Epidermal muscle attachment site-specific target gene expression and interference with myotube guidance in response to ectopic stripe expression in the developing Drosophila epidermis

Exoskeletal cuticle proteins enable Drosophila locomotion

Exo- and Endoskeleton function enables muscle-mediated locomotion in animals. In mammals, the defective protein matrix of bones found in systematic skeletal disorders such as osteoporosis causes fractures and severe skeletal deformations under high muscle tension. We identified an analogous mechanism for integrating muscle-mediated tension into the apical extracellular matrix (aECM) of the invertebrate body wall exoskeleton. Obstructor chitin-binding proteins, the chitin deacetylases, Chitinases, and the matrix-protecting proteins Knickkopf and Retroactive are epidermally expressed during late embryogenesis. Their control of forming epidermal chitinous structures protects the exoskeletal aECM from collapsing when embryos start moving and hatch as larvae. In a larval locomotion assay we tested the function of these cuticle related genes. Gene mutations and knockdowns caused changes in normal movement behavior and lower the speed of larvae. Moreover, we found that the transmembrane Zona Pellucida domain protein Piopio provides the adhesion between the epidermal apical membrane and the overlaying chitinous aECM in a matriptase-dependent manner. A failure of Pio and chitin-associated proteins leads to exoskeletal deformations and detachment from the epidermal membrane, destabilizing muscle forces and impairing larval mobility. Our data identifies a protein network that transforms the chitinous aECM into a stable exoskeleton that directly resists muscle impact at epidermal tendon cells, thereby serving locomotion. Demonstrating the importance of these proteins in producing aECM as a three-dimensional cuticular scaffold for exoskeletal function opens up opportunities for the development of biomimetic applications of synthetic materials. STATEMENT OF SIGNIFICANCE: Chitin-based materials include hydrogels, microcapsules, membranous films, sponges, tubes, and various porous structures. In nature, chitin structures form cuticles, which serves as the exoskeleton of arthropods. Using Drosophila melanogaster, we have performed systematic analyses to identify the proteins and enzymes that organize chitin polymers in 3D structures of the cuticle exoskeleton. Three-dimensional laser-scanning and ultrastructural electron microscopy revealed deformations of the cuticle structure, lack of cellular cuticle adhesion, and overall changes in the flexibility of the chitin-based material, leading to insufficient function of the exoskeleton. Components such as the identified proteins and enzymes, which play a unique role in the organization of the chitin fibers and the formation of the exoskeleton, offer suitable materials for tissue engineering for biomimetic applications.

Developmental origin of tendon diversity in Drosophila melanogaster

Myogenesis is a developmental process that is largely conserved in both Drosophila and higher organisms. Consequently, the fruit fly is an excellent in vivo model for identifying the genes and mechanisms involved in muscle development. Moreover, there is growing evidence indicating that specific conserved genes and signaling pathways govern the formation of tissues that connect the muscles to the skeleton. In this review, we present an overview of the different stages of tendon development, from the specification of tendon progenitors to the assembly of a stable myotendinous junction across three different myogenic contexts in Drosophila: larval, flight and leg muscle development. We underline the different aspects of tendon cell specification and differentiation in embryo and during metamorphosis that result into tendon morphological and functional diversity.

Transcriptomic and Genetic Analyses Identify the Krüppel-Like Factor Dar1 as a New Regulator of Tube-Shaped Long Tendon Development

To ensure locomotion and body stability, the active role of muscle contractions relies on a stereotyped muscle pattern set in place during development. This muscle patterning requires a precise assembly of the muscle fibers with the skeleton via a specialized connective tissue, the tendon. Like in vertebrate limbs, Drosophila leg muscles make connections with specific long tendons that extend through different segments. During the leg disc development, cell precursors of long tendons rearrange and collectively migrate to form a tube-shaped structure. A specific developmental program underlies this unique feature of tendon-like cells in the Drosophila model. We provide for the first time a transcriptomic profile of leg tendon precursors through fluorescence-based cell sorting. From promising candidates, we identified the Krüppel-like factor Dar1 as a critical actor of leg tendon development. Specifically expressed in the leg tendon precursors, loss of dar1 disrupts actin-rich filopodia formation and tendon elongation. Our findings show that Dar1 acts downstream of Stripe and is required to set up the correct number of tendon progenitors.

A Large Scale Systemic RNAi Screen in the Red Flour Beetle Tribolium castaneum Identifies Novel Genes Involved in Insect Muscle Development

Although muscle development has been widely studied in Drosophila melanogaster there are still many gaps in our knowledge, and it is not known to which extent this knowledge can be transferred to other insects. To help in closing these gaps we participated in a large-scale RNAi screen that used the red flour beetle, Tribolium castaneum, as a screening platform. The effects of systemic RNAi were screened upon double-stranded RNA injections into appropriate muscle-EGFP tester strains. Injections into pupae were followed by the analysis of the late embryonic/early larval muscle patterns, and injections into larvae by the analysis of the adult thoracic muscle patterns. Herein we describe the results of the first-pass screens with pupal and larval injections, which covered ∼8,500 and ∼5,000 genes, respectively, of a total of ∼16,500 genes of the Tribolium genome. Apart from many genes known from Drosophila as regulators of muscle development, a collection of genes previously unconnected to muscle development yielded phenotypes in larval body wall and leg muscles as well as in indirect flight muscles. We then present the main candidates from the pupal injection screen that remained after being processed through a series of verification and selection steps. Further, we discuss why distinct though overlapping sets of genes are revealed by the Drosophila and Tribolium screening approaches.

Odd-skipped and Stripe act downstream of Notch to promote the morphogenesis of long appendicular tendons in Drosophila

Multiple tissue interactions take place during the development of the limb musculoskeletal system. While appendicular myogenesis has been extensively studied, development of connective tissue associated with muscles has received less attention. In the developing Drosophila leg, tendon-like connective tissue arises from clusters of epithelial cells that invaginate into the leg cavity and then elongate to form internal tube-shape structures along which muscle precursors are distributed. Here we show that stripe-positive appendicular precursors of tendon-like connective tissue are set up among intersegmental leg joint cells expressing odd-skipped genes, and that Notch signaling is necessary and locally sufficient to trigger stripe expression. This study also finds that odd-skipped genes and stripe are both required downstream of Notch to promote morphogenesis of tube-shaped internal tendons of the leg.

Summary: In this paper, we show that Notch promotes the tendon development by inducing Stripe expression in leg discs and that both Stripe and Odd-skipped are required to form tube-like tendons.

Coordinated Development of Muscles and Tendon-Like Structures: Early Interactions in the Drosophila Leg

The formation of the musculoskeletal system is a remarkable example of tissue assembly. In both vertebrates and invertebrates, precise connectivity between muscles and skeleton (or exoskeleton) via tendons or equivalent structures is fundamental for movement and stability of the body. The molecular and cellular processes underpinning muscle formation are well-established and significant advances have been made in understanding tendon development. However, the mechanisms contributing to proper connection between these two tissues have received less attention. Observations of coordinated development of tendons and muscles suggest these tissues may interact during the different steps in their development. There is growing evidence that, depending on animal model and muscle type, these interactions can take place from progenitor induction to the final step of the formation of the musculoskeletal system. Here, we briefly review and compare the mechanisms behind muscle and tendon interaction throughout the development of vertebrates and Drosophila before going on to discuss our recent findings on the coordinated development of muscles and tendon-like structures in Drosophila leg. By altering apodeme formation (the functional Drosophila equivalent of tendons in vertebrates) during the early steps of leg development, we affect the spatial localization of subsequent myoblasts. These findings provide the first evidence of the developmental impact of early interactions between muscle and tendon-like precursors, and confirm the appendicular Drosophila muscle system as a valuable model for studying these processes.

Molecular regulation of tendon cell fate during development

Although there have been several advances identifying novel mediators of tendon induction, differentiation, and patterning, much of the basic landscape of tendon biology from developmental stages onward remain almost completely undefined. During the New Frontiers in Tendon Research meeting, a group of developmental biologists with expertise across musculoskeletal disciplines identified key challenges for the tendon development field. The tools generated and the molecular regulators identified in developmental research have enhanced mechanistic studies in tendon injury and repair, both by defining benchmarks for success, as well as informing regenerative strategies. To address the needs of the orthopedic research community, this review will therefore focus on three key areas in tendon development that may have critical implications for the fields of tendon repair/regeneration and tendon tissue engineering, including functional markers of tendon cell identity, signaling regulators of tendon induction and differentiation, and in vitro culture models for tendon cell differentiation. Our goal is to provide a useful list of the currently known molecular players and their function in tendon differentiation within the context of development.

Hedgehog is a positive regulator of FGF signalling during embryonic tracheal cell migration

Cell migration is a widespread and complex process that is crucial for morphogenesis and for the underlying invasion and metastasis of human cancers. During migration, cells are steered toward target sites by guidance molecules that induce cell direction and movement through complex intracellular mechanisms. The spatio-temporal regulation of the expression of these guidance molecules is of extreme importance for both normal morphogenesis and human disease. One way to achieve this precise regulation is by combinatorial inputs of different transcription factors. Here we used Drosophila melanogaster mutants with migration defects in the ganglionic branches of the tracheal system to further clarify guidance regulation during cell migration. By studying the cellular consequences of overactivated Hh signalling, using ptc mutants, we found that Hh positively regulates Bnl/FGF levels during embryonic stages. Our results show that Hh modulates cell migration non-autonomously in the tissues surrounding the action of its activity. We further demonstrate that the Hh signalling pathway regulates bnl expression via Stripe (Sr), a zinc-finger transcription factor with homology to the Early Growth Response (EGR) family of vertebrate transcription factors. We propose that Hh modulates embryonic cell migration by participating in the spatio-temporal regulation of bnl expression in a permissive mode. By doing so, we provide a molecular link between the activation of Hh signalling and increased chemotactic responses during cell migration.

Plasticity of both planar cell polarity and cell identity during the development of Drosophila

Drosophila has helped us understand the genetic mechanisms of pattern formation. Particularly useful have been those organs in which different cell identities and polarities are displayed cell by cell in the cuticle and epidermis (Lawrence, 1992; Bejsovec and Wieschaus, 1993; Freeman, 1997). Here we use the pattern of larval denticles and muscle attachments and ask how this pattern is maintained and renewed over the larval moult cycles. During larval growth each epidermal cell increases manyfold in size but neither divides nor dies. We follow individuals from moult to moult, tracking marked cells and find that, as cells are repositioned and alter their neighbours, their identities change to compensate and the pattern is conserved. Single cells adopting a new fate may even acquire a new polarity: an identified cell that makes a forward-pointing denticle in the first larval stage may make a backward-pointing denticle in the second and third larval stages. DOI: http://dx.doi.org/10.7554/eLife.01569.001.

"Importin" signaling roles for import proteins: the function of Drosophila importin-7 (DIM-7) in muscle-tendon signaling

The formation of a mature myotendinous junction (MTJ) between a muscle and its site of attachment is a highly regulated process that involves myofiber migration, cell-cell signaling, and culminates with the stable adhesion between the adjacent muscle-tendon cells. Improper establishment or maintenance of muscle-tendon attachment sites results in a decrease in force generation during muscle contraction and progressive muscular dystrophies in vertebrate models. Many studies have demonstrated the important role of the integrins and integrin-associated proteins in the formation and maintenance of the MTJ. We recently demonstrated that moleskin (msk), the gene that encodes for Drosophila importin-7 (DIM-7), is required for the proper formation of muscle-tendon adhesion sites in the developing embryo. Further studies demonstrated an enrichment of DIM-7 to the ends of muscles where the muscles attach to their target tendon cells. Genetic analysis supports a model whereby msk is required in the muscle and signals via the secreted epidermal growth factor receptor (Egfr) ligand Vein to regulate tendon cell maturation. These data demonstrate a novel role for the canonical nuclear import protein DIM-7 in establishment of the MTJ.

WNT5 interacts with the Ryk receptors doughnut and derailed to mediate muscle attachment site selection in Drosophila melanogaster

In recent years a number of the genes that regulate muscle formation and maintenance in higher organisms have been identified. Studies employing invertebrate and vertebrate model organisms have revealed that many of the genes required for early mesoderm specification are highly conserved throughout evolution. Less is known about the molecules that mediate the steps subsequent to myogenesis, e. g. myotube guidance and attachment to tendon cells. We use the stereotypic pattern of the Drosophila embryonic body wall musculature in genetic approaches to identify novel factors required for muscle attachment site selection. Here, we show that Wnt5 is needed in this process. The lateral transverse muscles frequently overshoot their target attachment sites and stably attach at novel epidermal sites in Wnt5 mutant embryos. Restoration of WNT5 expression in either the muscle or the tendon cell rescues the mutant phenotype. Surprisingly, the novel attachment sites in Wnt5 mutants frequently do not express the Stripe (SR) protein which has been shown to be required for terminal tendon cell differentiation. A muscle bypass phenotype was previously reported for embryos lacking the WNT5 receptor Derailed (DRL). drl and Wnt5 mutant embryos also exhibit axon path finding errors. DRL belongs to the conserved Ryk receptor tyrosine kinase family which includes two other Drosophila orthologs, the Doughnut on 2 (DNT) and Derailed-2 (DRL-2) proteins. We generated a mutant allele of dnt and find that dnt, but not Drl-2, mutant embryos also show a muscle bypass phenotype. Genetic interaction experiments indicate that drl and dnt act together, likely as WNT5 receptors, to control muscle attachment site selection. These results extend previous findings that at least some of the molecular pathways that guide axons towards their targets are also employed for guidance of muscle fibers to their appropriate attachment sites.

The muscle pattern of the Drosophila abdomen depends on a subdivision of the anterior compartment of each segment

In the past, segments were defined by landmarks such as muscle attachments, notably by Snodgrass, the king of insect anatomists. Here, we show how an objective definition of a segment, based on developmental compartments, can help explain the dorsal abdomen of adult Drosophila. The anterior (A) compartment of each segment is subdivided into two domains of cells, each responding differently to Hedgehog. The anterior of these domains is non-neurogenic and clones lacking Notch develop normally; this domain can express stripe and form muscle attachments. The posterior domain is neurogenic and clones lacking Notch do not form cuticle; this domain is unable to express stripe or form muscle attachments. The posterior (P) compartment does not form muscle attachments. Our in vivo films indicate that early in the pupa the anterior domain of the A compartment expresses stripe in a narrowing zone that attracts the extending myotubes and resolves into the attachment sites for the dorsal abdominal muscles. We map the tendon cells precisely and show that all are confined to the anterior domain of A. It follows that the dorsal abdominal muscles are intersegmental, spanning from one anterior domain to the next. This view is tested and supported by clones that change cell identity or express stripe ectopically. It seems that growing myotubes originate in posterior A and extend forwards and backwards until they encounter and attach to anterior A cells. The dorsal adult muscles are polarised in the anteroposterior axis: we disprove the hypothesis that muscle orientation depends on genes that define planar cell polarity in the epidermis.

Moleskin is essential for the formation of the myotendinous junction in Drosophila

It is the precise connectivity between skeletal muscles and their corresponding tendon cells to form a functional myotendinous junction (MTJ) that allows for the force generation required for muscle contraction and organismal movement. The Drosophila MTJ is composed of secreted extracellular matrix (ECM) proteins deposited between integrin-mediated hemi-adherens junctions on the surface of muscle and tendon cells. In this paper, we have identified a novel, cytoplasmic role for the canonical nuclear import protein Moleskin (Msk) in Drosophila embryonic somatic muscle attachment. Msk protein is enriched at muscle attachment sites in late embryogenesis and msk mutant embryos exhibit a failure in muscle-tendon cell attachment. Although the muscle-tendon attachment sites are reduced in size, components of the integrin complexes and ECM proteins are properly localized in msk mutant embryos. However, msk mutants fail to localize phosphorylated focal adhesion kinase (pFAK) to the sites of muscle-tendon cell junctions. In addition, the tendon cell specific proteins Stripe (Sr) and activated mitogen-activated protein kinase (MAPK) are reduced in msk mutant embryos. Our rescue experiments demonstrate that Msk is required in the muscle cell, but not in the tendon cells. Moreover, muscle attachment defects due to loss of Msk are rescued by an activated form of MAPK or the secreted epidermal growth factor receptor (Egfr) ligand Vein. Taken together, these findings provide strong evidence that Msk signals non-autonomously through the Vein-Egfr signaling pathway for late tendon cell late differentiation and/or maintenance.

EGR1 and EGR2 involvement in vertebrate tendon differentiation

The molecules involved in vertebrate tendon formation during development remain largely unknown. To date, only two DNA-binding proteins have been identified as being involved in vertebrate tendon formation, the basic helix-loop-helix transcription factor Scleraxis and, recently, the Mohawk homeobox gene. We investigated the involvement of the early growth response transcription factors Egr1 and Egr2 in vertebrate tendon formation. We established that Egr1 and Egr2 expression in tendon cells was correlated with the increase of collagen expression during tendon cell differentiation in embryonic limbs. Vertebrate tendon differentiation relies on a muscle-derived FGF (fibroblast growth factor) signal. FGF4 was able to activate the expression of Egr genes and that of the tendon-associated collagens in chick limbs. Egr gene misexpression experiments using the chick model allowed us to establish that either Egr gene has the ability to induce de novo expression of the reference tendon marker scleraxis, the main tendon collagen Col1a1, and other tendon-associated collagens Col3a1, Col5a1, Col12a1, and Col14a1. Mouse mutants for Egr1 or Egr2 displayed reduced amounts of Col1a1 transcripts and a decrease in the number of collagen fibrils in embryonic tendons. Moreover, EGR1 and EGR2 trans-activated the mouse Col1a1 proximal promoter and were recruited to the tendon regulatory regions of this promoter. These results identify EGRs as novel DNA-binding proteins involved in vertebrate tendon differentiation by regulating type I collagen production.

Vestigial is required during late-stage muscle differentiation in Drosophila melanogaster embryos

The Drosophila member of the vestigial-like gene family (vestigial) is known primarily as a transcriptional activator that defines cell identity during Drosophila wing differentiation. We show that during embryo development Vestigial also has a role during specification of muscle–muscle attachments in ventral longitudinal muscles.

The somatic muscles of Drosophila develop in a complex pattern that is repeated in each embryonic hemi-segment. During early development, progenitor cells fuse to form a syncytial muscle, which further differentiates via expression of muscle-specific factors that induce specific responses to external signals to regulate late-stage processes such as migration and attachment. Initial communication between somatic muscles and the epidermal tendon cells is critical for both of these processes. However, later establishment of attachments between longitudinal muscles at the segmental borders is largely independent of the muscle–epidermal attachment signals, and relatively little is known about how this event is regulated. Using a combination of null mutations and a truncated version of Sd that binds Vg but not DNA, we show that Vestigial (Vg) is required in ventral longitudinal muscles to induce formation of stable intermuscular attachments. In several muscles, this activity may be independent of Sd. Furthermore, the cell-specific differentiation events induced by Vg in two cells fated to form attachments are coordinated by Drosophila epidermal growth factor signaling. Thus, Vg is a key factor to induce specific changes in ventral longitudinal muscles 1–4 identity and is required for these cells to be competent to form stable intermuscular attachments with each other.

Cytoskeletal remodeling during myotube assembly and guidance: coordinating the actin and microtubule networks

The formation of a multinucleated muscle fiber from individual myoblasts is a complex morphological event that requires dramatic cytoskeletal rearrangements. This multistep process includes myoblast fusion, myotube migration and elongation, myotube target recognition, and finally attachment to form a stable adhesion complex. Many of the studies directed towards understanding the developmental process of muscle morphogenesis at the cellular level have relied on forward genetic screens in model systems such as Drosophila melanogaster for mutations affecting individual stages in myogenesis. Through the analyses of these gene products, proteins that regulate the actin or microtubule cytoskeleton have emerged as important players in each of these steps. We recently demonstrated that RacGAP50C, an essential protein that functions as a cytoskeletal regulator during cell division, also plays an important role in organizing the polarized microtubule network in the elongating myotube. Here we review the current literature regarding Drosophila myogenesis and illustrate several steps of muscle development with respect to the diverse roles that the cytoskeleton plays during this process. Furthermore, we discuss the significance of cytoskeletal coordination during these multiple steps.

LRT, a tendon-specific leucine-rich repeat protein, promotes muscle-tendon targeting through its interaction with Robo

Correct muscle migration towards tendon cells, and the adhesion of these two cell types, form the basis for contractile tissue assembly in the Drosophila embryo. While molecules promoting the attraction of muscles towards tendon cells have been described, signals involved in the arrest of muscle migration following the arrival of myotubes at their corresponding tendon cells have yet to be elucidated. Here, we describe a novel tendon-specific transmembrane protein, which we named LRT due to the presence of a leucine-rich repeat domain (LRR) in its extracellular region. Our analysis suggests that LRT acts non-autonomously to better target the muscle and/or arrest its migration upon arrival at its corresponding tendon cell. Muscles in embryos lacking LRT exhibited continuous formation of membrane extensions despite arrival at their corresponding tendon cells, and a partial failure of muscles to target their correct tendon cells. In addition, overexpression of LRT in tendon cells often stalled muscles located close to the tendon cells. LRT formed a protein complex with Robo, and we detected a functional genetic interaction between Robo and LRT at the level of muscle migration behavior. Taken together, our data suggest a novel mechanism by which muscles are targeted towards tendon cells as a result of LRT-Robo interactions. This mechanism may apply to the Robo-dependent migration of a wide variety of cell types.

Metamodels and phylogenetic replication: a systematic approach to the evolution of developmental pathways

Molecular genetic analysis of phenotypic variation has revealed many examples of evolutionary change in the developmental pathways that control plant and animal morphology. A major challenge is to integrate the information from diverse organisms and traits to understand the general patterns of developmental evolution. This integration can be facilitated by evolutionary metamodels-traits that have undergone multiple independent changes in different species and whose development is controlled by well-studied regulatory pathways. The metamodel approach provides the comparative equivalent of experimental replication, allowing us to test whether the evolution of each developmental pathway follows a consistent pattern, and whether different pathways are predisposed to different modes of evolution by their intrinsic organization. A review of several metamodels suggests that the structure of developmental pathways may bias the genetic basis of phenotypic evolution, and highlights phylogenetic replication as a value-added approach that produces deeper insights into the mechanisms of evolution than single-species analyses.

The transmembrane protein Perdido interacts with Grip and integrins to mediate myotube projection and attachment in the Drosophilaembryo

The molecular mechanisms underlying muscle guidance and formation of myotendinous junctions are poorly understood both in vertebrates and in Drosophila. We have identified a novel gene that is essential for Drosophila embryonic muscles to form proper projections and stable attachments to epidermal tendon cells. Loss-of-function of this gene - which we named perdido (perd)-results in rounded, unattached muscles. perd is expressed prior to myoblast fusion in a subset of muscle founder cells, and it encodes a conserved single-pass transmembrane cell adhesion protein that contains laminin globular extracellular domains and a small intracellular domain with a C-terminal PDZ-binding consensus sequence. Biochemical experiments revealed that the Perd intracellular domain interacts directly with one of the PDZ domains of the Glutamate receptor interacting protein (Grip), another factor required for formation of proper muscle projections. In addition, Perd is necessary to localize Grip to the plasma membrane of developing myofibers. Using a newly developed, whole-embryo RNA interference assay to analyze genetic interactions, perd was shown to interact not only with Grip but also with multiple edematous wings, which encodes one subunit of the αPS1-βPS integrin expressed in tendon cells. These experiments uncovered a previously unrecognized role for the αPS1-βPS integrin in the formation of muscle projections during early stages of myotendinous junction development. We propose that Perd regulates projection of myotube processes toward and subsequent differentiation of the myotendinous junction by priming formation of a protein complex through its intracellular interaction with Grip and its transient engagement with the tendon cell-expressed laminin-bindingαPS1-βPS integrin.

AlphaPS2 integrin-mediated muscle attachment in Drosophila requires the ECM protein Thrombospondin

During Drosophila embryogenesis, the attachment of somatic muscles to epidermal tendon cells requires heterodimeric PS-integrin proteins (alpha- and beta-subunits). The alpha-subunits are expressed complementarily, either tendon cell- or muscle-specific, whereas the beta-integrin subunit is expressed in both tissues. Mutations of beta-integrin cause a severe muscle detachment phenotype, whereas alpha-subunit mutations have weaker or only larval muscle detachment phenotypes. Furthermore, mutations of extracellular matrix (ECM) proteins known to act as integrin binding partners have comparatively weak effects only, suggesting the presence of additional integrin binding ECM proteins required for proper muscle attachment. Here, we report that mutations in the Drosophila gene thrombospondin (tsp) cause embryonic muscle detachment. tsp is specifically expressed in both developing and mature epidermal tendon cells. Its initial expression in segment border cells, the tendon precursors, is under the control of hedgehog-dependent signaling, whereas tsp expression in differentiated tendon cells depends on the transcription factor encoded by stripe. In the absence of tsp activity, no aspect of muscle pattern formation as well as the initial contact between muscle and tendon cells nor muscle-to-muscle attachments are affected. However, when muscle contractions occur during late embryogenesis, muscles detach from the tendon cells. The Tsp protein is localized to the tendon cell ECM where muscles attach. Genetic interaction studies indicate that Tsp specifically interacts with the alphaPS2 integrin and that this interaction is needed to withstand the forces of muscle contractions at the tendon cells.

Morphology of the prosomal endoskeleton of Scorpiones (Arachnida) and a new hypothesis for the evolution of cuticular cephalic endoskeletons in arthropods

Skeletomuscular anatomy of the scorpion prosoma is examined in an attempt to explain the evolution of two endoskeletal features, a muscular diaphragm dividing the prosoma and opisthosoma and cuticular epistomal entapophyses with a uniquely complex arrangement of muscles, tendons and ligaments. Both structures appear to be derived from modifications of the mesodermal intersegmental endoskeleton that is primitive for all major arthropod groups. The scorpion diaphragm is a compound structure comprising axial muscles and pericardial ligaments of segments VI to VIII and extrinsic muscles of leg 4 brought into contact by longitudinal reduction of segment VII and integrated into a continuous subvertical sheet. This finding reconciles a long-standing conflict between one interpretation of opisthosomal segmentation based on scorpion embryology and another derived from comparative skeletomuscular anatomy. A new evolutionary-developmental mechanism is proposed to account for the complex morphology of the epistomal entapophyses. Each entapophysis receives 14 muscles and tendons that in other taxa would attach to the anterior connective endoskeleton in the same relative positions. This observation suggests that the embryological precursor to the connective endoskeleton can initiate and guide ectodermal invagination and thereby serve as a spatial template for the development of cuticular apodemes. This mesoderm-template model of ectodermal invagination is potentially applicable to all arthropods and may explain structural diversity and convergence in cephalic apodemes throughout the group. The model is used to interpret the cephalic endoskeletons of two non-chelicerate arthropods, Archaeognatha (Hexapoda) and Symphyla (Myriapoda), to demonstrate the generality of the model.

Thrombospondin-mediated adhesion is essential for the formation of the myotendinous junction in Drosophila

Organogenesis of the somatic musculature in Drosophila is directed by the precise adhesion between migrating myotubes and their corresponding ectodermally derived tendon cells. Whereas the PS integrins mediate the adhesion between these two cell types, their extracellular matrix (ECM) ligands have been only partially characterized. We show that the ECM protein Thrombospondin (Tsp), produced by tendon cells, is essential for the formation of the integrin-mediated myotendinous junction. Tsp expression is induced by the tendon-specific transcription factor Stripe, and accumulates at the myotendinous junction following the association between the muscle and the tendon cell. In tsp mutant embryos, migrating somatic muscles fail to attach to tendon cells and often form hemiadherens junctions with their neighboring muscle cells, resulting in nonfunctional somatic musculature. Talin accumulation at the cytoplasmic faces of the muscles and tendons is greatly reduced, implicating Tsp as a potential integrin ligand. Consistently, purified Tsp C-terminal domain polypeptide mediates spreading of PS2 integrin-expressing S2 cells in a KGD- and PS2-integrin-dependent manner. We propose a model in which the myotendinous junction is formed by the specific association of Tsp with multiple muscle-specific PS2 integrin receptors and a subsequent consolidation of the junction by enhanced tendon-specific production of Tsp secreted into the junctional space.

Muscle-dependent maturation of tendon cells is induced by post-transcriptional regulation of stripeA

Terminal differentiation of single cells selected from a group of equivalent precursors may be random, or may be regulated by external signals. In the Drosophila embryo, maturation of a single tendon cell from a field of competent precursors is triggered by muscle-dependent signaling. The transcription factor Stripe was reported to induce both the precursor cell phenotype, as well as the terminal differentiation of muscle-bound tendons. The mechanism by which Stripe activates these distinct differentiation programs remained unclear. Here, we demonstrate that each differentiation state is associated with a distinct Stripe isoform and that the Stripe isoforms direct different transcriptional outputs. Importantly, the transition to the mature differentiation state is triggered post-transcriptionally by enhanced production of the stripeA splice variant, which is typical of the tendon mature state. This elevation is mediated by the RNA-binding protein How(S), with levels sensitive to muscle-dependent signals. In how mutant embryos the expression of StripeA is significantly reduced, while overexpression of How(S) enhances StripeA protein as well as mRNA levels in embryos. Analysis of the expression of a stripeA minigene in S-2 cells suggests that this elevation may be due to enhanced splicing of stripeA. Consistently, stripeA mRNA is specifically reduced in embryos mutant for the splicing factor Crn, which physically interacts with How(S). Thus, we demonstrate a mechanism by which tendon cell terminal differentiation is maintained and reinforced by the approaching muscle.

Gain-of-function screen for genes that affect Drosophila muscle pattern formation

This article reports the production of an EP-element insertion library with more than 3,700 unique target sites within the Drosophila melanogaster genome and its use to systematically identify genes that affect embryonic muscle pattern formation. We designed a UAS/GAL4 system to drive GAL4-responsive expression of the EP-targeted genes in developing apodeme cells to which migrating myotubes finally attach and in an intrasegmental pattern of cells that serve myotubes as a migration substrate on their way towards the apodemes. The results suggest that misexpression of more than 1.5% of the Drosophila genes can interfere with proper myotube guidance and/or muscle attachment. In addition to factors already known to participate in these processes, we identified a number of enzymes that participate in the synthesis or modification of protein carbohydrate side chains and in Ubiquitin modifications and/or the Ubiquitin-dependent degradation of proteins, suggesting that these processes are relevant for muscle pattern formation.

Muscle pattern formation during embryogenesis requires the activity of a distinct network of genes. In the model organism Drosophila, this process involves the determination of stem-cell-like muscle founder cells, their differentiation, and their attraction to tendon-like epidermal cells, termed apodemes, to which the muscles attach. In order to systematically identify genes involved in these processes, a collection of fruit fly strains was generated that can be used for the ectopic expression of more than 3,700 individual fruit fly genes in a spatiotemporally restricted manner. In order to address muscle pattern formation, the collection was used to express the genes in the developing apodemes and in a series of distinct epidermal cells that serve as migration substrate for developing muscles towards the apodemes. In addition to already known factors, some 60 novel gene activities were found to interfere under these circumstances with the formation of the muscle pattern. In addition to providing a most valuable tool for the Drosophila community of researchers, the results provide a framework for a detailed analysis of the gene network and insight into molecular mechanisms underlying embryonic muscle pattern formation.

Drosophila starvin encodes a tissue-specific BAG-domain protein required for larval food uptake

We describe a developmental, genetic, and molecular analysis of the sole Drosophila member of the BAG family of genes, which is implicated in stress response and survival in mammalian cells. We show that the gene, termed starvin (stv), is expressed in a highly tissue-specific manner, accumulating primarily in tendon cells following germ-band retraction and later in somatic muscles and the esophagus during embryonic stage 15. We show that stv expression falls within known tendon and muscle cell transcriptional regulatory cascades, being downstream of stripe, but not of another tendon transcriptional regulator, delilah, and downstream of the muscle regulator, mef-2. We generated a series of stv alleles and, surprisingly, given the muscle and tendon-specific embryonic expression of stv, found that the gross morphology and function of somatic muscles is normal in stv mutants. Nonetheless, stv mutant larvae exhibit a striking and fully penetrant mutant phenotype of failure to grow after hatching and a severely impaired ability to take up food. Our study provides the first report of an essential, developmentally regulated BAG-family gene.

Ab initio prediction of transcription factor targets using structural knowledge

Current approaches for identification and detection of transcription factor binding sites rely on an extensive set of known target genes. Here we describe a novel structure-based approach applicable to transcription factors with no prior binding data. Our approach combines sequence data and structural information to infer context-specific amino acid–nucleotide recognition preferences. These are used to predict binding sites for novel transcription factors from the same structural family. We demonstrate our approach on the Cys₂His₂ Zinc Finger protein family, and show that the learned DNA-recognition preferences are compatible with experimental results. We use these preferences to perform a genome-wide scan for direct targets of Drosophila melanogaster Cys₂His₂ transcription factors. By analyzing the predicted targets along with gene annotation and expression data we infer the function and activity of these proteins.

Cells respond to dynamic changes in their environment by invoking various cellular processes, coordinated by a complex regulatory program. A main component of this program is the regulation of transcription, which is mainly accomplished by transcription factors that bind the DNA in the vicinity of genes. To better understand transcriptional regulation, advanced computational approaches are needed for linking between transcription factors and their targets. The authors describe a novel approach by which the binding site of a given transcription factor can be characterized without previous experimental binding data. This approach involves learning a set of context-specific amino acid–nucleotide recognition preferences that, when combined with the sequence and structure of the protein, can predict its specific binding preferences. Applying this approach to the Cys₂His₂ Zinc Finger protein family demonstrated its genome-wide potential by automatically predicting the direct targets of 29 regulators in the genome of the fruit fly Drosophila melanogaster. At present, with the availability of many genome sequences, there are numerous proteins annotated as transcription factors based on their sequence alone. This approach offers a promising direction for revealing the targets of these factors and for understanding their roles in the cellular network.

Coordinated development of muscles and tendons of the Drosophila leg

Since Miller's morphological description, the Drosophila leg musculature and its formation has not been revisited. Here, using a set of GFP markers and confocal microscopy, we analyse Drosophila leg muscle development, and describe all the muscles and tendons present in the adult leg. Importantly, we provide for the first time evidence for tendons located internally within leg segments. By visualising muscle and tendon precursors, we demonstrate that leg muscle development is closely associated with the formation of internal tendons. In the third instars discs, in the vicinity of tendon progenitors, some Twist-positive myoblasts start to express the muscle founder cell marker dumbfounded (duf). Slightly later, in the early pupa, epithelial tendon precursors invaginate inside the developing leg segments, giving rise to the internal string-like tendons. The tendon-associated duf-lacZ-expressing muscle founders are distributed along the invaginating tendon precursors and then fuse with surrounding myoblasts to form syncytial myotubes. At mid-pupation, these myotubes grow towards their epithelial insertion sites, apodemes, and form links between internally located tendons and the leg epithelium. This leads to a stereotyped pattern of multifibre muscles that ensures movement of the adult leg.

Terminal tendon cell differentiation requires the glide/gcm complex

Locomotion relies on stable attachment of muscle fibres to their target sites, a process that allows for muscle contraction to generate movement. Here, we show that glide/gcm and glide2/gcm2, the fly glial cell determinants, are expressed in a subpopulation of embryonic tendon cells and required for their terminal differentiation. By using loss-of-function approaches, we show that in the absence of both genes, muscle attachment to tendon cells is altered, even though the molecular cascade induced by stripe, the tendon cell determinant, is normal. Moreover, we show that glide/gcm activates a new tendon cell gene independently of stripe. Finally, we show that segment polarity genes control the epidermal expression of glide/gcm and determine, within the segment,whether it induces glial or tendon cell-specific markers. Thus, under the control of positional cues, glide/gcm triggers a new molecular pathway involved in terminal tendon cell differentiation, which allows the establishment of functional muscle attachment sites and locomotion.

Recruitment of ectodermal attachment cells via an EGFR-dependent mechanism during the organogenesis of Drosophila proprioceptors

Drosophila proprioceptors (chordotonal organs) are structured as a linear array of four lineage-related cells: a neuron, a glial cell, and two accessory cells, called cap and ligament, between which the neuron is stretched. To function properly as stretch receptors, chordotonal organs must be stably anchored at both edges. The cap cells are anchored to the cuticle through specialized lineage-related attachment cells. However, the mechanism by which the ligament cells at the other edge of the organ attach is not known. Here, we report the identification of specialized attachment cells that anchor the ligament cells of pentascolopidial chordotonal organs (lch5) to the cuticle. The ligament attachment cells are recruited by the approaching ligament cells upon reaching their attachment site, through an EGFR-dependent mechanism. Molecular characterization of lch5 attachment cells demonstrated that they share significant properties with Drosophila tendon cells and with mammalian proprioceptive organs.

Muscle building; mechanisms of myotube guidance and attachment site selection

The complex muscle patterns of higher organisms arise as migrating myoblasts are guided toward and connect with specific attachment sites. We review here the current understanding of myotube migration, focusing on its dynamic nature and the few molecular cues that have been identified to date. Much of this knowledge comes from studies in Drosophila, where powerful methods for in vivo imaging and genetic manipulation can be used to tackle this important but largely unsolved problem in developmental biology.

Mutual exclusion of sensory bristles and tendons on the notum of dipteran flies

BACKGROUND

Genes of the achaete-scute complex encode transcription factors whose activity regulates the development of neural cells. The spatially restricted expression of achaete-scute on the mesonotum of higher flies governs the development and positioning of the large sensory bristles. On the scutum the bristles are arranged into conserved patterns, based on an ancestral arrangement of four longitudinal rows. This pattern appears to date back to the origin of cyclorraphous flies about 100-140 million years ago. The origin of the four-row bauplan, which is independent of body size, and the reasons for its conservation, are not known.

RESULTS

We report that tendons for attachment of the indirect flight muscles are invariably located between the bristle rows of the scutum throughout the Diptera. Tendon development depends on the activity of a transcription factor encoded by the gene stripe. In Drosophila, stripe and achaete-scute have separate expression domains, leading to spatial segregation of tendon precursors and bristle precursors. Furthermore the products of these genes act antagonistically: ectopic sr expression prevents bristle development and ectopic sc expression prevents normal muscle attachment. The product of stripe acts downstream of Achaete-Scute and interferes with the development of bristle precursors.

CONCLUSIONS

The pattern of flight muscles has changed little throughout the Diptera and we argue that the sites of muscle attachment may have constrained the positioning of bristles during the course of evolution. This could account for the pattern of four bristle rows on the scutum.

Heparan sulfate proteoglycan syndecan promotes axonal and myotube guidance by slit/robo signaling

Slit, the ligand for the Roundabout (Robo) receptors, is secreted from midline cells of the Drosophila central nervous system (CNS). It acts as a short-range repellent that controls midline crossing of axons and allows growth cones to select specific pathways along each side of the midline. In addition, Slit directs the migration of muscle precursors and ventral branches of the tracheal system, showing that it provides long-range activity beyond the limit of the developing CNS. Biochemical studies suggest that guidance activity requires cell-surface heparan sulfate to promote binding of mammalian Slit/Robo homologs. Here, we report that the Drosophila homolog of Syndecan (reviewed in ), a heparan sulfate proteoglycan (HSPG), is required for proper Slit signaling. We generated syndecan (sdc) mutations and show that they affect all aspects of Slit activity and cause robo-like phenotypes. sdc interacts genetically with robo and slit, and double mutations cause a synergistic strengthening of the single-mutant phenotypes. The results suggest that Syndecan is a necessary component of Slit/Robo signaling and is required in the Slit target cells.

A glutamate receptor-interacting protein homolog organizes muscle guidance in Drosophila

During Drosophila embryogenesis, developing muscles extend growth-cone-like structures to navigate toward specific epidermal attachment sites. Here, we show that the homolog of Glutamate Receptor-Interacting Proteins (DGrip) acts as a key component of proper muscle guidance. Mutations in dgrip impair patterning of ventral longitudinal muscles (VLMs), whereas lateral transverse muscles (LTMs) that attach to intrasegmental attachment sites develop normally. Myoblast fusion, stabilization of muscle contacts, and general muscle function are not impaired in the absence of DGrip. Instead, the proper formation of cellular extensions during guidance fails in dgrip mutant VLMs. DGrip protein concentrates at the ends of VLMs while these muscles guide toward segment border attachment sites. Conversely, LTMs overexpressing DGrip form ectopic cellular extensions that can cause attachment of these muscles to other muscles at segment borders. Our data suggest that DGrip participates in the reception of an attractive signal that emanates from the epidermal attachment sites to direct the motility of developing muscles. This dgrip phenotype should be valuable to study mechanistic principles of Grip function.

Stripe provides cues synergizing with branchless to direct tracheal cell migration

The Drosophila tracheal system is an interconnected tubular respiratory network, which is formed by directed stereotypic migration and fusion of branches. Cell migration and specification are determined by combinatorial signaling of several morphogens secreted from the ectoderm. We report the discovery of a group of ectodermal cells, marked by Stripe (Sr) expression, that coordinates tracheal cell migration in the dorsoventral axis. Sr, an EGR family transcription factor, is known to regulate muscle migration. In this study, we show that Sr ectodermal cells also provide signals that are utilized for tracheal migration. These cues are separated in the time course of embryonic development. Initially, tendon-precursor cells are in close proximity to the tracheal cells, and later, when tracheal migration is complete, the muscles displace the trachea and attach to the tendon cells. sr-mutant embryos exhibit defects in migration of all tracheal branches. Although the FGF ligand Branchless (Bnl) is expressed in a subset of tendon-precursor cells independently of Sr, Bnl functions cooperatively with proteins induced by Sr in attraction of tracheal branches.

Direct flight muscles in Drosophila develop from cells with characteristics of founders and depend on DWnt-2 for their correct patterning

The direct flight muscles (DFMs) of Drosophila allow for the fine control of wing position necessary for flight. In DWnt-2 mutant flies, certain DFMs are either missing or fail to attach to the correct epithelial sites. Using a temperature-sensitive allele, we show that DWnt-2 activity is required only during pupation for correct DFM patterning. DWnt-2 is expressed in the epithelium of the wing hinge primordium during pupation. This expression is in the vicinity of the developing DFMs, as revealed by expression of the muscle founder cell-specific gene dumbfounded in DFM precursors. The observation that a gene necessary for embryonic founder cell function is expressed in the DFM precursors suggests that these cells may have a similar founder cell role. Although the expression pattern of DWnt-2 suggests that it could influence epithelial cells to differentiate into attachment sites for muscle, the expression of stripe, a transcription factor necessary for epithelial cells to adopt an attachment cell fate, is unaltered in the mutant. Ectopic expression of DWnt-2 in the wing hinge during pupation can also create defects in muscle patterning without alterations in stripe expression. We conclude that DWnt-2 promotes the correct patterning of DFMs through a mechanism that is independent of the attachment site differentiation initiated by stripe.

Divide and conquer: pattern formation in Drosophila embryonic epidermis

The pattern of differentiated cell types within tissues and organs is often established by organizers, the localized sources of secreted ligands. Although the mechanisms underlying organizer function have been extensively studied, only in a few cases is it clear how an organizer ultimately controls each individual cell's fate across a field of progenitor cells. One of these cases involves the establishment of a precise pattern of cell differentiation across the embryonic epidermis in Drosophila. Here, we review several recent reports that help to elucidate the regulatory principles used to control this pattern. Because organizers are conserved, the same fundamental principles might operate in other organizers.

Distinct signals generate repeating striped pattern in the embryonic parasegment

How repeating striped patterns arise across cellular fields is unclear. To address this we examined the repeating pattern of Stripe (Sr) expression across the parasegment (PS) in Drosophila. This pattern is generated in two steps. First, the ligands Hedgehog (Hh) and Wingless (Wg) subdivide the PS into smaller territories. Second, the ligands Hh, Spitz (Spi), and Wg each emanate from a specific territory and induce Sr expression in an adjacent territory. We also show that the width of Sr expression is determined by signaling strength. Finally, an enhancer trap in the sr gene detects the response to Spi and Wg, but not to Hh, implying the existence of separable control elements in the sr gene. Thus, a distinct inductive event is used to initiate each element of the repeating striped pattern.

Apterous mediates development of direct flight muscles autonomously and indirect flight muscles through epidermal cues

Two physiologically distinct types of muscles, the direct and indirect flight muscles, develop from myoblasts associated with the Drosophila wing disc. We show that the direct flight muscles are specified by the expression of Apterous, a Lim homeodomain protein, in groups of myoblasts. This suggests a mechanism of cell-fate specification by labelling groups of fusion competent myoblasts, in contrast to mechanisms in the embryo, where muscle cell fate is specified by single founder myoblasts. In addition, Apterous is expressed in the developing adult epidermal muscle attachment sites. Here, it functions to regulate the expression of stripe, a gene that is an important element of early patterning of muscle fibres, from the epidermis. Our results, which may have broad implications, suggest novel mechanisms of muscle patterning in the adult, in contrast to embryonic myogenesis.

Epidermal tendon cells require Broad Complex function for correct attachment of the indirect flight muscles in Drosophila melanogaster

Drosophila Broad Complex, a primary response gene in the ecdysone cascade, encodes a family of zinc-finger transcription factors essential for metamorphosis. Broad Complex mutations of the rbp complementation group disrupt attachment of the dorsoventral indirect flight muscles during pupal development. We previously demonstrated that isoform BRC-Z1 mediates the muscle attachment function of rbp(+) and is expressed in both developing muscle fibers and their epidermal attachment sites. We now report two complementary studies to determine the cellular site and mode of action of rbp(+) during maturation of the myotendinous junctions of dorsoventral indirect flight muscles. First, genetic mosaics, produced using the paternal loss method, revealed that the muscle attachment phenotype is determined primarily by the genotype of the dorsal epidermis, with the muscle fiber and the ventral epidermis exerting little or no influence. When the dorsal epidermis was mutant, the vast majority of muscles detached or chose ectopic attachment sites, regardless of the muscle genotype. Conversely, wild-type dorsal epidermis could support attachment of mutant muscles. Second, ultrastructural analysis corroborated and extended these results, revealing defective and delayed differentiation of rbp mutant epidermal tendon cells in the dorsal attachment sites. Tendon cell processes, the stress-bearing links between the epidermis and muscle, were reduced in number and showed delayed appearance of microtubule bundles. In contrast, mutant muscle and ventral epidermis resembled the wild type. In conclusion, BRC-Z1 acts in the dorsal epidermis to ensure differentiation of the myotendinous junction. By analogy with the cell-cell interaction essential for embryonic muscle attachment, we propose that BRC-Z1 regulates one or more components of the epidermal response to a signal from the developing muscle.

The balance between two isoforms of the Drosophila RNA-binding protein how controls tendon cell differentiation

In Drosophila, a tendon cell is selected from a group of equipotent precursors following its interaction with a muscle cell. This interaction results in elevated levels of the transcription factor Stripe in the future tendon cells. Here we show that the balance between two distinct forms of the RNA-binding protein How maintains low levels of Stripe at the precursor stage and high levels in the mature tendon. The long, nuclear-specific protein How(L) downregulates Stripe protein levels at the precursor stage by binding stripe mRNA and inhibiting its nuclear export. This inhibition is likely to be counteracted by the short How(S) protein, present in both nucleus and cytoplasm, which is upregulated in the muscle-bound tendon cell following EGF receptor activation.

Kakapo, a novel cytoskeletal-associated protein is essential for the restricted localization of the neuregulin-like factor, vein, at the muscle-tendon junction site

In the Drosophila embryo, the correct association of muscles with their specific tendon cells is achieved through reciprocal interactions between these two distinct cell types. Tendon cell differentiation is initiated by activation of the EGF-receptor signaling pathway within these cells by Vein, a neuregulin-like factor secreted by the approaching myotube. Here, we describe the cloning and the molecular and genetic analyses of kakapo, a Drosophila gene, expressed in the tendons, that is essential for muscle-dependent tendon cell differentiation. Kakapo is a large intracellular protein and contains structural domains also found in cytoskeletal-related vertebrate proteins (including plakin, dystrophin, and Gas2 family members). kakapo mutant embryos exhibit abnormal muscle-dependent tendon cell differentiation. A major defect in the kakapo mutant tendon cells is the failure of Vein to be localized at the muscle-tendon junctional site; instead, Vein is dispersed and its levels are reduced. This may lead to aberrant differentiation of tendon cells and consequently to the kakapo mutant deranged somatic muscle phenotype.