Pericardin, aDrosophilatype IV collagen-like protein is involved in the morphogenesis and maintenance of the heart epithelium during dorsal ectoderm closure

Extracellular matrix and its receptors in Drosophila neural development

Extracellular matrix (ECM) and matrix receptors are intimately involved in most biological processes. The ECM plays fundamental developmental and physiological roles in health and disease, including processes underlying the development, maintenance, and regeneration of the nervous system. To understand the principles of ECM-mediated functions in the nervous system, genetic model organisms like Drosophila provide simple, malleable, and powerful experimental platforms. This article provides an overview of ECM proteins and receptors in Drosophila. It then focuses on their roles during three progressive phases of neural development: (1) neural progenitor proliferation, (2) axonal growth and pathfinding, and (3) synapse formation and function. Each section highlights known ECM and ECM-receptor components and recent studies done in mutant conditions to reveal their in vivo functions, all illustrating the enormous opportunities provided when merging work on the nervous system with systematic research into ECM-related gene functions.

Loss of PTB or negative regulation of Notch mRNA reveals distinct zones of Notch and actin protein accumulation in Drosophila embryo

Polypyrimidine Tract Binding (PTB) protein is a regulator of mRNA processing and translation. Genetic screens and studies of wing and bristle development during the post-embryonic stages of Drosophila suggest that it is a negative regulator of the Notch pathway. How PTB regulates the Notch pathway is unknown. Our studies of Drosophila embryogenesis indicate that (1) the Notch mRNA is a potential target of PTB, (2) PTB and Notch functions in the dorso-lateral regions of the Drosophila embryo are linked to actin regulation but not their functions in the ventral region, and (3) the actin-related Notch activity in the dorso-lateral regions might require a Notch activity at or near the cell surface that is different from the nuclear Notch activity involved in cell fate specification in the ventral region. These data raise the possibility that the Drosophila embryo is divided into zones of different PTB and Notch activities based on whether or not they are linked to actin regulation. They also provide clues to the almost forgotten role of Notch in cell adhesion and reveal a role for the Notch pathway in cell fusions.

Syndecan contributes to heart cell specification and lumen formation during Drosophila cardiogenesis

The transmembrane proteoglycan Syndecan contributes to cell surface signaling of diverse ligands in mammals, yet in Drosophila, genetic evidence links Syndecan only to the Slit receptor Roundabout and to the receptor tyrosine phosphatase LAR. Here we characterize the requirement for syndecan in the determination and morphogenesis of the Drosophila heart, and reveal two phases of activity, indicating that Syndecan is a co-factor in at least two signaling events in this tissue. There is a stochastic failure to determine heart cell progenitors in a subset of abdominal hemisegments in embryos mutant for syndecan, and subsequent to Syndecan depletion by RNA interference. This phenotype is sensitive to gene dosage in the FGF receptor (Heartless), its ligand, Pyramus, as well as BMP-ligand Decapentaplegic (Dpp) and co-factor Sara. Syndecan is also required for lumen formation during assembly of the heart vessel, a phenotype shared with mutations in the Slit and Integrin signaling pathways. Phenotypic interactions of syndecan with slit and Integrin mutants suggest intersecting function, consistent with Syndecan acting as a co-receptor for Slit in the Drosophila heart.

Drosophila models of cardiac disease

The fruit fly Drosophila melanogaster has emerged as a useful model for cardiac diseases, both developmental abnormalities and adult functional impairment. Using the tools of both classical and molecular genetics, the study of the developing fly heart has been instrumental in identifying the major signaling events of cardiac field formation, cardiomyocyte specification, and the formation of the functioning heart tube. The larval stage of fly cardiac development has become an important model system for testing isolated preparations of living hearts for the effects of biological and pharmacological compounds on cardiac activity. Meanwhile, the recent development of effective techniques to study adult cardiac performance in the fly has opened new uses for the Drosophila model system. The fly system is now being used to study long-term alterations in adult performance caused by factors such as diet, exercise, and normal aging. The fly is a unique and valuable system for the study of such complex, long-term interactions, as it is the only invertebrate genetic model system with a working heart developmentally homologous to the vertebrate heart. Thus, the fly model combines the advantages of invertebrate genetics (such as large populations, facile molecular genetic techniques, and short lifespan) with physiological measurement techniques that allow meaningful comparisons with data from vertebrate model systems. As such, the fly model is well situated to make important contributions to the understanding of complicated interactions between environmental factors and genetics in the long-term regulation of cardiac performance.

Drosophila laminins act as key regulators of basement membrane assembly and morphogenesis

Laminins are heterotrimeric molecules found in all basement membranes. In mammals, they have been involved in diverse developmental processes, from gastrulation to tissue maintenance. The Drosophila genome encodes two laminin alpha chains, one beta and one Gamma, which form two distinct laminin trimers. So far, only mutations affecting one or other trimer have been analysed. In order to study embryonic development in the complete absence of laminins, we mutated the gene encoding the sole laminin beta chain in Drosophila, LanB1, so that no trimers can be made. We show that LanB1 mutant embryos develop until the end of embryogenesis. Electron microscopy analysis of mutant embryos reveals that the basement membranes are absent and the remaining extracellular material appears disorganised and diffuse. Accordingly, abnormal accumulation of major basement membrane components, such as Collagen IV and Perlecan, is observed in mutant tissues. In addition, we show that elimination of LanB1 prevents the normal morphogenesis of most organs and tissues, including the gut, trachea, muscles and nervous system. In spite of the above structural roles for laminins, our results unravel novel functions in cell adhesion, migration and rearrangement. We propose that while an early function of laminins in gastrulation is not conserved in Drosophila and mammals, their function in basement membrane assembly and organogenesis seems to be maintained throughout evolution.

Bithorax complex genes control alary muscle patterning along the cardiac tube of Drosophila

Cardiac specification models are widely utilized to provide insight into the expression and function of homologous genes and structures in humans. In Drosophila, contractions of the alary muscles control hemolymph inflow and support the cardiac tube, however embryonic development of these muscles remain largely understudied. We found that alary muscles in Drosophila embryos appear as segmental pairs, attaching dorsally at the seven-up (svp) expressing pericardial cells along the cardiac dorsal vessel, and laterally to the body wall. Normal patterning of alary muscles along the dorsal vessel was found to be a function of the Bithorax Complex genes abdominal-A (abd-A) and Ultrabithorax (Ubx) but not of the orphan nuclear receptor gene svp. Ectopic expression of either abd-A or Ubx resulted in an increase in the number of alary muscle pairs from seven to 10, and also produced a general elongation of the dorsal vessel. A single knockout of Ubx resulted in a reduced number of alary muscles. Double knockouts of both Ubx and abd-A prevented alary muscles from developing normally and from attaching to the dorsal vessel. These studies demonstrate an additional facet of muscle development that depends upon the Hox genes, and define for the first time mechanisms that impact development of this important subset of muscles.

Non-autonomous modulation of heart rhythm, contractility and morphology in adult fruit flies

The outermost layer of the vertebrate heart originates from migratory mesothelial cells (epicardium) that give rise to coronary vascular smooth muscles and fibroblasts. The role of the epicardium in myocardial morphogenesis and establishment of normal heart function is still largely unknown. Here, we use Drosophila to investigate non-autonomous influences of epicardial-like tissue surrounding the heart tube on the structural and functional integrity of the myocardium. It has previously been shown that during Drosophila heart formation, mesodermal expression of the homeobox transcription factor even-skipped (eve) is required for specification of a subset of non-myocardial progenitors in the precardiac mesoderm. These progenitors may share some similarities with the vertebrate epicardium. To investigate a non-autonomous epicardial-like influence on myocardial physiology, we studied the consequences of reduced mesodermal Eve expression and epi/pericardial cell numbers on the maturation of the myocardial heart tube, its contractility, and acquisition of a normal heart rhythm in the Drosophila model. Targeting the eve repressor ladybird early (lbe) with the minimal eve mesodermal enhancer efficiently eliminates the mesodermal Eve lineages. These flies exhibit defects in heart structure, including a reduction in systolic and diastolic diameter (akin to 'restrictive cardiomyopathy'). They also exhibit an elevated incidence of arrhythmias and intermittent asystoles, as well as compromised performance under stress. These abnormalities are restored by eve reexpression or by lbe-RNAi co-overexpression. The data suggest that adult heart function in Drosophila is likely to be modulated non-autonomously, possibly by paracrine influences from neighboring cells, such as the epi/pericardium. Thus, Drosophila may serve as a model for finding genetic effectors of epicardial-myocardial interactions relevant to higher organisms.

Cardiac gene regulatory networks in Drosophila

The Drosophila system has proven a powerful tool to help unlock the regulatory processes that occur during specification and differentiation of the embryonic heart. In this review, we focus upon a temporal analysis of the molecular events that result in heart formation in Drosophila, with a particular emphasis upon how genomic and other cutting-edge approaches are being brought to bear upon the subject. We anticipate that systems-level approaches will contribute greatly to our comprehension of heart development and disease in the animal kingdom.

Antagonistic function of Lmd and Zfh1 fine tunes cell fate decisions in the Twi and Tin positive mesoderm of Drosophila melanogaster

In this study we show that cell fate decisions in the dorsal and lateral mesoderm of Drosophila melanogaster depend on the antagonistic action of the Gli-like transcription factor Lame duck (Lmd) and the zinc finger homeodomain factor Zfh1. Lmd expression leads to the reduction of Zfh1 positive cell types, thereby restricting the number of Odd-skipped (Odd) positive and Tinman (Tin) positive pericardial cells in the dorsal mesoderm. In more lateral regions, ectopic activation of Zfh1 or loss of Lmd leads to an excess of adult muscle precursor (AMP) like cells. We also observed that Lmd is co-expressed with Tin in the early dorsal mesoderm and leads to a reduction of Tin expression in cells destined to become dorsal fusion competent myoblasts (FCMs). In the absence of Lmd function, these cells remain Tin positive and develop as Tin positive pericardial cells although they do not express Zfh1. We show further that Tin repression and pericardial restriction in the dorsal mesoderm facilitated by Lmd is instructed by a late Decapentaplegic (Dpp) signal that is abolished in embryos carrying the disk region mutation dpp(d6).

Heterotrimeric G proteins regulate a noncanonical function of septate junction proteins to maintain cardiac integrity in Drosophila

The gene networks regulating heart morphology and cardiac integrity are largely unknown. We previously reported a role for the heterotrimeric G protein gamma subunit 1 (Ggamma1) in mediating cardial-pericardial cell adhesion in Drosophila. Here we show G-oalpha47A and Gbeta13F cooperate with Ggamma1 to maintain cardiac integrity. Cardial-pericardial cell adhesion also relies on the septate junction (SJ) proteins Neurexin-IV (Nrx-IV), Sinuous, Coracle, and Nervana2, which together function in a common pathway with Ggamma1. Furthermore, Ggamma1 signaling is required for proper SJ protein localization, and loss of at least one SJ protein, Nrx-IV, induces cardiac lumen collapse. These results are surprising because the embryonic heart lacks SJs and suggest that SJ proteins perform noncanonical functions to maintain cardiac integrity in Drosophila. Our findings unveil the components of a previously unrecognized network of genes that couple G protein signaling with structural constituents of the heart.

Cardiac expression of the Drosophila Transglutaminase (CG7356) gene is directly controlled by myocyte enhancer factor-2

The myocyte enhancer factor-2 (MEF2) family of transcription factors plays key roles in the activation of muscle structural genes. In Drosophila, MEF2 accumulates at high levels in the embryonic muscles, where it activates target genes throughout the mesoderm. Here, we identify the Transglutaminase gene (Tg; CG7356) as a direct transcriptional target of MEF2 in the cardiac musculature. Tg is expressed in cells forming the inflow tracts of the dorsal vessel, and we identify the enhancer responsible for this expression. The enhancer contains three binding sites for MEF2, and can be activated by MEF2 in tissue culture and in vivo. Moreover, loss of MEF2 function, or removal of the MEF2 binding sites from the enhancer, results in loss of Tg expression. These studies identify a new MEF2 target in the cardiac musculature. These studies provide a possible mechanism for the activation of transglutaminase genes.

Genetic control of cell morphogenesis during Drosophila melanogaster cardiac tube formation

Tubulogenesis is an essential component of organ development, yet the underlying cellular mechanisms are poorly understood. We analyze here the formation of the Drosophila melanogaster cardiac lumen that arises from the migration and subsequent coalescence of bilateral rows of cardioblasts. Our study of cell behavior using three-dimensional and time-lapse imaging and the distribution of cell polarity markers reveals a new mechanism of tubulogenesis in which repulsion of prepatterned luminal domains with basal membrane properties and cell shape remodeling constitute the main driving forces. Furthermore, we identify a genetic pathway in which roundabout, slit, held out wings, and dystroglycan control cardiac lumen formation by establishing nonadherent luminal membranes and regulating cell shape changes. From these data we propose a model for D. melanogaster cardiac lumen formation, which differs, both at a cellular and molecular level, from current models of epithelial tubulogenesis. We suggest that this new example of tube formation may be helpful in studying vertebrate heart tube formation and primary vasculogenesis.

The Drosophila wing hearts originate from pericardial cells and are essential for wing maturation

In addition to the heart proper, insects possess wing hearts in the thorax to ensure regular hemolymph flow through the narrow wings. In Drosophila, the wing hearts consist of two bilateral muscular pumps of unknown origin. Here, we present the first developmental study on these organs and report that the wing hearts originate from eight embryonic progenitor cells arising in two pairs in parasegments 4 and 5. These progenitors represent a so far undescribed subset of the Even-skipped positive pericardial cells (EPC) and are characterized by the early loss of tinman expression in contrast to the continuously Tinman positive classical EPCs. Ectopic expression of Tinman in the wing heart progenitors omits organ formation, indicating a crucial role for Tinman during progenitor specification. The subsequent postembryonic development is a highly dynamic process, which includes proliferation and two relocation events. Adults lacking wing hearts display a severe wing phenotype and are unable to fly. The phenotype is caused by omitted clearance of the epidermal cells from the wings during maturation, which inhibits the formation of a flexible wing blade. This indicates that wing hearts are required for proper wing morphogenesis and functionality.

The Drosophila Hand gene is required for remodeling of the developing adult heart and midgut during metamorphosis

The Hand proteins of the bHLH family of transcriptional factors play critical roles in vertebrate cardiogenesis. In Drosophila, the single orthologous Hand gene is expressed in the developing embryonic dorsal vessel (heart), lymph glands, circular visceral musculature, and a subset of CNS cells. We demonstrate that the absence of Hand activity causes semilethality during the early larval instars. The dorsal vessel and midgut musculature are unaffected in null mutant embryos, but in a large fraction the lymph glands are missing. However, homozygous adult flies lacking Hand possess morphologically abnormal dorsal vessels characterized by a disorganized myofibrillar structure, reduced systolic and diastolic diameter, and abnormal heartbeat contractions, and suffer from premature lethality. In addition, their midguts are highly deformed; in the most severe cases, there is midgut blockage and a massive excess of ectopic peritrophic membrane tubules exiting a rupture in an anterior midgut bulge. Nevertheless, the visceral musculature appears to be relatively normal. Based on these phenotypes, we conclude that the expression of the Drosophila Hand gene in the dorsal vessel and circular visceral muscles is mainly required during pupal stages, when Hand participates in the proper hormone-dependent remodeling of the larval aorta into the adult heart and in the normal morphogenesis of the adult midgut endoderm during metamorphosis.

Post-embryonic pericardial cells of Drosophila are required for overcoming toxic stress but not for cardiac function or adult development

The Drosophila heart is composed of two cell types: cardioblasts (CB) and pericardial cells (PC). Whereas CBs act to maintain rhythmic contractions, the functions of accessory PCs are not clear. The close association between these two cell types has led to speculation of a cardio-regulatory role for PCs. However, we find that viability and cardiac function are normal in larvae following post-embryonic ablation of PCs by induced cell death. Removal of PCs during the larval instars or before metamorphosis results in viable and fertile adults. Interestingly, such animals have a reduced lifespan and increased sensitivity to toxic chemicals. Thus, although PCs may have an embryonic role in cardiogenesis, they do not appear to play a part later in cardiac function as suggested. However, the role of PCs in the uptake and sequestering of toxins, their sensitivity to toxic stress and the decreased lifespan of animals without PCs indicate the importance of PCs in organismal homeostasis.

Defective decapentaplegic signaling results in heart overgrowth and reduced cardiac output in Drosophila

During germ-band extension, Decapentaplegic (Dpp) signals from the dorsal ectoderm to maintain Tinman (Tin) expression in the underlying mesoderm. This signal specifies the cardiac field, and homologous genes (BMP2/4 and Nkx2.5) perform this function in mammals. We showed previously that a second Dpp signal from the dorsal ectoderm restricts the number of pericardial cells expressing the transcription factor Zfh1. Here we report that, via Zfh1, the second Dpp signal restricts the number of Odd-skipped-expressing and the number of Tin-expressing pericardial cells. Dpp also represses Tin expression independently of Zfh1, implicating a feed-forward mechanism in the regulation of Tin pericardial cell number. In the adjacent dorsal muscles, Dpp has the opposite effect. Dpp maintains Krüppel and Even-skipped expression required for muscle development. Our data show that Dpp refines the cardiac field by limiting the number of pericardial cells. This maintains the boundary between pericardial and dorsal muscle cells and defines the size of the heart. In the absence of the second Dpp signal, pericardial cells overgrow and this significantly reduces larval cardiac output. Our study suggests the existence of a second round of BMP signaling in mammalian heart development and that perhaps defects in this signal play a role in congenital heart defects.

Heart development in Drosophila

The Drosophila heart, also called the dorsal vessel, is an organ for hemolymph circulation that resembles the vertebrate heart at its transient linear tube stage. Dorsal vessel morphogenesis shares several similarities with early events of vertebrate heart development and has proven to be an insightful system for the study of cardiogenesis due to its relatively simple structure and the productive use of Drosophila genetic approaches. In this review, we summarize published findings on Drosophila heart development in terms of the regulators and genetic pathways required for cardiac cell specification and differentiation, and organ formation and function. Emerging genome-based strategies should further facilitate the use of Drosophila as an advantageous system in which to identify previously unknown genes and regulatory networks essential for normal cardiac development and function.

Exploring in vitro roles of siRNA in cardiovascular disease

RNA interference (RNAi) is an adaptive defense mechanism through which double stranded RNAs silence cognate genes in a sequence-specific manner. It has been employed widely as a powerful tool in functional genomics studies, target validation and therapeutic product development. Similarly, the application of small interfering RNA (siRNA) to the silencing of the disease-causing genes involved in cardiovascular diseases has made great progress. In this overview, we attempt to provide a brief outline of the current understanding of the mechanism of RNAi and its potential application to the cardiovascular system, with particular emphasis on its ability to identify the pathophysiological function of genes related to several important cardiovascular disorders. The prospects of RNAi-based therapeutics, as well as the advantages and potential problems, are also discussed.

Lateral positioning at the dorsal midline: Slit and Roundabout receptors guide Drosophila heart cell migration

Heart morphogenesis requires the coordinated regulation of cell movements and cell-cell interactions between distinct populations of cardiac precursor cells. Little is known about the mechanisms that organize cardiac cells into this complex structure. In this study, we analyzed the role of Slit, an extracellular matrix protein and its transmembrane receptors Roundabout (Robo) and Roundabout2 (Robo2) during morphogenesis of the Drosophila heart tube, a process analogous to early heart formation in vertebrates. During heart assembly, two types of progenitor cells align into rows and coordinately migrate to the dorsal midline of the embryo, where they merge to assemble a linear heart tube. Here we show that cardiac-specific expression of Slit is required to maintain adhesion between cells within each row during dorsal migration. Moreover, differential Robo expression determines the relative distance each row is positioned from the dorsal midline. The innermost CBs express only Robo, whereas the flanking pericardial cells express both receptors. Removal of robo2 causes pericardial cells to shift toward the midline, whereas ectopic robo2 in CBs drives them laterally, resulting in an unfused heart tube. We propose a model in which Slit has a dual role during assembly of the linear heart tube, functioning to regulate both cell positioning and adhesive interactions between migrating cardiac precursor cells.

The ADAM metalloprotease Kuzbanian is crucial for proper heart formation in Drosophila melanogaster

We have screened a collection of EMS mutagenized fly lines in order to identify genes involved in cardiogenesis. In the present work, we have studied a group of alleles exhibiting a hypertrophic heart. Our analysis revealed that the ADAM protein (A Disintegrin And Metalloprotease) Kuzbanian, which is the functional homologue of the vertebrate ADAM10, is crucial for proper heart formation. ADAMs are a family of transmembrane proteins that play a critical role during the proteolytic conversion (shedding) of membrane bound proteins to soluble forms. Enzymes harboring a sheddase function recently became candidates for causing several congenital diseases, like distinct forms of the Alzheimer disease. ADAMs play also a pivotal role during heart formation and vascularisation in vertebrates, therefore mutations in ADAM genes potentially could cause congenital heart defects in humans. In Drosophila, the zygotic loss of an active form of the Kuzbanian protein results in a dramatic excess of cardiomyocytes, accompanied by a loss of pericardial cells. Our data presented herein suggest that Kuzbanian acts during lateral inhibition within the cardiac primordium. Furthermore we discuss a second function of Kuzbanian in heart cell morphogenesis.

Slit coordinates cardiac morphogenesis in Drosophila

Slit is a secreted guidance cue that conveys repellent or attractive signals from target and guidepost cells. In Drosophila, responsive cells express one or more of three Robo receptors. The cardial cells of the developing heart express both Slit and Robo2. This is the first report of coincident expression of a Robo and its ligand. In slit mutants, cardial cell alignment, polarization and uniform migration are disrupted. The heart phenotype of robo2 mutants is similar, with fewer migration defects. In the guidance of neuronal growth cones in Drosophila, there is a phenotypic interaction between slit and robo heterozygotes, and also with genes required for Robo signaling. In contrast, in the heart, slit has little or no phenotypic interaction with Robo-related genes, including Robo2, Nck2, and Disabled. However, there is a strong phenotypic interaction with Integrin genes and their ligands, including Laminin and Collagen, and intracellular messengers, including Talin and ILK. This indicates that Slit participates in adhesion or adhesion signaling during heart development.

Expression, regulation, and requirement of the toll transmembrane protein during dorsal vessel formation in Drosophila melanogaster

Early heart development in Drosophila and vertebrates involves the specification of cardiac precursor cells within paired progenitor fields, followed by their movement into a linear heart tube structure. The latter process requires coordinated cell interactions, migration, and differentiation as the primitive heart develops toward status as a functional organ. In the Drosophila embryo, cardioblasts emerge from bilateral dorsal mesoderm primordia, followed by alignment as rows of cells that meet at the midline and morph into a dorsal vessel. Genes that function in coordinating cardioblast organization, migration, and assembly are integral to heart development, and their encoded proteins need to be understood as to their roles in this vital morphogenetic process. Here we prove the Toll transmembrane protein is expressed in a secondary phase of heart formation, at lateral cardioblast surfaces as they align, migrate to the midline, and form the linear tube. The Toll dorsal vessel enhancer has been characterized, with its activity controlled by Dorsocross and Tinman transcription factors. Consistent with the observed protein expression pattern, phenotype analyses demonstrate Toll function is essential for normal dorsal vessel formation. Such findings implicate Toll as a critical cell adhesion molecule in the alignment and migration of cardioblasts during dorsal vessel morphogenesis.

Evidence for a fruit fly hemangioblast and similarities between lymph-gland hematopoiesis in fruit fly and mammal aorta-gonadal-mesonephros mesoderm

The Drosophila melanogaster lymph gland is a hematopoietic organ and, together with prospective vascular cells (cardioblasts) and excretory cells (pericardial nephrocytes), arises from the cardiogenic mesoderm. Clonal analysis provided evidence for a hemangioblast 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 mesoderm.

Drosophila cardiac tube organogenesis requires multiple phases of Hox activity

The segmented Drosophila linear cardiac tube originates from two cell lineages that give rise to the anterior aorta (AA) and the posterior cardiac tube. The three Hox genes of the Bithorax Complex as well as Antennapedia (Antp) have been shown to be expressed in the posterior cardiac tube, while no Hox gene is expressed in the anterior aorta. We show that the cells of the whole tube adopt the anterior aorta identity in the complete absence of Hox function. Conversely, ectopic expression of Antp, Ultrabithorax (Ubx), or abdominal-A (abd-A) transformed the anterior aorta into posterior cardiac tube by all available criteria, indicating an equivalent early function in their ability to direct a posterior cardiac tube lineage. We further demonstrate that Hox genes act in a subsequent step during cardiac tube organogenesis, specifically on the differentiation of posterior cardiac tube myocytes. In addition, while some of these functions are fulfilled equally well by any one of the three Hox genes, some others are specific to a given Hox. Notably, the gene encoding the anion transporter Na+-Driven Anion Exchanger 1 behaves as a Hox differential transcriptional target and is activated by abd-A in the heart and repressed by Ubx in the posterior aorta. This analysis illustrates the mechanisms by which Hox genes can orchestrate organogenesis and, in particular, allows a clear uncoupling of the different phases of Hox activity in this process.

Embryonic enhancers in the dpp disk region regulate a second round of Dpp signaling from the dorsal ectoderm to the mesoderm that represses Zfh-1 expression in a subset of pericardial cells

During germ band elongation, widespread decapentaplegic (dpp) expression in the dorsal ectoderm patterns the underlying mesoderm. These Dpp signals specify cardial and pericardial cell fates in the developing heart. At maximum germ band extension, dpp dorsal ectoderm expression becomes restricted to the dorsal-most or leading edge cells (LE). A second round of Dpp signaling then specifies cell shape changes in ectodermal cells leading to dorsal closure. Here we show that a third round of dpp dorsal ectoderm expression initiates during germ band retraction. This round of dpp expression is also restricted to LE cells but Dpp signaling specifies the repression of the transcription factor Zfh-1 in a subset of pericardial cells in the underlying mesoderm. Surprisingly, we found that cis-regulatory sequences that activate the third round of dpp dorsal ectoderm expression are found in the dpp disk region. We also show that the activation of this round of dpp expression is dependent upon prior Dpp signals, the signal transducer Medea, and possibly release from dTCF-mediated repression. Our results demonstrate that a second round of Dpp signaling from the dorsal ectoderm to the mesoderm is required to pattern the developing heart and that this round of dpp expression may be activated by combinatorial interactions between Dpp and Wingless.

Developmental control of cell morphogenesis: a focus on membrane growth

To date, the role of transport and insertion of membrane in the control of membrane remodelling during cell and tissue morphogenesis has received little attention. In contrast, the contributions of cytoskeletal rearrangements and both intercellular and cell-substrate attachments have been the focus of many studies. Here, we review work from many developmental systems that highlights the importance of polarized membrane growth and suggests a general model for the role of endocytic recycling during cell morphogenesis. We also address how the spatio-temporal control of membrane insertion during development can account for various classes of tissue rearrangements. We suggest that tubulogenesis, tissue spreading and cell intercalation stem mostly from a remarkably small number of cell intrinsic surface remodelling events that confer on cells different modes of migratory behaviours.

Homeotic genes autonomously specify the anteroposterior subdivision of the Drosophila dorsal vessel into aorta and heart

The embryonic dorsal vessel in Drosophila possesses anteroposterior polarity and is subdivided into two chamber-like portions, the aorta in the anterior and the heart in the posterior. The heart portion features a wider bore as compared with the aorta and develops inflow valves (ostia) that allow the pumping of hemolymph from posterior toward the anterior. Here, we demonstrate that homeotic selector genes provide positional information that determines the anteroposterior subdivision of the dorsal vessel. Antennapedia (Antp), Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B) are expressed in distinct domains along the anteroposterior axis within the dorsal vessel, and, in particular, the domain of abd-A expression in cardioblasts and pericardial cells coincides with the heart portion. We provide evidence that loss of abd-A function causes a transformation of the heart into aorta, whereas ectopic expression of abd-A in more anterior cardioblasts causes the aorta to assume heart-like features. These observations suggest that the spatially restricted expression and activity of abd-A determine heart identities in cells of the posterior portion of the dorsal vessel. We also show that Abd-B, which at earlier stages is expressed posteriorly to the cardiogenic mesoderm, represses cardiogenesis. In light of the developmental and morphological similarities between the Drosophila dorsal vessel and the primitive heart tube in early vertebrate embryos, these data suggest that Hox genes may also provide important anteroposterior cues during chamber specification in the developing vertebrate heart.