The Drosophila neuregulin homolog Vein mediates inductive interactions between myotubes and their epidermal attachment cells

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.

Activation of EGFR signaling by Tc-Vein and Tc-Spitz regulates the metamorphic transition in the red flour beetle Tribolium castaneum

Animal development relies on a sequence of specific stages that allow the formation of adult structures with a determined size. In general, juvenile stages are dedicated mainly to growth, whereas last stages are devoted predominantly to the maturation of adult structures. In holometabolous insects, metamorphosis marks the end of the growth period as the animals stops feeding and initiate the final differentiation of the tissues. This transition is controlled by the steroid hormone ecdysone produced in the prothoracic gland. In Drosophila melanogaster different signals have been shown to regulate the production of ecdysone, such as PTTH/Torso, TGFß and Egfr signaling. However, to which extent the roles of these signals are conserved remains unknown. Here, we study the role of Egfr signaling in post-embryonic development of the basal holometabolous beetle Tribolium castaneum. We show that Tc-Egfr and Tc-pointed are required to induced a proper larval-pupal transition through the control of the expression of ecdysone biosynthetic genes. Furthermore, we identified an additional Tc-Egfr ligand in the Tribolium genome, the neuregulin-like protein Tc-Vein (Tc-Vn), which contributes to induce larval-pupal transition together with Tc-Spitz (Tc-Spi). Interestingly, we found that in addition to the redundant role in the control of pupa formation, each ligand possesses different functions in organ morphogenesis. Whereas Tc-Spi acts as the main ligand in urogomphi and gin traps, Tc-Vn is required in wings and elytra. Altogether, our findings show that in Tribolium, post-embryonic Tc-Egfr signaling activation depends on the presence of two ligands and that its role in metamorphic transition is conserved in holometabolous insects.

Muscle-tendon Crosstalk During Muscle Wasting

In organisms from flies to mammals, the initial formation of a functional tendon is completely dependent on chemical signals from muscle (myokines). However, how myokines affect the maturation, maintenance, and regeneration of tendons as a function of age is completely unstudied. Here we discuss the role of four myokines - fibroblast growth factors (FGF), myostatin, the secreted protein acidic and rich in cysteine (SPARC), and miR-29 - in tendon development and hypothesize a role for these factors in the progressive changes in tendon structure and function as a result of muscle wasting (disuse, aging and disease). Because of the close relationship between mechanical loading and muscle and tendon regulation, disentangling muscle-tendon crosstalk from simple mechanical loading is experimentally quite difficult. Therefore, we propose an experimental framework that hopefully will be useful in demonstrating muscle-tendon crosstalk in vivo. Though understudied, the promise of a better understanding of muscle-tendon crosstalk is the development of new interventions that will improve tendon development, regeneration, and function throughout the lifespan.

Extracellular matrix at the muscle - tendon interface: functional roles, techniques to explore and implications for regenerative medicine

The muscle-tendon interface is an anatomically specialized region that is involved in the efficient transmission of force from muscle to tendon. Due to constant exposure to loading, the interface is susceptible to injury. Current treatment methods do not meet the socioeconomic demands of reduced recovery time without compromising the risk of reinjury, requiring the need for developing alternative strategies. The extracellular matrix (ECM) present in muscle, tendon, and at the interface of these tissues consists of unique molecules that play significant roles in homeostasis and repair. Better, understanding the function of the ECM during development, injury, and aging has the potential to unearth critical missing information that is essential for accelerating the repair at the muscle-tendon interface. Recently, advanced techniques have emerged to explore the ECM for identifying specific roles in musculoskeletal biology. Simultaneously, there is a tremendous increase in the scope for regenerative medicine strategies to address the current clinical deficiencies. Advancements in ECM research can be coupled with the latest regenerative medicine techniques to develop next generation therapies that harness ECM for treating defects at the muscle-tendon interface. The current work provides a comprehensive review on the role of muscle and tendon ECM to provide insights about the role of ECM in the muscle-tendon interface and discusses the latest research techniques to explore the ECM to gathered information for developing regenerative medicine strategies.

Abelson tyrosine-protein kinase 2 regulates myoblast proliferation and controls muscle fiber length

Muscle fiber length is nearly uniform within a muscle but widely different among different muscles. We show that Abelson tyrosine-protein kinase 2 (Abl2) has a key role in regulating myofiber length, as a loss of Abl2 leads to excessively long myofibers in the diaphragm, intercostal and levator auris muscles but not limb muscles. Increased myofiber length is caused by enhanced myoblast proliferation, expanding the pool of myoblasts and leading to increased myoblast fusion. Abl2 acts in myoblasts, but as a consequence of expansion of the diaphragm muscle, the diaphragm central tendon is reduced in size, likely contributing to reduced stamina of Abl2 mutant mice. Ectopic muscle islands, each composed of myofibers of uniform length and orientation, form within the central tendon of Abl2^+/− mice. Specialized tendon cells, resembling tendon cells at myotendinous junctions, form at the ends of these muscle islands, suggesting that myofibers induce differentiation of tendon cells, which reciprocally regulate myofiber length and orientation.

Mechanical Control of Myotendinous Junction Formation and Tendon Differentiation during Development

The development of the musculoskeletal system is a great model to study the interplay between chemical and mechanical inter-tissue signaling in cell adhesion, tissue morphogenesis and differentiation. In both vertebrates and invertebrates (e.g., Drosophila melanogaster) the formation of muscle-tendon interaction generates mechanical forces which are required for myotendinous junction maturation and tissue differentiation. In addition, these forces must be withstood by muscles and tendons in order to prevent detachment from each other, deformation or even losing their integrity. Extracellular matrix remodeling at the myotendinous junction is key to resist mechanical load generated by muscle contraction. Recent evidences in vertebrates indicate that mechanical forces generated during junction formation regulate chemical signaling leading to extracellular matrix remodeling, however, the mechanotransduction mechanisms associated to this response remains elusive. In addition to extracellular matrix remodeling, the ability of Drosophila tendon-cells to bear mechanical load depends on rearrangement of tendon cell cytoskeleton, thus studying the molecular mechanisms involved in this process is critical to understand the contribution of mechanical forces to the development of the musculoskeletal system. Here, we review recent findings regarding the role of chemical and mechanical signaling in myotendinous junction formation and tendon differentiation, and discuss molecular mechanisms of mechanotransduction that may allow tendon cells to withstand mechanical load during development of the musculoskeletal system.

Drosophila Sulf1 is required for the termination of intestinal stem cell division during regeneration

Stem cell division is activated to trigger regeneration in response to tissue damage. The molecular mechanisms by which this stem cell mitotic activity is properly repressed at the end of regeneration are poorly understood. Here, we show that a specific modification of heparan sulfate is crucial for regulating Drosophila intestinal stem cell (ISC) division during normal midgut homeostasis and regeneration. Loss of the extracellular heparan sulfate endosulfatase Sulf1 resulted in increased ISC division during normal homeostasis, which was caused by upregulation of mitogenic signaling including the JAK-STAT, EGFR and Hedgehog pathways. Using a regeneration model, we found that ISCs failed to properly halt division at the termination stage in Sulf1 mutants, showing that Sulf1 is required for terminating ISC division at the end of regeneration. We propose that post-transcriptional regulation of mitogen signaling by heparan sulfate structural modifications provides a new regulatory step for precise temporal control of stem cell activity during regeneration.

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.

Neurological dysfunctions associated with altered BACE1-dependent Neuregulin-1 signaling

Inhibition of BACE1 is being pursued as a therapeutic target to treat patients suffering from Alzheimer's disease because BACE1 is the sole β-secretase that generates β-amyloid peptide. Knowledge regarding other cellular functions of BACE1 is therefore critical for the safe use of BACE1 inhibitors in human patients. Neuregulin-1 (Nrg1) is a BACE1 substrate and BACE1 cleavage of Nrg1 is critical for signaling functions in myelination, remyelination, synaptic plasticity, normal psychiatric behaviors, and maintenance of muscle spindles. This review summarizes the most recent discoveries associated with BACE1-dependent Nrg1 signaling in these areas. This body of knowledge will help to provide guidance for preventing unwanted Nrg1-based side effects following BACE1 inhibition in humans. To initiate its signaling cascade, membrane anchored Neuregulin (Nrg), mainly type I and III β1 Nrg1 isoforms and Nrg3, requires ectodomain shedding. BACE1 is one of such indispensable sheddases to release the functional Nrg signaling fragment. The dependence of Nrg on the cleavage by BACE1 is best manifested by disrupting the critical role of Nrg in the control of axonal myelination, schizophrenic behaviors as well as the formation and maintenance of muscle spindles.

Functional Conservation of the Glide/Gcm Regulatory Network Controlling Glia, Hemocyte, and Tendon Cell Differentiation in Drosophila

High-throughput screens allow us to understand how transcription factors trigger developmental processes, including cell specification. A major challenge is identification of their binding sites because feedback loops and homeostatic interactions may mask the direct impact of those factors in transcriptome analyses. Moreover, this approach dissects the downstream signaling cascades and facilitates identification of conserved transcriptional programs. Here we show the results and the validation of a DNA adenine methyltransferase identification (DamID) genome-wide screen that identifies the direct targets of Glide/Gcm, a potent transcription factor that controls glia, hemocyte, and tendon cell differentiation in Drosophila. The screen identifies many genes that had not been previously associated with Glide/Gcm and highlights three major signaling pathways interacting with Glide/Gcm: Notch, Hedgehog, and JAK/STAT, which all involve feedback loops. Furthermore, the screen identifies effector molecules that are necessary for cell-cell interactions during late developmental processes and/or in ontogeny. Typically, immunoglobulin (Ig) domain-containing proteins control cell adhesion and axonal navigation. This shows that early and transiently expressed fate determinants not only control other transcription factors that, in turn, implement a specific developmental program but also directly affect late developmental events and cell function. Finally, while the mammalian genome contains two orthologous Gcm genes, their function has been demonstrated in vertebrate-specific tissues, placenta, and parathyroid glands, begging questions on the evolutionary conservation of the Gcm cascade in higher organisms. Here we provide the first evidence for the conservation of Gcm direct targets in humans. In sum, this work uncovers novel aspects of cell specification and sets the basis for further understanding of the role of conserved Gcm gene regulatory cascades.

A Tendon Cell Specific RNAi Screen Reveals Novel Candidates Essential for Muscle Tendon Interaction

Tendons are fibrous connective tissue which connect muscles to the skeletal elements thus acting as passive transmitters of force during locomotion and provide appropriate body posture. Tendon-derived cues, albeit poorly understood, are necessary for proper muscle guidance and attachment during development. In the present study, we used dorsal longitudinal muscles of Drosophila and their tendon attachment sites to unravel the molecular nature of interactions between muscles and tendons. We performed a genetic screen using RNAi-mediated knockdown in tendon cells to find out molecular players involved in the formation and maintenance of myotendinous junction and found 21 candidates out of 2507 RNAi lines screened. Of these, 19 were novel molecules in context of myotendinous system. Integrin-βPS and Talin, picked as candidates in this screen, are known to play important role in the cell-cell interaction and myotendinous junction formation validating our screen. We have found candidates with enzymatic function, transcription activity, cell adhesion, protein folding and intracellular transport function. Tango1, an ER exit protein involved in collagen secretion was identified as a candidate molecule involved in the formation of myotendinous junction. Tango1 knockdown was found to affect development of muscle attachment sites and formation of myotendinous junction. Tango1 was also found to be involved in secretion of Viking (Collagen type IV) and BM-40 from hemocytes and fat cells.

Tendon development and diseases

Tendon is a uniaxial connective tissue component of the musculoskeletal system. Tendon is involved in force transmission between muscle and bone. Tendon injury is very common and debilitating but tendon repair remains a clinical challenge for orthopedic medicine. In vertebrates, tendon is mainly composed of type I collagen fibrils, displaying a parallel organization along the tendon axis. The tendon-specific spatial organization of type I collagen provides the mechanical properties for tendon function. In contrast to other components of the musculoskeletal system, tendon biology is poorly understood. An important goal in tendon biology is to understand the mechanisms involved in the production and assembly of type I collagen fibrils during development, postnatal formation, and healing processes in order to design new therapies for tendon repair. In this review we highlight the current understanding of the molecular and mechanical signals known to be involved in tenogenesis during development, and how development provides insights into tendon healing processes. WIREs Dev Biol 2016, 5:5-23. doi: 10.1002/wdev.201 For further resources related to this article, please visit the WIREs website.

Morphogenesis of the somatic musculature in Drosophila melanogaster

In Drosophila melanogaster, the somatic muscle system is first formed during embryogenesis, giving rise to the larval musculature. Later during metamorphosis, this system is destroyed and replaced by an entirely new set of muscles in the adult fly. Proper formation of the larval and adult muscles is critical for basic survival functions such as hatching and crawling (in the larva), walking and flying (in the adult), and feeding (at both larval and adult stages). Myogenesis, from mononucleated muscle precursor cells to multinucleated functional muscles, is driven by a number of cellular processes that have begun to be mechanistically defined. Once the mesodermal cells destined for the myogenic lineage have been specified, individual myoblasts fuse together iteratively to form syncytial myofibers. Combining cytoplasmic contents demands a level of intracellular reorganization that, most notably, leads to redistribution of the myonuclei to maximize internuclear distance. Signaling from extending myofibers induces terminal tendon cell differentiation in the ectoderm, which results in secure muscle-tendon attachments that are critical for muscle contraction. Simultaneously, muscles become innervated and undergo sarcomerogenesis to establish the contractile apparatus that will facilitate movement. The cellular mechanisms governing these morphogenetic events share numerous parallels to mammalian development, and the basic unit of all muscle, the myofiber, is conserved from flies to mammals. Thus, studies of Drosophila myogenesis and comparisons to muscle development in other systems highlight conserved regulatory programs of biomedical relevance to general muscle biology and studies of muscle disease.

Axonal wrapping in the Drosophila PNS is controlled by glia-derived neuregulin homolog Vein

Efficient neuronal conductance requires that axons are insulated by glial cells. For this, glial membranes need to wrap around axons. Invertebrates show a relatively simple extension of glial membranes around the axons, resembling Remak fibers formed by Schwann cells in the mammalian peripheral nervous system. To unravel the molecular pathways underlying differentiation of glial cells that provide axonal wrapping, we are using the genetically amenable Drosophila model. At the end of larval life, the wrapping glia differentiates into very large cells, spanning more than 1 mm of axonal length. The extension around axonal membranes is not influenced by the caliber of the axon or its modality. Using cell type-specific gene knockdown we show that the extension of glial membranes around the axons is regulated by an autocrine activation of the EGF receptor through the neuregulin homolog Vein. This resembles the molecular mechanism employed during cell-autonomous reactivation of glial differentiation after injury in mammals. We further demonstrate that Vein, produced by the wrapping glia, also regulates the formation of septate junctions in the abutting subperineurial glia. Moreover, the wrapping glia indirectly controls the proliferation of the perineurial glia. Thus, the wrapping glia appears center stage to orchestrate the development of the different glial cell layers in a peripheral nerve.

βν integrin inhibits chronic and high level activation of JNK to repress senescence phenotypes in Drosophila adult midgut

Proper control of adult stem cells including their proliferation and differentiation is crucial in maintaining homeostasis of well-organized tissues/organs throughout an organism's life. The Drosophila adult midgut has intestinal stem cells (ISCs), which have been exploited as a simple model system to investigate mechanisms controlling adult tissue homeostasis. Here, we found that a viable mutant of βν integrin (βint-ν), encoding one of two Drosophila integrin β subunits, showed a short midgut and abnormal multilayered epithelia accompanied by an increase in ISC proliferation and misdifferentiation defects. The increase in ISC proliferation and misdifferentiation was due to frequent ISC duplication expanding a pool of ISCs, which was caused by depression of the Notch signalling, and up-regulation of unpaired (upd), a gene encoding an extracellular ligand in the JAK/STAT signalling pathway. In addition, we observed that abnormally high accumulation of filamentous actin (F-actin) was caused in the βint-ν mutant enterocytes. Furthermore, the defects were rescued by suppressing c-Jun N-terminal kinase (JNK) signalling, which was up-regulated in a manner correlated with the defect levels in the above-mentioned βint-ν mutant phenotype. These symptoms observed in young βint-ν mutant midgut were very similar to those in the aged midgut in wild type. Our results suggested that βint-ν has a novel function for the Drosophila adult midgut homeostasis under normal conditions and provided a new insight into possible age-related diseases caused by latent abnormality of an integrin function.

Drosophila importin-7 functions upstream of the Elmo signaling module to mediate the formation and stability of muscle attachments

Establishment and maintenance of stable muscle attachments is essential for coordinated body movement. Studies in Drosophila have pioneered a molecular understanding of the morphological events in the conserved process of muscle attachment formation, including myofiber migration, muscle-tendon signaling, and stable junctional adhesion between muscle cells and their corresponding target insertion sites. In both Drosophila and vertebrate models, integrin complexes play a key role in the biogenesis and stability of muscle attachments through the interactions of integrins with extracellular matrix (ECM) ligands. We show that Drosophila importin-7 (Dim7) is an upstream regulator of the conserved Elmo-Mbc→Rac signaling pathway in the formation of embryonic muscle attachment sites (MASs). Dim7 is encoded by the moleskin (msk) locus and was identified as an Elmo-interacting protein. Both Dim7 and Elmo localize to the ends of myofibers coincident with the timing of muscle-tendon attachment in late myogenesis. Phenotypic analysis of elmo mutants reveal muscle attachment defects similar to those previously described for integrin mutants. Furthermore, Elmo and Dim7 interact both biochemically and genetically in the developing musculature. The muscle detachment phenotype resulting from mutations in the msk locus can be rescued by components in the Elmo signaling pathway, including the Elmo-Mbc complex, an activated Elmo variant, or a constitutively active form of Rac. In larval muscles, the localization of Dim7 and activated Elmo to the sites of muscle attachment is attenuated upon RNAi knockdown of integrin heterodimer complex components. Our results show that integrins function as upstream signals to mediate Dim7-Elmo enrichment to the MASs.

"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.

The development of the myotendinous junction. A review

The myotendinous junction (MTJ) is a complex specialized region located at the muscle-tendon interface that represents the primary site of force transmission. Despite their different embryologic origins, muscle and tendon morphogenesis occurs in close spatial and temporal association. After muscle attachment, muscle and tendon constitute a dynamic and functional integrated unit that transduces muscle contraction force to the skeletal system. We review here the current understanding of MTJ formation describing changes during morphogenesis and focusing on the crosstalk between muscle and tendon cells that leads to the development of a functional MTJ. Molecules involved in the formation of the linkage, both at the tendon side and at the muscle side of the junction are described. Much of this knowledge comes from studies using different animal models such as mice, zebrafish and Drosophila where powerful methods for in vivo imaging and genetic manipulations can be used to enlighten this developmental process.

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.

Diversification of muscle types in Drosophila: upstream and downstream of identity genes

Understanding gene regulatory pathways underlying diversification of cell types during development is one of the major challenges in developmental biology. Progressive specification of mesodermal lineages that are at the origin of body wall muscles in Drosophila embryos has been extensively studied during past years, providing an attractive framework for dissecting cell type diversification processes. In particular, it has been found that muscle founder cells that are at the origin of individual muscles display specific expression of transcription factors that control diversification of muscle types. These factors, encoded by genes collectively called muscle identity genes, are activated in discrete subsets of muscle founders. As a result, each founder cell is thought to carry a unique combinatorial code of identity gene expression. Considering this, to define temporally and spatially restricted expression of identity genes, a set of coordinated upstream regulatory inputs is required. But also, to realize the identity program and to form specific muscle types with distinct properties, an efficient battery of downstream identity gene targets needs to be activated. Here we review how the specificity of expression and action of muscle identity genes is acquired.

Combinatorial coding of Drosophila muscle shape by Collier and Nautilus

The diversity of Drosophila muscles correlates with the expression of combinations of identity transcription factors (iTFs) in muscle progenitors. Here, we address the question of when and how a combinatorial code is translated into muscle specific properties, by studying the roles of the Collier and Nautilus iTFs that are expressed in partly overlapping subsets of muscle progenitors. We show that the three dorso-lateral (DL) progenitors which express Nautilus and Collier are specified in a fixed temporal sequence and that each expresses additionally other, distinct iTFs. Removal of Collier leads to changes in expression of some of these iTFs and mis-orientation of several DL muscles, including the dorsal acute DA3 muscle which adopts a DA2 morphology. Detailed analysis of this transformation revealed the existence of two steps in the attachment of elongating muscles to specific tendon cells: transient attachment to alternate tendon cells, followed by a resolution step selecting the final sites. The multiple cases of triangular-shaped muscles observed in col mutant embryos indicate that transient binding of elongating muscle to exploratory sites could be a general feature of the developing musculature. In nau mutants, the DA3 muscle randomly adopts the attachment sites of the DA3 or DO5 muscles that derive from the same progenitor, resulting in a DA3, DO5-like or bifid DA3-DO5 orientation. In addition, nau mutant embryos display thinner muscle fibres. Together, our data show that the sequence of expression and combinatorial activities of Col and Nau control the pattern and morphology of DL muscles.

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.

Born to run: creating the muscle fiber

From the muscles that control the blink of your eye to those that allow you to walk, the basic architecture of muscle is the same: muscles consist of bundles of the unit muscle cell, the muscle fiber. The unique morphology of the individual muscle fiber is dictated by the functional demands necessary to generate and withstand the forces of contraction, which in turn leads to movement. Contractile muscle fibers are elongated, syncytial cells, which interact with both the nervous and skeletal systems to govern body motion. In this review, we focus on three key cell-cell and cell-matrix contact processes, that are necessary to create this exquisitely specialized cell: cell fusion, cell elongation, and establishment of a myotendinous junction. We address these processes by highlighting recent findings from the Drosophila model system.

Spatial regulation of cell adhesion in the Drosophila wing is mediated by Delilah, a potent activator of βPS integrin expression

In spite of our conceptual view of how differential gene expression is used to define different cell identities, we still do not understand how different cell identities are translated into actual cell properties. The example discussed here is that of the fly wing, which is composed of two main cell types: vein and intervein cells. These two cell types differ in many features, including their adhesive properties. One of the major differences is that intervein cells express integrins, which are required for the attachment of the two wing layers to each other, whereas vein cells are devoid of integrin expression. The major signaling pathways that divide the wing to vein and intervein domains have been characterized. However, the genetic programs that execute these two alternative differentiation programs are still very roughly drawn. Here we identify the bHLH protein Delilah (Dei) as a mediator between signaling pathways that specify intervein cell-fate and one of the most significant realizators of this fate, βPS integrin. Dei's expression is restricted to intervein territories where it acts as a potent activator of βPS integrin expression. In the absence of normal Dei activity the level of βPS integrin is reduced, leading to a failure of adhesion between the dorsal and ventral wing layers and a consequent formation of wing blisters. The effect of Dei on βPS expression is not restricted to the wing, suggesting that Dei functions as a general genetic switch, which is turned on wherever a sticky cell-identity is determined and integrin-based adhesion is required.

Connecting muscles to tendons: tendons and musculoskeletal development in flies and vertebrates

The formation of the musculoskeletal system represents an intricate process of tissue assembly involving heterotypic inductive interactions between tendons, muscles and cartilage. An essential component of all musculoskeletal systems is the anchoring of the force-generating muscles to the solid support of the organism: the skeleton in vertebrates and the exoskeleton in invertebrates. Here, we discuss recent findings that illuminate musculoskeletal assembly in the vertebrate embryo, findings that emphasize the reciprocal interactions between the forming tendons, muscle and cartilage tissues. We also compare these events with those of the corresponding system in the Drosophila embryo, highlighting distinct and common pathways that promote efficient locomotion while preserving the form of the organism.

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.

Non-cell-autonomous control of denticle diversity in the Drosophila embryo

Certain Drosophila embryonic epidermal cells construct actin-based protrusions, called denticles, which exhibit stereotyped, column-specific differences in size, density and hook orientation. This precise denticle pattern is conserved throughout all drosophilids yet studied, and screening for mutations that affect this pattern has been used to identify genes involved in development and signaling. However, how column-specific differences are specified and the mechanism(s) involved have remained elusive. Here, we show that the transcription factor Stripe is required for multiple aspects of this column-specific denticle pattern, including denticle hook orientation. The induction of stripe expression in certain denticle field cells appears to be the primary mechanism by which developmental pathways assign denticle hook orientation. Furthermore, we show that the cytoskeletal linker protein Short stop (Shot) functions both cell-autonomously and non-autonomously to specify denticle hook orientation via interaction with the microtubule cytoskeleton. We propose that stripe mediates its effect on hook orientation, in part, via upregulation of shot.

Combinatorial patterning mechanisms in the Drosophila embryo

The classical concept of the morphogen gradient proposes that small differences in the levels of a signalling molecule or transcription factor are responsible for producing a continuous spectrum of distinctive cellular identities across a naïve field of cells. In this review, we discuss how the Dorsal gradient controls the dorsal-ventral patterning of the early Drosophila embryo. This gradient extends from the ventral midline of the embryo into dorso-lateral regions, encompassing a cross-sectional field of approximately 20 cells. There is no evidence that these cells acquire distinctive identities due to subtle changes in the nuclear concentrations of the Dorsal protein. Rather, a variety of evidence suggests that the Dorsal gradient generates just three primary thresholds of gene activity. High levels activate gene expression in the presumptive mesoderm, while intermediate and low levels activate gene expression in the ventral and dorsal neurogenic ectoderm, respectively. We discuss how these primary readouts of the gradient establish localized domains of cell signalling, which work in a combinatorial manner with transcriptional networks to produce complex patterns of gene expression and tissue differentiation.

How the Dorsal gradient works: insights from postgenome technologies

Gradients of extracellular signaling molecules and transcription factors are used in a variety of developmental processes, including the patterning of the Drosophila embryo, the establishment of diverse neuronal cell types in the vertebrate neural tube, and the anterior-posterior patterning of vertebrate limbs. Here, we discuss how a gradient of the maternal transcription factor Dorsal produces complex patterns of gene expression across the dorsal-ventral (DV) axis of the early Drosophila embryo. The identification of 60-70 Dorsal target genes, along with the characterization of approximately 35 associated regulatory DNAs, suggests that there are at least six different regulatory codes driving diverse DV expression profiles.

Genetic control of muscle development: learning from Drosophila

Muscle development involves a complex sequence of time and spatially regulated cellular events leading to the formation of highly specialised syncytial muscle cells displaying a common feature, the capacity of contraction. Analyses of mechanisms controlling muscle development reveals that the main steps of muscle formation including myogenic determination, diversification of muscle precursors, myoblast fusion and terminal differentiation involve the actions of evolutionarily conserved genes. Thus dissecting the genetic control of muscle development in simple model organisms appears to be an attractive way to get insights into core genetic cascade that orchestrate myogenesis. In this respect, particularly insightful have been data generated using Drosophila as a model system. Notably, the interplay between intrinsic and extrinsic cues that determine the early myogenic decisions leading to the specification of muscle progenitors and those controlling myoblasts fusion are much better characterised in Drosophila than in vertebrate species. Also, adult Drosophila myogenesis, which leads to the formation of vertebrate-like multi-fibre muscles, emerges as a particularly well-adapted system to study normal and aberrant muscle development.

The EGFR ligands Spitz and Keren act cooperatively in the Drosophila eye

The EGFR signalling cascade is responsible for coordinating a wide variety of events during Drosophila eye development. It remains something of a mystery how it is that cells are able to interpret the signal so as to choose the appropriate response from the battery of possibilities: division, differentiation, cell shape change and so on. Since the cascade is essentially linear below the receptor, different cellular responses cannot be regulated by alternative signal transduction pathways. The main diversity lies upstream, in the multiple activating ligands. Spitz, Gurken and Vein have been long studied, but little is known about the physiological functions of the fourth ligand, Keren, although various roles have been predicted based on the differences between mutants in the known ligands and those of the receptor. Here, we have isolated a mutant in the keren gene, and demonstrate that Keren does indeed participate in EGFR signalling in the eye, where it acts redundantly with Spitz to control R8 spacing, cell clustering and survival. Thus, specificity cannot be determined by ligand choice, and must instead be a consequence of cell-intrinsic factors, although we speculate that there may be some quantitative differences in signalling elicited by the two ligands.

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.

Drosophila Rolling pebbles colocalises and putatively interacts with alpha-Actinin and the Sls isoform Zormin in the Z-discs of the sarcomere and with Dumbfounded/Kirre, alpha-Actinin and Zormin in the terminal Z-discs

The rolling pebbles gene of Drosophila encodes two proteins, one of which, Rols7, is essential for myoblast fusion. In addition, Rols 7 is expressed during myofibrillogenesis and in the mature muscles. Here it overlaps with alpha-Actinin (alpha-Actn) and the N-terminus of D-Titin/Kettin/Zormin in the Z-line of the sarcomeres. In the attachment sites of the somatic muscles, Rols7 and the immunoglobulin superfamily protein Dumbfounded/Kin of irreC (Duf/Kirre) colocalise. As Duf/Kirre is detectable only transiently, it may be involved in establishing the first contact of the outgrowing muscle fiber to the epidermal attachment site. We propose that Rols7 and Duf/Kirre link the terminal Z-disc to the cell membrane by direct interaction. This is supported by the fact that in yeast two hybrid assays the tetratricopeptide repeat E (TPR E) of Rols7 shows interaction with the intracellular domain of Duf/Kirre. The colocalisation of Rols7 with alpha-Actn and with D-Titin/Kettin/Zormin in the Z-dics is reflected in interactions with different domains of Rols7 in this assay. In summary, these data show that besides the role in myoblast fusion, Rols7 is a scaffold protein during myofibrillogenesis and in the Z-line of the sarcomere as well as in the terminal Z-disc linking the muscle to the epidermal attachment sites.

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.

Regulation of the Drosophila epidermal growth factor-ligand vein is mediated by multiple domains

Vein (Vn), a ligand for the Drosophila epidermal growth factor receptor (Egfr), has a complex structure including a PEST, Ig, and EGF domain. We analyzed the structure-function relationships of Vn by assaying deletion mutants. The results show that each conserved domain influences Vn activity. A PEST deletion increases Vn potency and genetic evidence suggests that Vn is regulated by proteasomal degradation. The Ig deletion causes toxic effects not seen following expression of native Vn, but the Ig domain is not required for Vn localization or for the activation of Egfr signaling in wing vein patterning. Remarkably, when the EGF domain is deleted, Vn functions as a dominant negative ligand, implying that Vn normally physically interacts with another factor to promote its activity. We identified additional highly conserved sequences and found several regions that affect Vn potency and one that may mediate the effect of dominant negative Vn molecules. Together the results show that the activity of Vn is controlled both positively and negatively, demonstrating the existence of additional levels at which Egfr signaling can be regulated.

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.