mixed lineages. Future genetic screens aimed at finding components in early lymph-gland development will probably identify additional pathways and strategies important for vertebrate hematopoiesis.
Fly strains and antibodies.
We used the following D. melanogaster lines and antibodies (α): tin346, UAS-tin and α-Tin (gifts from M. Frasch, Mount Sinai School of Medicine, New York, New York, USA), Act5C-FRT-Draf-FRT-tau-lacZ and α-Odd (gifts from J. Skeath, Washington University School of Medicine, St. Louis, Missouri, USA), srp6G (a gift from R. Reuter, University Tuebingen, Germany), UAS-srp and α-Srp (gifts from D. Hoshizaki, University of Nevada, Las Vegas, USA), htlAB42 (a gift from A.M. Michelson, Harvard Medical School, Boston, Massachusetts, USA), pnrvx6 and α-Pnr (gifts from G. Morata, Universidad Autonoma de Madrid, Spain), UAS-pnr (a gift from M. Haenlin, CNRS/UPS, France), mef2-Gal4 (a gift from E. Chen, University of Texas Southwestern Medical Center, Dallas, Texas), Ntsl (a gift from R. Cagan, Washington University School of Medicine, St. Louis, Missouri, USA), α-Odd (a gift from D. Kosman, University of California, San Diego), α-Prc (a gift from D. Gratecos, Centre Univ. Marseille, France), α-Dmef2 (a gift from H.T. Nguyen, Albert Einstein College of Medicine, New York, New York, USA) and β-galactosidase (Promega). The other fly stocks (dppH46, Df(1)N81K, wgCX4, UAS-Nintra, UAS-Ubx and Ubx9.22 Sbsbd) were from the Bloomington Stock Center. We carried out histological studies and electron microscopy as described28.
We created random lacZ-expressing clones in the cardiogenic region by using the FLP/FRT lineage-tracing system7 with some modifications. We heat-shocked hs-flp Act5C-FRT-Draf-FRT-tau-lacZ embryos, aged 3–4 h, for 30 min at 37 °C to induce Flp recombinase. We then shifted them back to 18 °C and aged them until stages 15–16. We fixed and immunostained the embryos by standard methods.
At appropriate stages, as indicated in the text, we collected Nts1 embryos at 18 °C, reared them for 2 h at the restrictive temperature (31 °C) and then transferred them to the permissive temperature (18 °C) until stages 15–16. We used wild-type embryos (Oregon R) subjected to the same temperature regimens as controls.
Evans, C.J., Hartenstein, V. & Banerjee, U. Thicker than blood: conserved mechanisms in Drosophila and vertebrate hematopoiesis. Dev. Cell 5, 673–690 (2003).
Murray, P.D.F. The development in vitro of the blood of the early chick embryo. Proc. R. Soc. Lond. B 11, 497–521 (1932).
Choi, K., Kennedy, M., Kazarov, A., Papadimitriou, J.C. & Keller, G. A common precursor for hematopoietic and endothelial cells. Development 125, 725–732 (1998).
Medvinsky, A. & Dzierzak, E. Definitive hematopoiesis is autonomously initiated by the AGM region. Cell 86, 897–906 (1996).
Ema, M. et al. Combinatorial effects of the Flk1 and Tal1 on the vascular and hematopoietic development in the mouse. Genes Dev. 17, 380–393 (2003).
Crossley, A.C. The ultrastructure and function of pericardial cells and other nephrocytes in an insect: Calliphora erythrocephala. Tissue Cell 4, 529–560 (1972).
Ward, E.J. & Skeath, J.B. Characterization of a novel subset of cardiac cells and their progenitors in the Drosophila embryo. Development 127, 4959–4969 (2000).
Alvarez, A.D., Shi, W., Wilson, B.A. & Skeath, J.B. Pannier and PointedP2 act sequentially to regulate Drosophila heart development. Development 130, 3015–3026 (2003).
Klinedinst, S.L. & Bodmer, R. GATA factor Pannier is required to establish competence for heart progenitor formation. Development 130, 3027–3038 (2003).
Beiman, M., Shilo, B.Z. & Volk, T. Heartless, a Drosophila FGF receptor homolog, is essential for cell migration and establishment of several mesodermal lineages. Genes Dev. 10, 2993–3002 (1996).
Frasch, M. Induction of visceral and cardiac mesoderm by ectodermal Dpp in the early Drosophila embryo. Nature 374, 464–467 (1995).
Wu, X., Golden, K. & Bodmer, R. Heart development in Drosophila requires the segment polarity gene wingless. Dev. Biol. 169, 619–628 (1995).
Hartenstein, A.Y., Rugendorff, A., Tepass, U. & Hartenstein, V. The function of the neurogenic genes during epithelial development in the Drosophila embryo. Development 116, 1203–1220 (1992).
Rehorn, K.P., Thelen, H., Michelson, A.M. & Reuter, R. A molecular aspect of hematopoiesis and endoderm development common to vertebrates and Drosophila. Development 122, 4023–4031 (1996).
Lebestky, T., Chang, T., Hartenstein, V. & Banerjee, U. Specification of Drosophila hematopoietic lineage by conserved transcription factors. Science 288, 146–149 (2000).
Chartier, A., Zaffran, S., Astier, M., Semeriva, M. & Gratecos, D. Pericardin, a Drosophila type IV collagen-like protein is involved in the morphogenesis and maintenance of the heart epithelium during dorsal ectoderm closure. Development 129, 3241–3253 (2002).
Yin, Z., Xu, X.L. & Frasch, M. Regulation of the Twist target gene tinman by modular cis-regulatory elements during early mesoderm development. Development 124, 4971–4982 (1997).
Hartenstein, V. & Posakony, J.W. A dual function of the Notch gene in Drosophila sensillum development. Dev. Biol. 142, 13–30 (1990).
Lo, P.C., Skeath, J.B., Gajewski, K., Schulz, R.A. & Frasch, M. Homeotic genes autonomously specify the anteroposterior subdivision of the Drosophila dorsal vessel into aorta and heart. Dev. Biol. 251, 307–319 (2002).
Nishikawa, M. et al. Role of the microenvironment of the embryonic aorta-gonad-mesonephros region in hematopoiesis. Ann. NY Acad. Sci. 938, 109–116 (2001).
Marshall, C.J., Kinnon, C. & Thrasher, A.J. Polarized expression of bone morphogenetic protein-4 in the human aorta-gonad-mesonephros region. Blood 96, 1591–1593 (2000).
Orelio, C. & Dzierzak, E. Identification of 2 novel genes developmentally regulated in the mouse aorta-gonad-mesonephros region. Blood 101, 2246–2249 (2003).
Fossett, N. & Schulz, R.A. Functional conservation of hematopoietic factors in Drosophila and vertebrates. Differentiation 69, 83–90 (2001).
Kumano, K. et al. Notch1 but not Notch2 is essential for generating hematopoietic stem cells from endothelial cells. Immunity 18, 699–711 (2003).
Lawson, N.D. et al. Notch signaling is required for arterial-venous differentiation during embryonic vascular development. Development 128, 3675–3678 (2001).
Schroeder, T. et al. Recombination signal sequence-binding protein Jκ alters mesodermal cell fate decisions by suppressing cardiomyogenesis. Proc. Natl. Acad. Sci. USA 100, 4018–4023 (2003).
Fehling, H.J. et al. Tracking mesoderm induction and its specification to the hemangioblast during embryonic stem cell differentiation. Development 130, 4217–4227 (2003).
Tepass, U., Fessler, L.I., Aziz, A. & Hartenstein, V. Embryonic origin of hemocytes and their relationship to cell death in Drosophila. Development 120, 1829–1837 (1994).
We thank R. Cagan, J.A. Campos-Ortega, M. Cho, M. Frasch, D. Gratecos, M. Haenlin, D. Hoshizaki, K. Matthews, A.M. Michelson, G. Morata, H.T. Nguyen, R. Reuter, D. Kosman and J. Skeath for fly stocks and antibodies; and members of the laboratories of V.H. and U.B., particularly S.-H. Jung, for help and advice on the manuscript.
The authors declare no competing financial interests.
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Mandal, L., Banerjee, U. & Hartenstein, V. Evidence for a fruit fly hemangioblast and similarities between lymph-gland hematopoiesis in fruit fly and mammal aorta-gonadal-mesonephros mesoderm. Nat Genet 36, 1019–1023 (2004). https://doi.org/10.1038/ng1404
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