The FGF8-related signals Pyramus and Thisbe promote pathfinding, substrate adhesion, and survival of migrating longitudinal gut muscle founder cells

FGF8 Protects Against Polymicrobial Sepsis by Enhancing the Host's Anti-infective Immunity

BACKGROUND

Sepsis is characterized by a life-threatening syndrome caused by an unbalanced host response to infection. Fibroblast growth factor 8 (FGF8) has been newly identified to play important roles in inflammation and innate immunity, but its role in host response to sepsis is undefined.

METHODS

A cecal ligation and puncture (CLP)-induced mouse sepsis model was established to evaluate the immunomodulatory function of FGF8 during sepsis. The underlying molecular mechanisms were elucidated by cell models using relevant molecular biology experiments. The clinical value of FGF8 in the adjuvant diagnosis of sepsis was evaluated using clinical samples.

RESULTS

FGF8 protein concentrations were elevated in CLP-induced septic mice compared to controls. In vivo, FGF8 blockade using anti-FGF8 antibody significantly increased mortality and bacterial burden and was paralleled by significantly aggravated tissue injury after CLP. Therapeutic administration of recombinant FGF8 (rFGF8) improved the bacterial clearance and mortality of septic mice in a FGFR1-dependent manner. In vitro, FGF8 directly enhanced bacterial phagocytosis and killing of macrophages by enhancing the phosphorylation of the ERK1/2 signaling pathway, which could be abrogated with the ERK1/2 pathway inhibitor U0126. Clinically, serum FGF8 levels in both adult and pediatric patients with sepsis in an intensive care unit were significantly higher than those in healthy controls.

CONCLUSIONS

These results present a previously unrecognized role of FGF8 in improving survival of sepsis by enhancing host immune defense. Therefore, targeting FGF8 may provide new strategies for the diagnosis and immunotherapy of sepsis.

The Alk receptor tyrosine kinase regulates Sparkly, a novel activity regulating neuropeptide precursor in the Drosophila central nervous system

Numerous roles for the Alk receptor tyrosine kinase have been described in Drosophila, including functions in the central nervous system (CNS), however the molecular details are poorly understood. To gain mechanistic insight, we employed Targeted DamID (TaDa) transcriptional profiling to identify targets of Alk signaling in the larval CNS. TaDa was employed in larval CNS tissues, while genetically manipulating Alk signaling output. The resulting TaDa data were analyzed together with larval CNS scRNA-seq datasets performed under similar conditions, identifying a role for Alk in the transcriptional regulation of neuroendocrine gene expression. Further integration with bulk and scRNA-seq datasets from larval brains in which Alk signaling was manipulated identified a previously uncharacterized Drosophila neuropeptide precursor encoded by CG4577 as an Alk signaling transcriptional target. CG4577, which we named Sparkly (Spar), is expressed in a subset of Alk-positive neuroendocrine cells in the developing larval CNS, including circadian clock neurons. In agreement with our TaDa analysis, overexpression of the Drosophila Alk ligand Jeb resulted in increased levels of Spar protein in the larval CNS. We show that Spar protein is expressed in circadian (clock) neurons, and flies lacking Spar exhibit defects in sleep and circadian activity control. In summary, we report a novel activity regulating neuropeptide precursor gene that is regulated by Alk signaling in the Drosophila CNS.

The RNF220 domain nuclear factor Teyrha-Meyrha (Tey) regulates the migration and differentiation of specific visceral and somatic muscles in Drosophila

Development of the visceral musculature of the Drosophila midgut encompasses a closely coordinated sequence of migration events of cells from the trunk and caudal visceral mesoderm that underlies the formation of the stereotypic orthogonal pattern of circular and longitudinal midgut muscles. Our study focuses on the last step of migration and morphogenesis of longitudinal visceral muscle precursors and shows that these multinucleated precursors utilize dynamic filopodial extensions to migrate in dorsal and ventral directions over the forming midgut tube. The establishment of maximal dorsoventral distances from one another, and anteroposterior alignments, lead to the equidistant coverage of the midgut with longitudinal muscle fibers. We identify Teyrha-Meyhra (Tey), a tissue-specific nuclear factor related to the RNF220 domain protein family, as a crucial regulator of this process of muscle migration and morphogenesis that is further required for proper differentiation of longitudinal visceral muscles. In addition, Tey is expressed in a single somatic muscle founder cell in each hemisegment, regulates the migration of this founder cell, and is required for proper pathfinding of its developing myotube to specific myotendinous attachment sites.

Drosophila FGFR/Htl signaling shapes embryonic glia to phagocytose apoptotic neurons

Glial phagocytosis of apoptotic neurons is crucial for development and proper function of the central nervous system. Relying on transmembrane receptors located on their protrusions, phagocytic glia recognize and engulf apoptotic debris. Like vertebrate microglia, Drosophila phagocytic glial cells form an elaborate network in the developing brain to reach and remove apoptotic neurons. However, the mechanisms controlling creation of the branched morphology of these glial cells critical for their phagocytic ability remain unknown. Here, we demonstrate that during early embryogenesis, the Drosophila fibroblast growth factor receptor (FGFR) Heartless (Htl) and its ligand Pyramus are essential in glial cells for the formation of glial extensions, the presence of which strongly affects glial phagocytosis of apoptotic neurons during later stages of embryonic development. Reduction in Htl pathway activity results in shorter lengths and lower complexity of glial branches, thereby disrupting the glial network. Our work thus illuminates the important role Htl signaling plays in glial subcellular morphogenesis and in establishing glial phagocytic ability.

Longitudinal visceral muscles in Drosophila fully dedifferentiate and fragment prior to their reestablishment during metamorphosis

Although the Drosophila longitudinal visceral muscles have been shown to undergo major morphological changes during the transition from larval to adult gut musculature, there have been conflicting views as to whether these muscles persist as such during metamorphosis or whether they are built anew (Klapper 2000; Aghajanian et al. 2016). Here we present our independent analysis using HLH54Fb-eGFP as a cell type specific marker, which strengthens the proposition by Aghajanian et al. (2016) that the syncytial larval longitudinal gut muscles completely dedifferentiate and fragment into mononucleated myoblasts during pupariation before they fuse again and redifferentiate to form the adult longitudinal gut muscles.

BMP-gated cell-cycle progression drives anoikis during mesenchymal collective migration

Tissue homeostasis involves the elimination of abnormal cells to avoid compromised patterning and function. Although quality control through cell competition is well studied in epithelial tissues, it is unknown if and how homeostasis is regulated in mesenchymal collectives. Here, we demonstrate that collectively migrating Drosophila muscle precursors utilize both fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) signaling to promote homeostasis via anoikis, a form of cell death in response to substrate de-adhesion. Cell-cycle-regulated expression of the cell death gene head involution defective is responsible for caudal visceral mesoderm (CVM) anoikis. The secreted BMP ligand drives cell-cycle progression via a visceral mesoderm-specific cdc25/string enhancer to synchronize collective proliferation, as well as apoptosis of cells that have lost access to substrate-derived FGF. Perturbation of BMP-dependent cell-cycle progression is sufficient to confer anoikis resistance to mismigrating cells and thus facilitate invasion of other tissues. This BMP-gated cell-cycle checkpoint defines a quality control mechanism during mesenchymal collective migration.

Putting in the Erk: Growth factor signaling and mesoderm morphogenesis

It has long been known that FGF signaling contributes to mesoderm formation, a germ layer found in triploblasts that is composed of highly migratory cells that give rise to muscles and to the skeletal structures of vertebrates. FGF signaling activates several pathways in the developing mesoderm, including transient activation of the Erk pathway, which triggers mesodermal fate specification through the induction of the gene brachyury and activates morphogenetic programs that allow mesodermal cells to position themselves in the embryo. In this review, we discuss what is known about the generation and interpretation of transient Erk signaling in mesodermal tissues across species. We focus specifically on mechanisms that translate the level and duration of Erk signaling into cell fate and cell movement instructions and discuss strategies for further interrogating the role that Erk signaling dynamics play in mesodermal gastrulation and morphogenesis.

Simulation and in vivo experimentation predict AdamTS-A location of function during caudal visceral mesoderm (CVM) migration in Drosophila

BACKGROUND

Caudal visceral mesoderm (CVM) cells migrate as a loose collective along the trunk visceral mesoderm (TVM) and are surrounded by extracellular matrix (ECM). In this study, we examined how one extracellular protease, AdamTS-A, facilitates CVM migration.

RESULTS

A comparison of mathematical simulation to experimental results suggests that location of AdamTS-A action in CVM cells is on the sides of the cell not in contact with the TVM, predominantly at the CVM-ECM interface. CVM migration from a top-down view showed CVM cells migrating along the outside of the TVM substrate in the absence of AdamTS-A. Moreover, over-expression of AdamTS-A resulted in similar, but milder, mis-migration of the CVM. These results contrast with the salivary gland where AdamTS-A is proposed to cleave connections at the trailing edge of migrating cells. Subcellular localization of GFP-tagged AdamTS-A suggests this protease is not limited to functioning at the trailing edge of CVM cells.

CONCLUSION

Using both in vivo experimentation and mathematical simulations, we demonstrated that AdamTS-A cleaves connections between CVM cells and the ECM on all sides not attached to the TVM. Clearly, AdamTS-A has a more expansive role around the entire cell in cleaving cell-ECM attachments in cells migrating as a loose collective. This article is protected by copyright. All rights reserved.

Collective Migrations of Drosophila Embryonic Trunk and Caudal Mesoderm-Derived Muscle Precursor Cells

Mesoderm migration in the Drosophila embryo is a highly conserved, complex process that is required for the formation of specialized tissues and organs, including the somatic and visceral musculature. In this FlyBook chapter, we will compare and contrast the specification and migration of cells originating from the trunk and caudal mesoderm. Both cell types engage in collective migrations that enable cells to achieve new positions within developing embryos and form distinct tissues. To start, we will discuss specification and early morphogenetic movements of the presumptive mesoderm, then focus on the coordinate movements of the two subtypes trunk mesoderm and caudal visceral mesoderm, ending with a comparison of these processes including general insights gained through study.

Cell interactions in collective cell migration

Collective cell migration is the coordinated movement of a physically connected group of cells and is a prominent driver of development and metastasis. Interactions between cells within migrating collectives, and between migrating cells and other cells in the environment, play key roles in stimulating motility, steering and sometimes promoting cell survival. Similarly, diverse heterotypic interactions and collective behaviors likely contribute to tumor metastasis. Here, we describe a sampling of cells that migrate collectively in vivo, including well-established and newer examples. We focus on the under-appreciated property that many - perhaps most - collectively migrating cells move as cooperating groups of distinct cell types.

Sticking to a plan: adhesion and signaling control spatial organization of cells within migrating collectives

Collective cell migration is required in a vast array of biological phenomena, including organogenesis and embryonic development. The mechanisms that underlie collective cell migration not only involve the morphogenetic changes associated with single cell migration, but also require the maintenance of cell-cell junctions during movement. Additionally, cell shape changes and polarity must be coordinated in a multicellular manner in order to preserve directional movement in the migrating cohort, and often relates to multiple functions of common signaling pathways. In this review, we summarize the current understanding of the mechanisms underlying higher order tissue organization during migration, with particular focus on the interplay between cell adhesion and signaling that we propose can be tuned to support different types of collective movements.

Migrating cells control morphogenesis of substratum serving as track to promote directional movement of the collective

In Drosophila embryos, caudal visceral mesoderm (CVM) cells undergo bilateral migration along the trunk visceral mesoderm (TVM) in order to form midgut muscles. Mutation of FGF receptor Heartless (Htl) has been shown to cause CVM migration defects, particularly midline crossing of the bilateral groups. Here, we show that htl mutants also exhibit TVM defects including contralateral merging. Both CVM mismigration and TVM contralateral merging are attenuated by restoring FGF signaling specifically in the CVM, suggesting that migrating CVM cells influence TVM morphogenesis; however, the inverse, supplying FGF to the TVM, does not rescue CVM mismigration. In addition, we show that FGF regulates integrin expression in both tissues, but only providing a source of integrin specifically to the TVM attenuates the contralateral merging phenotype. Finally, we demonstrate that the CVM influences cell shape in the TVM, and a loss of CVM results in TVM morphological defects. In summary, this study provides insight into how a migrating collective of cells can influence their tissue substrate and supports the view that morphogenesis of tissues during development is interdependent.

Outstanding questions in developmental ERK signaling

The extracellular signal-regulated kinase (ERK) pathway leads to activation of the effector molecule ERK, which controls downstream responses by phosphorylating a variety of substrates, including transcription factors. Crucial insights into the regulation and function of this pathway came from studying embryos in which specific phenotypes arise from aberrant ERK activation. Despite decades of research, several important questions remain to be addressed for deeper understanding of this highly conserved signaling system and its function. Answering these questions will require quantifying the first steps of pathway activation, elucidating the mechanisms of transcriptional interpretation and measuring the quantitative limits of ERK signaling within which the system must operate to avoid developmental defects.

Hematopoietic cell-derived RELMα regulates hookworm immunity through effects on macrophages

Resistin-like molecule α (RELMα) is a highly secreted protein in type 2 (Th2) cytokine-induced inflammation including helminth infection and allergy. In infection with Nippostrongylus brasiliensis (Nb), RELMα dampens Th2 inflammatory responses. RELMα is expressed by immune cells, and by epithelial cells (EC); however, the functional impact of immune versus EC-derived RELMα is unknown. We generated bone marrow (BM) chimeras that were RELMα deficient (RELMα^{-/ -} ) in BM or non BM cells and infected them with Nb. Non BM RELMα^-/- chimeras had comparable inflammatory responses and parasite burdens to RELMα^+/+ mice. In contrast, both RELMα^-/- and BM RELMα^-/- mice exhibited increased Nb-induced lung and intestinal inflammation, correlated with elevated Th2 cytokines and Nb killing. CD11c⁺ lung macrophages were the dominant BM-derived source of RELMα and can mediate Nb killing. Therefore, we employed a macrophage-worm co-culture system to investigate whether RELMα regulates macrophage-mediated Nb killing. Compared to RELMα^{+ /+} macrophages, RELMα^-/- macrophages exhibited increased binding to Nb and functionally impaired Nb development. Supplementation with recombinant RELMα partially reversed this phenotype. Gene expression analysis revealed that RELMα decreased cell adhesion and Fc receptor signaling pathways, which are associated with macrophage-mediated helminth killing. Collectively, these studies demonstrate that BM-derived RELMα is necessary and sufficient to dampen Nb immune responses, and identify that one mechanism of action of RELMα is through inhibiting macrophage recruitment and interaction with Nb. Our findings suggest that RELMα acts as an immune brake that provides mutually beneficial effects for the host and parasite by limiting tissue damage and delaying parasite expulsion.

Diversification of heart progenitor cells by EGF signaling and differential modulation of ETS protein activity

For coordinated circulation, vertebrate and invertebrate hearts require stereotyped arrangements of diverse cell populations. This study explores the process of cardiac cell diversification in the Drosophila heart, focusing on the two major cardioblast subpopulations: generic working myocardial cells and inflow valve-forming ostial cardioblasts. By screening a large collection of randomly induced mutants, we identified several genes involved in cardiac patterning. Further analysis revealed an unexpected, specific requirement of EGF signaling for the specification of generic cardioblasts and a subset of pericardial cells. We demonstrate that the Tbx20 ortholog Midline acts as a direct target of the EGFR effector Pointed to repress ostial fates. Furthermore, we identified Edl/Mae, an antagonist of the ETS factor Pointed, as a novel cardiac regulator crucial for ostial cardioblast specification. Combining these findings, we propose a regulatory model in which the balance between activation of Pointed and its inhibition by Edl controls cardioblast subtype-specific gene expression.

Organs contain many different kinds of cells, each specialised to perform a particular role. The fruit fly heart, for example, has two types of muscle cells: generic heart muscle cells and ostial heart muscle cells. The generic cells contract to force blood around the body, whilst the ostial cells form openings that allow blood to enter the heart. Though both types of cells carry the same genetic information, each uses a different combination of active genes to perform their role.

During development, the cells must decide whether to become generic or ostial. They obtain signals from other cells in and near the developing heart, and respond by turning genes on or off. The response uses proteins called transcription factors, which bind to regulatory portions of specific genes.

The sequence of signals and transcription factors that control the fate of developing heart muscle cells was not known. So Schwarz et al. examined the process using a technique called a mutagenesis screen. This involved triggering random genetic mutations and looking for flies with defects in their heart muscle cells. Matching the defects to the mutations revealed genes responsible for heart development.

Schwarz et al. found that for cells to develop into generic heart muscle cells, a signal called epidermal growth factor (EGF) switches on a transcription factor called Pointed in the cells. Pointed then turns on another transcription factor that switches off the genes for ostial cells. Conversely, ostial heart muscle cells develop when a protein called ‘ETS-domain lacking’ (Edl) interferes with Pointed, allowing the ostial genes to remain on. The balance between Pointed and Edl controls which type of heart cell each cell will become.

Many cells in other tissues in fruit flies also produce the Pointed and Edl proteins and respond to EGF signals. This means that this system may help to decide the fate of cells in other organs. The EGF signaling system is also present in other animals, including humans. Future work could reveal whether the same molecular decision making happens in our own hearts.

Myotube migration to cover and shape the testis of Drosophila depends on Heartless, Cadherin/Catenin, and myosin II

During Drosophila metamorphosis, nascent testis myotubes migrate from the prospective seminal vesicle of the genital disc onto pupal testes and then further to cover the testes with multinucleated smooth-like muscles. Here we show that DWnt2 is likely required for determination of testis-relevant myoblasts on the genital disc. Knock down of fibroblast growth factor receptor (FGFR) heartless by RNAi and a dominant-negative version revealed multiple functions of Heartless, namely regulation of the amount of myoblasts on the genital disc, connection of seminal vesicles and testes, and migration of muscles along the testes. Live imaging indicated that the downstream effector Stumps is required for migration of testis myotubes on the testis towards the apical tip. After myoblast fusion, myosin II is needed for migration of nascent testis myotubes, in which Thisbe-dependent fibroblast growth factor (FGF) signaling is activated. Cadherin-N is essential for connecting these single myofibers and for creating a firm testis muscle sheath that shapes and stabilizes the testis tubule. Based on these results, we propose a model for the migration of testis myotubes in which nascent testis myotubes migrate as a collective onto and along the testis, dependent on FGF-regulated expression of myosin II.

Summary:Drosophila testes and mammalian seminiferous tubules are surrounded by a muscle layer. Drosophila myotubes migrate towards testes in dependence of the FGF receptor Heartless, myosin II and Cadherin-N.

The migrations of Drosophila muscle founders and primordial germ cells are interdependent

Caudal visceral mesoderm (CVM) cells migrate from posterior to anterior of the Drosophila embryo as two bilateral streams of cells to support the specification of longitudinal muscles along the midgut. To accomplish this long-distance migration, CVM cells receive input from their environment, but little is known about how this collective cell migration is regulated. In a screen we found that wunen mutants exhibit CVM cell migration defects. Wunens are lipid phosphate phosphatases known to regulate the directional migration of primordial germ cells (PGCs). PGC and CVM cell types interact while PGCs are en route to the somatic gonadal mesoderm, and previous studies have shown that CVM impacts PGC migration. In turn, we found here that CVM cells exhibit an affinity for PGCs, localizing to the position of PGCs whether mislocalized or trapped in the endoderm. In the absence of PGCs, CVM cells exhibit subtle changes, including more cohesive movement of the migrating collective, and an increased number of longitudinal muscles is found at anterior sections of the larval midgut. These data demonstrate that PGC and CVM cell migrations are interdependent and suggest that distinct migrating cell types can coordinately influence each other to promote effective cell migration during development.

Comparative analysis of gene expression profiles for several migrating cell types identifies cell migration regulators

Cell migration is an instrumental process that ensures cells are properly positioned to support the specification of distinct tissue types during development. To provide insight, we used fluorescence activated cell sorting (FACS) to isolate two migrating cell types from the Drosophila embryo: caudal visceral mesoderm (CVM) cells, precursors of longitudinal muscles of the gut, and hemocytes (HCs), the Drosophila equivalent of blood cells. ~350 genes were identified from each of the sorted samples using RNA-seq, and in situ hybridization was used to confirm expression within each cell type or, alternatively, within other interacting, co-sorted cell types. To start, the two gene expression profiling datasets were compared to identify cell migration regulators that are potentially generally-acting. 73 genes were present in both CVM cell and HC gene expression profiles, including the transcription factor zinc finger homeodomain-1 (zfh1). Comparisons with gene expression profiles of Drosophila border cells that migrate during oogenesis had a more limited overlap, with only the genes neyo (neo) and singed (sn) found to be expressed in border cells as well as CVM cells and HCs, respectively. Neo encodes a protein with Zona pellucida domain linked to cell polarity, while sn encodes an actin binding protein. Tissue specific RNAi expression coupled with live in vivo imaging was used to confirm cell-autonomous roles for zfh1 and neo in supporting CVM cell migration, whereas previous studies had demonstrated a role for Sn in supporting HC migration. In addition, comparisons were made to migrating cells from vertebrates. Seven genes were found expressed by chick neural crest cells, CVM cells, and HCs including extracellular matrix (ECM) proteins and proteases. In summary, we show that genes shared in common between CVM cells, HCs, and other migrating cell types can help identify regulators of cell migration. Our analyses show that neo in addition to zfh1 and sn studied previously impact cell migration. This study also suggests that modification of the extracellular milieu may be a fundamental requirement for cells that undergo cell streaming migratory behaviors.

Genome-Wide Approaches to Drosophila Heart Development

The development of the dorsal vessel in Drosophila is one of the first systems in which key mechanisms regulating cardiogenesis have been defined in great detail at the genetic and molecular level. Due to evolutionary conservation, these findings have also provided major inputs into studies of cardiogenesis in vertebrates. Many of the major components that control Drosophila cardiogenesis were discovered based on candidate gene approaches and their functions were defined by employing the outstanding genetic tools and molecular techniques available in this system. More recently, approaches have been taken that aim to interrogate the entire genome in order to identify novel components and describe genomic features that are pertinent to the regulation of heart development. Apart from classical forward genetic screens, the availability of the thoroughly annotated Drosophila genome sequence made new genome-wide approaches possible, which include the generation of massive numbers of RNA interference (RNAi) reagents that were used in forward genetic screens, as well as studies of the transcriptomes and proteomes of the developing heart under normal and experimentally manipulated conditions. Moreover, genome-wide chromatin immunoprecipitation experiments have been performed with the aim to define the full set of genomic binding sites of the major cardiogenic transcription factors, their relevant target genes, and a more complete picture of the regulatory network that drives cardiogenesis. This review will give an overview on these genome-wide approaches to Drosophila heart development and on computational analyses of the obtained information that ultimately aim to provide a description of this process at the systems level.

Cell death regulates muscle fiber number

Cell death can have both cell autonomous and non-autonomous roles in normal development. Previous studies have shown that the central cell death regulators grim and reaper are required for the developmentally important elimination of stem cells and neurons in the developing central nervous system (CNS). Here we show that cell death in the nervous system is also required for normal muscle development. In the absence of grim and reaper, there is an increase in the number of fibers in the ventral abdominal muscles in the Drosophila adult. This phenotype can be partially recapitulated by inhibition of cell death specifically in the CNS, indicating a non-autonomous role for neuronal death in limiting muscle fiber number. We also show that FGFs produced in the cell death defective nervous system are required for the increase in muscle fiber number. Cell death in the muscle lineage during pupal stages also plays a role in specifying fiber number. Our work suggests that FGFs from the CNS act as a survival signal for muscle founder cells. Thus, proper muscle fiber specification requires cell death in both the nervous system and in the developing muscle itself.

The extracellular protease stl functions to inhibit migration of v'ch1 sensory neuron during Drosophila embryogenesis

Proper migration of cells through the dense and complex extracellular matrix (ECM) requires constant restructuring of the ECM to allow cells to move forward in a smooth manner. This restructuring can occur through the action of extracellular enzymes. Among these extracellular enzymes is the ADAMTS (A Disintegrin And Metalloprotease with ThromboSpondin repeats) family of secreted extracellular proteases. Drosophila stl encodes an ADAMTS protease expressed in and around the peripheral nervous system (PNS) during embryogenesis. The absence of stl displayed one specific neuron, the v'ch1 sensory neuron, migrating to its target sooner than in wild type. During normal development, the v'ch1 sensory neuron migrates dorsally at the same time it is extending an axon ventrally toward the CNS. Surprisingly, in the absence of stl, the v'ch1 neuron migrated further dorsally as compared to the wild type at stage 15, but did not migrate past its correct target at stage 16, suggesting a novel role for this extracellular protease in inhibiting migration of this neuron past a certain point.

The single fgf receptor gene in the beetle Tribolium castaneum codes for two isoforms that integrate FGF8- and Branchless-dependent signals

The precise regulation of cell-cell communication by numerous signal-transduction pathways is fundamental for many different processes during embryonic development. One important signalling pathway is the evolutionary conserved fibroblast-growth-factor (FGF)-pathway that controls processes like cell migration, axis specification and mesoderm formation in vertebrate and invertebrate animals. In the model insect Drosophila, the FGF ligand / receptor combinations of FGF8 (Pyramus and Thisbe) / Heartless (Htl) and Branchless (Bnl) / Breathless (Btl) are required for the migration of mesodermal cells and for the formation of the tracheal network respectively with both the receptors functioning independently of each other. However, only a single fgf-receptor gene (Tc-fgfr) has been identified in the genome of the beetle Tribolium. We therefore asked whether both the ligands Fgf8 and Bnl could transduce their signal through a common FGF-receptor in Tribolium. Indeed, we found that the function of the single Tc-fgfr gene is essential for mesoderm differentiation as well as for the formation of the tracheal network during early development. Ligand specific RNAi for Tc-fgf8 and Tc-bnl resulted in two distinct non-overlapping phenotypes of impaired mesoderm differentiation and abnormal formation of the tracheal network in Tc-fgf8- and Tc-bnl(RNAi) embryos respectively. We further show that the single Tc-fgfr gene encodes at least two different receptor isoforms that are generated through alternative splicing. We in addition demonstrate through exon-specific RNAi their distinct tissue-specific functions. Finally, we discuss the structure of the fgf-receptor gene from an evolutionary perspective.

Cellular Mechanisms of Drosophila Heart Morphogenesis

Many of the major discoveries in the fields of genetics and developmental biology have been made using the fruit fly, Drosophila melanogaster. With regard to heart development, the conserved network of core cardiac transcription factors that underlies cardiogenesis has been studied in great detail in the fly, and the importance of several signaling pathways that regulate heart morphogenesis, such as Slit/Robo, was first shown in the fly model. Recent technological advances have led to a large increase in the genomic data available from patients with congenital heart disease (CHD). This has highlighted a number of candidate genes and gene networks that are potentially involved in CHD. To validate genes and genetic interactions among candidate CHD-causing alleles and to better understand heart formation in general are major tasks. The specific limitations of the various cardiac model systems currently employed (mammalian and fish models) provide a niche for the fly model, despite its evolutionary distance to vertebrates and humans. Here, we review recent advances made using the Drosophila embryo that identify factors relevant for heart formation. These underline how this model organism still is invaluable for a better understanding of CHD.

Ectopic expression screen identifies genes affecting Drosophila mesoderm development including the HSPG Trol

Gastrulation of the embryo involves coordinate cell movements likely supported by multiple signaling pathways, adhesion molecules, and extracellular matrix components. Fibroblast growth factors (FGFs) have a major role in Drosophila melanogaster mesoderm migration; however, few other inputs are known and the mechanism supporting cell movement is unclear. To provide insight, we performed an ectopic expression screen to identify secreted or membrane-associated molecules that act to support mesoderm migration. Twenty-four UAS insertions were identified that cause lethality when expressed in either the mesoderm (Twi-Gal4) or the ectoderm (69B-Gal4). The list was narrowed to a subset of 10 genes that were shown to exhibit loss-of-function mutant phenotypes specifically affecting mesoderm migration. These include the FGF ligand Pyramus, α-integrins, E-cadherin, Cueball, EGFR, JAK/STAT signaling components, as well as the heparan sulfate proteoglycan (HSPG) Terribly reduced optic lobes (Trol). Trol encodes the ortholog of mammalian HSPG Perlecan, a demonstrated FGF signaling cofactor. Here, we examine the role of Trol in Drosophila mesoderm migration and compare and contrast its role with that of Syndecan (Sdc), another HSPG previously implicated in this process. Embryos mutant for Trol or Sdc were obtained and analyzed. Our data support the view that both HSPGs function to support FGF-dependent processes in the early embryo as they share phenotypes with FGF mutants: Trol in terms of effects on mesoderm migration and caudal visceral mesoderm (CVM) migration and Sdc in terms of dorsal mesoderm specification. The differential roles uncovered for these two HSPGs suggest that HSPG cofactor choice may modify FGF-signaling outputs.

Distinct genetic programs guide Drosophila circular and longitudinal visceral myoblast fusion

BACKGROUND

The visceral musculature of Drosophila larvae comprises circular visceral muscles tightly interwoven with longitudinal visceral muscles. During myogenesis, the circular muscles arise by one-to-one fusion of a circular visceral founder cell (FC) with a visceral fusion-competent myoblast (FCM) from the trunk visceral mesoderm, and longitudinal muscles arise from FCs of the caudal visceral mesoderm. Longitudinal FCs migrate anteriorly under guidance of fibroblast growth factors during embryogenesis; it is proposed that they fuse with FCMs from the trunk visceral mesoderm to give rise to syncytia containing up to six nuclei.

RESULTS

Using fluorescence in situ hybridization and immunochemical analyses, we investigated whether these fusion events during migration use the same molecular repertoire and cellular components as fusion-restricted myogenic adhesive structure (FuRMAS), the adhesive signaling center that mediates myoblast fusion in the somatic mesoderm. Longitudinal muscles were formed by the fusion of one FC with Sns-positive FCMs, and defects in FCM specification led to defects in longitudinal muscle formation. At the fusion sites, Duf/Kirre and the adaptor protein Rols7 accumulated in longitudinal FCs, and Blow and F-actin accumulated in FCMs. The accumulation of these four proteins at the fusion sites argues for FuRMAS-like adhesion and signaling centers. Longitudinal fusion was disturbed in rols and blow single, and scar wip double mutants. Mutants of wasp or its interaction partner wip had no defects in longitudinal fusion.

CONCLUSIONS

Our results indicated that all embryonic fusion events depend on the same cell-adhesion molecules, but that the need for Rols7 and regulators of F-actin distinctly differs. Rols7 was required for longitudinal visceral and somatic myoblast fusion but not for circular visceral fusion. Importantly, longitudinal fusion depended on Kette and SCAR/Wave but was independent of WASp-dependent Arp2/3 activation. Thus, the complexity of the players involved in muscle formation increases from binucleated circular muscles to longitudinal visceral muscles to somatic muscles.

Distinct functions of the laminin β LN domain and collagen IV during cardiac extracellular matrix formation and stabilization of alary muscle attachments revealed by EMS mutagenesis in Drosophila

BACKGROUND

The Drosophila heart (dorsal vessel) is a relatively simple tubular organ that serves as a model for several aspects of cardiogenesis. Cardiac morphogenesis, proper heart function and stability require structural components whose identity and ways of assembly are only partially understood. Structural components are also needed to connect the myocardial tube with neighboring cells such as pericardial cells and specialized muscle fibers, the so-called alary muscles.

RESULTS

Using an EMS mutagenesis screen for cardiac and muscular abnormalities in Drosophila embryos we obtained multiple mutants for two genetically interacting complementation groups that showed similar alary muscle and pericardial cell detachment phenotypes. The molecular lesions underlying these defects were identified as domain-specific point mutations in LamininB1 and Cg25C, encoding the extracellular matrix (ECM) components laminin β and collagen IV α1, respectively. Of particular interest within the LamininB1 group are certain hypomorphic mutants that feature prominent defects in cardiac morphogenesis and cardiac ECM layer formation, but in contrast to amorphic mutants, only mild defects in other tissues. All of these alleles carry clustered missense mutations in the laminin LN domain. The identified Cg25C mutants display weaker and largely temperature-sensitive phenotypes that result from glycine substitutions in different Gly-X-Y repeats of the triple helix-forming domain. While initial basement membrane assembly is not abolished in Cg25C mutants, incorporation of perlecan is impaired and intracellular accumulation of perlecan as well as the collagen IV α2 chain is detected during late embryogenesis.

CONCLUSIONS

Assembly of the cardiac ECM depends primarily on laminin, whereas collagen IV is needed for stabilization. Our data underscore the importance of a correctly assembled ECM particularly for the development of cardiac tissues and their lateral connections. The mutational analysis suggests that the β6/β3/β8 interface of the laminin β LN domain is highly critical for formation of contiguous cardiac ECM layers. Certain mutations in the collagen IV triple helix-forming domain may exert a semi-dominant effect leading to an overall weakening of ECM structures as well as intracellular accumulation of collagen and other molecules, thus paralleling observations made in other organisms and in connection with collagen-related diseases.

Growth factors and early mesoderm morphogenesis: insights from the sea urchin embryo

The early morphogenesis of the mesoderm is critically important in establishing the body plan of the embryo. Recent research has led to a better understanding of the mechanisms that underlie this process, and growth factor signaling pathways have emerged as key regulators of the directional movements of mesoderm cells during gastrulation. In this review, we undertake a comparative analysis of the various essential functions of growth factor signaling pathways in regulating early mesoderm morphogenesis, with an emphasis on recent advances in the sea urchin embryo. We focus on the roles of the vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) pathways in the migration of primary mesenchyme cells and the formation of the embryonic endoskeleton. We compare the functions of VEGF and FGF in sea urchins with the roles that these and other growth factors play in regulating mesoderm migration during gastrulation in Drosophila and vertebrates.

Growth factor-mediated mesodermal cell guidance and skeletogenesis during sea urchin gastrulation

Growth factor signaling pathways provide essential cues to mesoderm cells during gastrulation in many metazoans. Recent studies have implicated the VEGF and FGF pathways in providing guidance and differentiation cues to primary mesenchyme cells (PMCs) during sea urchin gastrulation, although the relative contributions of these pathways and the cell behaviors they regulate are not fully understood. Here, we show that FGF and VEGF ligands are expressed in distinct domains in the embryonic ectoderm of Lytechinus variegatus. We find that PMC guidance is specifically disrupted in Lv-vegf3 morphants and these embryos fail to form skeletal elements. By contrast, PMC migration is unaffected in Lv-fgfa morphants, and well-patterned but shortened skeletal elements form. We use a VEGFR inhibitor, axitinib, to show that VEGF signaling is essential not only for the initial phase of PMC migration (subequatorial ring formation), but also for the second phase (migration towards the animal pole). VEGF signaling is not required, however, for PMC fusion. Inhibition of VEGF signaling after the completion of PMC migration causes significant defects in skeletogenesis, selectively blocking the elongation of skeletal rods that support the larval arms, but not rods that form in the dorsal region of the embryo. Nanostring nCounter analysis of ∼100 genes in the PMC gene regulatory network shows a decrease in the expression of many genes with proven or predicted roles in biomineralization in vegf3 morphants. Our studies lead to a better understanding of the roles played by growth factors in sea urchin gastrulation and skeletogenesis.

The Hydra FGFR, Kringelchen, partially replaces the Drosophila Heartless FGFR

Fibroblast growth factor receptors (FGFR) are highly conserved receptor tyrosine kinases, and evolved early in metazoan evolution. In order to investigate their functional conservation, we asked whether the Kringelchen FGFR in the freshwater polyp Hydra vulgaris, is able to functionally replace FGFR in fly embryos. In Drosophila, two endogenous FGFR, Breathless (Btl) and Heartless (Htl), ensure formation of the tracheal system and mesodermal cell migration as well as formation of the heart. Using UAS-kringelchen-5xmyc transgenic flies and targeted expression, we show that Kringelchen is integrated correctly into the cell membrane of mesodermal and tracheal cells in Drosophila. Nevertheless, Kringelchen expression driven in tracheal cells failed to rescue the btl (LG19) mutant. The Hydra FGFR was able to substitute for Heartless in the htl (AB42) null mutant; however, this occurred only during early mesodermal cell migration. Our data provide evidence for functional conservation of this early-diverged FGFR across these distantly related phyla, but also selectivity for the Htl FGFR in the Drosophila system.

Functions and Mechanisms of Fibroblast Growth Factor (FGF) Signalling in Drosophila melanogaster

Intercellular signalling via growth factors plays an important role in controlling cell differentiation and cell movements during the development of multicellular animals. Fibroblast Growth Factor (FGF) signalling induces changes in cellular behaviour allowing cells in the embryo to move, to survive, to divide or to differentiate. Several examples argue that FGF signalling is used in multi-step morphogenetic processes to achieve and maintain a transitional state of the cells required for the control of cell fate. In the genetic model Drosophila melanogaster, FGF signalling via the receptor tyrosine kinases Heartless (Htl) and Breathless (Btl) is particularly well studied. These FGF receptors affect gene expression, cell shape and cell–cell interactions during mesoderm layer formation, caudal visceral muscle (CVM) formation, tracheal morphogenesis and glia differentiation. Here, we will address the current knowledge of the biological functions of FGF signalling in the fly on the tissue, at a cellular and molecular level.

The role of FGF signaling in guiding coordinate movement of cell groups: guidance cue and cell adhesion regulator?

Cell migration influences cell-cell interactions to drive cell differentiation and organogenesis. To support proper development, cell migration must be regulated both temporally and spatially. Mesoderm cell migration in the Drosophila embryo serves as an excellent model system to study how cell migration is controlled and influences organogenesis. First, mesoderm spreading transforms the embryo into a multilayered form during gastrulation and, subsequently, cells originating from the caudal visceral mesoderm (CVM) migrate along the entire length of the gut. Here we review our studies, which have focused on the role of fibroblast growth factor (FGF) signaling, and compare and contrast these two different cell migration processes: mesoderm spreading and CVM migration. In both cases, FGF acts as a chemoattractant to guide cells’ directional movement but is likely not the only signal that serves this role. Furthermore, FGF likely modulates cell adhesion properties since FGF mutant phenotypes share similarities with those of cell adhesion molecules. Our working hypothesis is that levels of FGF signaling differentially influence cells’ response to result in either directional movement or changes in adhesive properties.