HLH54F is required for the specification and migration of longitudinal gut muscle founders from the caudal mesoderm of Drosophila

Two sequential gene expression programs bridged by cell division support long-distance collective cell migration

The precise assembly of tissues and organs relies on spatiotemporal regulation of gene expression to coordinate the collective behavior of cells. In Drosophila embryos, the midgut musculature is formed through collective migration of caudal visceral mesoderm (CVM) cells, but how gene expression changes as cells migrate is not well understood. Here, we have focused on ten genes expressed in the CVM and the cis-regulatory sequences controlling their expression. Although some genes are continuously expressed, others are expressed only early or late during migration. Late expression relates to cell cycle progression, as driving string/Cdc25 causes earlier division of CVM cells and accelerates the transition to late gene expression. In particular, we found that the cell cycle effector transcription factor E2F1 is a required input for the late gene CG5080. Furthermore, whereas late genes are broadly expressed in all CVM cells, early gene transcripts are polarized to the anterior or posterior ends of the migrating collective. We show this polarization requires transcription factors Snail, Zfh1 and Dorsocross. Collectively, these results identify two sequential gene expression programs bridged by cell division that support long-distance directional migration of CVM cells.

Single-cell transcriptomics illuminates regulatory steps driving anterior-posterior patterning of Drosophila embryonic mesoderm

Single-cell technologies promise to uncover how transcriptional programs orchestrate complex processes during embryogenesis. Here, we apply a combination of single-cell technology and genetic analysis to investigate the dynamic transcriptional changes associated with Drosophila embryo morphogenesis at gastrulation. Our dataset encompassing the blastoderm-to-gastrula transition provides a comprehensive single-cell map of gene expression across cell lineages validated by genetic analysis. Subclustering and trajectory analyses revealed a surprising stepwise progression in patterning to transition zygotic gene expression and specify germ layers as well as uncovered an early role for ecdysone signaling in epithelial-to-mesenchymal transition in the mesoderm. We also show multipotent progenitors arise prior to gastrulation by analyzing the transcription trajectory of caudal mesoderm cells, including a derivative that ultimately incorporates into visceral muscles of the midgut and hindgut. This study provides a rich resource of gastrulation and elucidates spatially regulated temporal transitions of transcription states during the process.

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.

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.

Visceral mesoderm signaling regulates assembly position and function of the Drosophila testis niche

Tissue homeostasis often requires a properly placed niche to support stem cells. Morphogenetic processes that position a niche are just being described. For the Drosophila testis, we recently showed that pro-niche cells, specified at disparate positions during early gonadogenesis, must assemble into one collective at the anterior of the gonad. We now find that Slit and FGF signals emanating from adjacent visceral mesoderm regulate assembly. In response to signaling, niche cells express islet, which we find is also required for niche assembly. Without signaling, niche cells specified furthest from the anterior are unable to migrate, remaining dispersed. The function of such niches is severely disrupted, with niche cells evading cell cycle quiescence, compromised in their ability to signal the incipient stem cell pool, and failing to orient stem cell divisions properly. Our work identifies both extrinsic signaling and intrinsic responses required for proper assembly and placement of the testis niche.

Drosophila melanogaster: A Model System to Study Distinct Genetic Programs in Myoblast Fusion

Muscle fibers are multinucleated cells that arise during embryogenesis through the fusion of mononucleated myoblasts. Myoblast fusion is a lifelong process that is crucial for the growth and regeneration of muscles. Understanding the molecular mechanism of myoblast fusion may open the way for novel therapies in muscle wasting and weakness. Recent reports in Drosophila and mammals have provided new mechanistic insights into myoblast fusion. In Drosophila, muscle formation occurs twice: during embryogenesis and metamorphosis. A fundamental feature is the formation of a cell–cell communication structure that brings the apposing membranes into close proximity and recruits possible fusogenic proteins. However, genetic studies suggest that myoblast fusion in Drosophila is not a uniform process. The complexity of the players involved in myoblast fusion can be modulated depending on the type of muscle that is formed. In this review, we introduce the different types of multinucleated muscles that form during Drosophila development and provide an overview in advances that have been made to understand the mechanism of myoblast fusion. Finally, we will discuss conceptual frameworks in cell–cell fusion in Drosophila and mammals.

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.

Expression of smooth muscle-like effectors and core cardiomyocyte regulators in the contractile papillae of Ciona

BACKGROUND

The evolution of vertebrate smooth muscles is obscured by lack of identifiable smooth muscle-like cells in tunicates, the invertebrates most closely related to vertebrates. A recent evolutionary model was proposed in which smooth muscles arose before the last bilaterian common ancestor, and were later diversified, secondarily lost or modified in the branches leading to extant animal taxa. However, there is currently no data from tunicates to support this scenario.

METHODS AND RESULTS

Here, we show that the axial columnar cells, a unique cell type in the adhesive larval papillae of the tunicate Ciona, are enriched for orthologs of vertebrate smooth/non-muscle-specific effectors of contractility, in addition to developing from progenitors that express conserved cardiomyocyte regulatory factors. We show that these cells contract during the retraction of the Ciona papillae during larval settlement and metamorphosis.

CONCLUSIONS

We propose that the axial columnar cells of Ciona are a myoepithelial cell type required for transducing external stimuli into mechanical forces that aid in the attachment of the motile larva to its final substrate. Furthermore, they share developmental and functional features with vertebrate myoepithelial cells, vascular smooth muscle cells, and cardiomyocytes. We discuss these findings in the context of the proposed models of vertebrate smooth muscle and cardiomyocyte evolution.

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.

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.

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.

Origin, Specification, and Plasticity of the Great Vessels of the Heart

The pharyngeal arch arteries (PAAs) are a series of paired embryonic blood vessels that give rise to several major arteries that connect directly to the heart. During development, the PAAs emerge from nkx2.5-expressing mesodermal cells and connect the dorsal head vasculature to the outflow tract of the heart. Despite their central role in establishing the circulatory system, the embryonic origins of the PAA progenitors are only coarsely defined, and the factors that specify them and their regenerative potential are unclear. Using fate mapping and mutant analysis, we find that PAA progenitors are derived from the tcf21 and nkx2.5 double-positive head mesoderm and require these two transcription factors for their specification and survival. Unexpectedly, cell ablation shows that the tcf21+; nkx2.5+ PAA progenitors are not required for PAA formation. We find that this compensation is due to the replacement of ablated tcf21+; nkx2.5+ PAA cells by endothelial cells from the dorsal head vasculature. Together, these studies assign the embryonic origin of the great vessel progenitors to the interface between the pharyngeal and cardiac mesoderm, identify the transcription factor code required for their specification, and reveal an unexpected plasticity in the formation of the great vessels.

Hox control of Drosophila larval anatomy; The Alary and Thoracic Alary-Related Muscles

The body plan of arthropods and vertebrates involves the formation of repetitive segments, which subsequently diversify to give rise to different body parts along the antero-posterior/rostro-caudal body axis. Anatomical variations between body segments are crucial for organ function and organismal fitness. Pioneering work in Drosophila has established that Hox transcription factors play key roles both in endowing initially identical segments with distinct identities and organogenesis. The focus of this review is on Alary Muscles (AMs) and the newly discovered Thoracic Alary-Related Muscles (TARMs). AMs and TARMs are thin muscles which together connect the circulatory system and different midgut regions to the exoskeleton, while intertwining with the respiratory tubular network. They were hypothesized to represent a new type of muscles with spring-like properties, maintaining internal organs in proper anatomical positions during larval locomotion. Both the morphology of TARMs relative to AMs, and morphogenesis of connected tissues is under Hox control, emphasizing the key role of Hox proteins in coordinating the anatomical development of the larva.

An Org-1-Tup transcriptional cascade reveals different types of alary muscles connecting internal organs in Drosophila

The T-box transcription factor Tbx1 and the LIM-homeodomain transcription factor Islet1 are key components in regulatory circuits that generate myogenic and cardiogenic lineage diversity in chordates. We show here that Org-1 and Tup, the Drosophila orthologs of Tbx1 and Islet1, are co-expressed and required for formation of the heart-associated alary muscles (AMs) in the abdomen. The same holds true for lineage-related muscles in the thorax that have not been described previously, which we name thoracic alary-related muscles (TARMs). Lineage analyses identified the progenitor cell for each AM and TARM. Three-dimensional high-resolution analyses indicate that AMs and TARMs connect the exoskeleton to the aorta/heart and to different regions of the midgut, respectively, and surround-specific tracheal branches, pointing to an architectural role in the internal anatomy of the larva. Org-1 controls tup expression in the AM/TARM lineage by direct binding to two regulatory sites within an AM/TARM-specific cis-regulatory module, tupAME. The contributions of Org-1 and Tup to the specification of Drosophila AMs and TARMs provide new insights into the transcriptional control of Drosophila larval muscle diversification and highlight new parallels with gene regulatory networks involved in the specification of cardiopharyngeal mesodermal derivatives in chordates.

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.

Myogenesis in the sea urchin embryo: the molecular fingerprint of the myoblast precursors

Background

In sea urchin larvae the circumesophageal fibers form a prominent muscle system of mesodermal origin. Although the morphology and later development of this muscle system has been well-described, little is known about the molecular signature of these cells or their precise origin in the early embryo. As an invertebrate deuterostome that is more closely related to the vertebrates than other commonly used model systems in myogenesis, the sea urchin fills an important phylogenetic gap and provides a unique perspective on the evolution of muscle cell development.

Results

Here, we present a comprehensive description of the development of the sea urchin larval circumesophageal muscle lineage beginning with its mesodermal origin using high-resolution localization of the expression of several myogenic transcriptional regulators and differentiation genes. A few myoblasts are bilaterally distributed at the oral vegetal side of the tip of the archenteron and first appear at the late gastrula stage. The expression of the differentiation genes Myosin Heavy Chain, Tropomyosin I and II, as well as the regulatory genes MyoD2, FoxF, FoxC, FoxL1, Myocardin, Twist, and Tbx6 uniquely identify these cells. Interestingly, evolutionarily conserved myogenic factors such as Mef2, MyoR and Six1/2 are not expressed in sea urchin myoblasts but are found in other mesodermal domains of the tip of the archenteron. The regulatory states of these domains were characterized in detail. Moreover, using a combinatorial analysis of gene expression we followed the development of the FoxF/FoxC positive cells from the onset of expression to the end of gastrulation. Our data allowed us to build a complete map of the Non-Skeletogenic Mesoderm at the very early gastrula stage, in which specific molecular signatures identify the precursors of different cell types. Among them, a small group of cells within the FoxY domain, which also express FoxC and SoxE, have been identified as plausible myoblast precursors. Together, these data support a very early gastrula stage segregation of the myogenic lineage.

Conclusions

From this analysis, we are able to precisely define the regulatory and differentiation signatures of the circumesophageal muscle in the sea urchin embryo. Our findings have important implications in understanding the evolution of development of the muscle cell lineage at the molecular level. The data presented here suggest a high level of conservation of the myogenic specification mechanisms across wide phylogenetic distances, but also reveal clear cases of gene cooption.

Genome-wide binding of the basic helix-loop-helix myogenic inhibitor musculin has substantial overlap with MyoD: implications for buffering activity

Background

Musculin (MSC) is a basic helix-loop-helix transcription factor that inhibits myogenesis during normal development and contributes to the differentiation defect in rhabdomyosarcoma. As one of many transcription factors that impede myogenesis, its binding on a genome-wide scale relative to the widespread binding of the myogenic factor MyoD is unknown.

Methods

Chromatin immunoprecipitation coupled to high-throughput sequencing was performed for endogenous MSC in rhabdomyosarcoma cells and its binding was compared to that of MyoD in the same type of cells.

Results

MSC binds throughout the genome, in a pattern very similar to MyoD. Its binding overlaps strongly with regions enriched for acetylated histone H4, as well as regions that score high for DNase hypersensitivity in human myoblasts. In contrast to MyoD, MSC has a more relaxed binding sequence preference in the nucleotides that flank the core E-box motif.

Conclusions

The myogenic inhibitor MSC binds throughout the genome of rhabdomyosarcoma cells, in a pattern highly similar to that of MyoD, suggesting a broad role in buffering the activity of MyoD in development and rhabdomyosarcomas.

Jeb/Alk signalling regulates the Lame duck GLI family transcription factor in the Drosophila visceral mesoderm

The Jelly belly (Jeb)/Anaplastic Lymphoma Kinase (Alk) signalling pathway regulates myoblast fusion in the circular visceral mesoderm (VM) of Drosophila embryos via specification of founder cells. However, only a limited number of target molecules for this pathway are described. We have investigated the role of the Lame Duck (Lmd) transcription factor in VM development in relationship to Jeb/Alk signal transduction. We show that Alk signalling negatively regulates Lmd activity post-transcriptionally through the MEK/MAPK (ERK) cascade resulting in a relocalisation of Lmd protein from the nucleus to cytoplasm. It has previously been shown that downregulation of Lmd protein is necessary for the correct specification of founder cells. In the visceral mesoderm of lmd mutant embryos, fusion-competent myoblasts seem to be converted to 'founder-like' cells that are still able to build a gut musculature even in the absence of fusion. The ability of Alk signalling to downregulate Lmd protein requires the N-terminal 140 amino acids, as a Lmd(141-866) mutant remains nuclear in the presence of active ALK and is able to drive robust expression of the Lmd downstream target Vrp1 in the developing VM. Our results suggest that Lmd is a target of Jeb/Alk signalling in the VM of Drosophila embryos.

The secreted AdamTS-A metalloprotease is required for collective cell migration

Members of the ADAMTS family of secreted metalloproteases play crucial roles in modulating the extracellular matrix (ECM) in development and disease. Here, we show that ADAMTS-A, the Drosophila ortholog of human ADAMTS 9 and ADAMTS 20, and of C. elegans GON-1, is required for cell migration during embryogenesis. AdamTS-A is expressed in multiple migratory cell types, including hemocytes, caudal visceral mesoderm (CVM), the visceral branch of the trachea (VBs) and the secretory portion of the salivary gland (SG). Loss of AdamTS-A causes defects in germ cell, CVM and VB migration and, depending on the tissue, AdamTS-A functions both autonomously and non-autonomously. In the highly polarized collective of the SG epithelium, loss of AdamTS-A causes apical surface irregularities and cell elongation defects. We provide evidence that ADAMTS-A is secreted into the SG lumen where it functions to release cells from the apical ECM, consistent with the defects observed in AdamTS-A mutant SGs. We show that loss of the apically localized protocadherin Cad99C rescues the SG defects, suggesting that Cad99C serves as a link between the SG apical membrane and the secreted apical ECM component(s) cleaved by ADAMTS-A. Our analysis of AdamTS-A function in the SG suggests a novel role for ADAMTS proteins in detaching cells from the apical ECM, facilitating tube elongation during collective cell migration.

Org-1 is required for the diversification of circular visceral muscle founder cells and normal midgut morphogenesis

The T-Box family of transcription factors plays fundamental roles in the generation of appropriate spatial and temporal gene expression profiles during cellular differentiation and organogenesis in animals. In this study we report that the Drosophila Tbx1 orthologue optomotor-blind-related-gene-1 (org-1) exerts a pivotal function in the diversification of circular visceral muscle founder cell identities in Drosophila. In embryos mutant for org-1, the specification of the midgut musculature per se is not affected, but the differentiating midgut fails to form the anterior and central midgut constrictions and lacks the gastric caeca. We demonstrate that this phenotype results from the nearly complete loss of the founder cell specific expression domains of several genes known to regulate midgut morphogenesis, including odd-paired (opa), teashirt (tsh), Ultrabithorax (Ubx), decapentaplegic (dpp) and wingless (wg). To address the mechanisms that mediate the regulatory inputs from org-1 towards Ubx, dpp, and wg in these founder cells we genetically dissected known visceral mesoderm specific cis-regulatory-modules (CRMs) of these genes. The analyses revealed that the activities of the dpp and wg CRMs depend on org-1, the CRMs are bound by Org-1 in vivo and their T-Box binding sites are essential for their activation in the visceral muscle founder cells. We conclude that Org-1 acts within a well-defined signaling and transcriptional network of the trunk visceral mesoderm as a crucial founder cell-specific competence factor, in concert with the general visceral mesodermal factor Biniou. As such, it directly regulates several key genes involved in the establishment of morphogenetic centers along the anteroposterior axis of the visceral mesoderm, which subsequently organize the formation of midgut constrictions and gastric caeca and thereby determine the morphology of the midgut.

What goes up must come down: transcription factors have their say in making ecdysone pulses

Insect metamorphosis is one of the most fascinating biological processes in the animal kingdom. The dramatic transition from an immature juvenile to a reproductive adult is under the control of the steroid hormone ecdysone, also known as the insect molting hormone. During Drosophila development, periodic pulses of ecdysone are released from the prothoracic glands, upon which the hormone is rapidly converted in peripheral tissues to its biologically active form, 20-hydroxyecdysone. Each hormone pulse has a unique profile and causes different developmental events, but we only have a rudimentary understanding of how the timing, amplitude, and duration of a given pulse are controlled. A key component involved in the timing of ecdysone pulses is PTTH, a brain-derived neuropeptide. PTTH stimulates ecdysone production through a Ras/Raf/ERK signaling cascade; however, comparatively little is known about the downstream targets of this pathway. In recent years, it has become apparent that transcriptional regulation plays a critical role in regulating the synthesis of ecdysone, but only one transcription factor has a well-defined link to PTTH. Interestingly, many of the ecdysteroidogenic transcription factors were originally characterized as primary response genes in the ecdysone signaling cascade that elicits the biological responses to the hormone in target tissues. To review these developments, we will first provide an overview of the transcription factors that act in the Drosophila ecdysone regulatory hierarchy. We will then discuss the roles of these transcriptional regulators in controlling ecdysone synthesis. In the last section, we will briefly outline transcription factors that likely have roles in regulating ecdysone synthesis but have not been formally identified as downstream effectors of ecdysone.

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.

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

Fibroblast growth factors (FGFs) frequently fulfill prominent roles in the regulation of cell migration in various contexts. In Drosophila, the FGF8-like ligands Pyramus (Pyr) and Thisbe (Ths), which signal through their receptor Heartless (Htl), are known to regulate early mesodermal cell migration after gastrulation as well as glial cell migration during eye development. Herein, we show that Pyr and Ths also exert key roles during the long-distance migration of a specific sub-population of mesodermal cells that migrate from the caudal visceral mesoderm within stereotypic bilateral paths along the trunk visceral mesoderm toward the anterior. These cells constitute the founder myoblasts of the longitudinal midgut muscles. In a forward genetic screen for regulators of this morphogenetic process we identified loss of function alleles for pyr. We show that pyr and ths are expressed along the paths of migration in the trunk visceral mesoderm and endoderm and act largely redundantly to help guide the founder myoblasts reliably onto and along their substrate of migration. Ectopically-provided Pyr and Ths signals can efficiently re-rout the migrating cells, both in the presence and absence of endogenous signals. Our data indicate that the guidance functions of these FGFs must act in concert with other important attractive or adhesive activities of the trunk visceral mesoderm. Apart from their guidance functions, the Pyr and Ths signals play an obligatory role for the survival of the migrating cells. Without these signals, essentially all of these cells enter cell death and detach from the migration substrate during early migration. We present experiments that allowed us to dissect the roles of these FGFs as guidance cues versus trophic activities during the migration of the longitudinal visceral muscle founders.

Synchronous and symmetric migration of Drosophila caudal visceral mesoderm cells requires dual input by two FGF ligands

Caudal visceral mesoderm (CVM) cells migrate synchronously towards the anterior of the Drosophila embryo as two distinct groups located on each side of the body, in order to specify longitudinal muscles that ensheath the gut. Little is known about the molecular cues that guide cells along this path, the longest migration of embryogenesis, except that they closely associate with trunk visceral mesoderm (TVM). The expression of the fibroblast growth factor receptor (FGFR) heartless and its ligands, pyramus (pyr) and thisbe (ths), within CVM and TVM cells, respectively, suggested FGF signaling may influence CVM cell guidance. In FGF mutants, CVM cells die before reaching the anterior region of the TVM. However, an earlier phenotype observed was that the two cell clusters lose direction and converge at the midline. Live in vivo imaging and tracking analyses identified that the movements of CVM cells were slower and no longer synchronous. Moreover, CVM cells were found to cross over from one group to the other, disrupting bilateral symmetry, whereas such mixing was never observed in wild-type embryos. Ectopic expression of either Pyr or Ths was sufficient to redirect CVM cell movement, but only when the endogenous source of these ligands was absent. Collectively, our results show that FGF signaling regulates directional movement of CVM cells and that native presentation of both FGF ligands together is most effective at attracting cells. This study also has general implications, as it suggests that the activity supported by two FGF ligands in concert differs from their activities in isolation.

PS integrins and laminins: key regulators of cell migration during Drosophila embryogenesis

During embryonic development, there are numerous cases where organ or tissue formation depends upon the migration of primordial cells. In the Drosophila embryo, the visceral mesoderm (vm) acts as a substrate for the migration of several cell populations of epithelial origin, including the endoderm, the trachea and the salivary glands. These migratory processes require both integrins and laminins. The current model is that αPS1βPS (PS1) and/or αPS3βPS (PS3) integrins are required in migrating cells, whereas αPS2βPS (PS2) integrin is required in the vm, where it performs an as yet unidentified function. Here, we show that PS1 integrins are also required for the migration over the vm of cells of mesodermal origin, the caudal visceral mesoderm (CVM). These results support a model in which PS1 might have evolved to acquire the migratory function of integrins, irrespective of the origin of the tissue. This integrin function is highly specific and its specificity resides mainly in the extracellular domain. In addition, we have identified the Laminin α1,2 trimer, as the key extracellular matrix (ECM) component regulating CVM migration. Furthermore, we show that, as it is the case in vertebrates, integrins, and specifically PS2, contributes to CVM movement by participating in the correct assembly of the ECM that serves as tracks for migration.