Drosophila myogenesis and insights into the role of nautilus

Cellular heterogeneity of the developing worker honey bee (Apis mellifera) pupa: a single cell transcriptomics analysis

It is estimated that animals pollinate 87.5% of flowering plants worldwide and that managed honey bees (Apis mellifera) account for 30-50% of this ecosystem service to agriculture. In addition to their important role as pollinators, honey bees are well-established insect models for studying learning and memory, behavior, caste differentiation, epigenetic mechanisms, olfactory biology, sex determination, and eusociality. Despite their importance to agriculture, knowledge of honey bee biology lags behind many other livestock species. In this study, we have used scRNA-Seq to map cell types to different developmental stages of the worker honey bee (prepupa at day 11 and pupa at day 15) and sought to determine their gene expression signatures. To identify cell-type populations, we examined the cell-to-cell network based on the similarity of the single-cells transcriptomic profiles. Grouping similar cells together we identified 63 different cell clusters of which 17 clusters were identifiable at both stages. To determine genes associated with specific cell populations or with a particular biological process involved in honey bee development, we used gene coexpression analysis. We combined this analysis with literature mining, the honey bee protein atlas, and gene ontology analysis to determine cell cluster identity. Of the cell clusters identified, 17 were related to the nervous system and sensory organs, 7 to the fat body, 19 to the cuticle, 5 to muscle, 4 to compound eye, 2 to midgut, 2 to hemocytes, and 1 to malpighian tubule/pericardial nephrocyte. To our knowledge, this is the first whole single-cell atlas of honey bees at any stage of development and demonstrates the potential for further work to investigate their biology at the cellular level.

Simultaneous cellular and molecular phenotyping of embryonic mutants using single-cell regulatory trajectories

Developmental progression and cellular diversity are largely driven by transcription factors (TFs); yet, characterizing their loss-of-function phenotypes remains challenging and often disconnected from their underlying molecular mechanisms. Here, we combine single-cell regulatory genomics with loss-of-function mutants to jointly assess both cellular and molecular phenotypes. Performing sci-ATAC-seq at eight overlapping time points during Drosophila mesoderm development could reconstruct the developmental trajectories of all major muscle types and reveal the TFs and enhancers involved. To systematically assess mutant phenotypes, we developed a single-nucleus genotyping strategy to process embryo pools of mixed genotypes. Applying this to four TF mutants could identify and quantify their characterized phenotypes de novo and discover new ones, while simultaneously revealing their regulatory input and mode of action. Our approach is a general framework to dissect the functional input of TFs in a systematic, unbiased manner, identifying both cellular and molecular phenotypes at a scale and resolution that has not been feasible before.

Acting on identity: Myoblast fusion and the formation of the syncytial muscle fiber

The study of Drosophila muscle development dates back to the middle of the last century. Since that time, Drosophila has proved to be an ideal system for studying muscle development, differentiation, function, and disease. As in humans, Drosophila muscle forms via a series of conserved steps, starting with muscle specification, myoblast fusion, attachment to tendon cells, interactions with motorneurons, and sarcomere and myofibril formation. The genes and mechanisms required for these processes share striking similarities to those found in humans. The highly tractable genetic system and imaging approaches available in Drosophila allow for an efficient interrogation of muscle biology and for application of what we learn to other systems. In this article, we review our current understanding of muscle development in Drosophila, with a focus on myoblast fusion, the process responsible for the generation of syncytial muscle cells. We also compare and contrast those genes required for fusion in Drosophila and vertebrates.

CF2 transcription factor is involved in the regulation of Mef2 RNA levels, nuclei number and muscle fiber size

CF2 and Mef2 influence a variety of developmental muscle processes at distinct stages of development. Nevertheless, the exact nature of the CF2-Mef2 relationship and its effects on muscle building remain yet to be resolved. Here, we explored the regulatory role of CF2 in the Drosophila embryo muscle formation. To address this question and not having proper null CF2 mutants we exploited loss or gain of function strategies to study the contribution of CF2 to Mef2 transcription regulation and to muscle formation. Our data point to CF2 as a factor involved in the regulation of muscle final size and/or the number of nuclei present in each muscle. This function is independent of its role as a Mef2 collaborative factor in the transcriptional regulation of muscle-structural genes. Although Mef2 expression patterns do not change, reductions or increases in parallel in CF2 and Mef2 transcript abundance were observed in interfered and overexpressed CF2 embryos. Since CF2 expression variations yield altered Mef2 expression levels but with correct spatio-temporal Mef2 expression patterns, it can be concluded that only the mechanism controlling expression levels is de-regulated. Here, it is proposed that CF2 regulates Mef2 expression through a Feedforward Loop circuit.

Myogenic regulatory transcription factors regulate growth in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a pediatric malignacy of muscle with myogenic regulatory transcription factors MYOD and MYF5 being expressed in this disease. Consensus in the field has been that expression of these factors likely reflects the target cell of transformation rather than being required for continued tumor growth. Here, we used a transgenic zebrafish model to show that Myf5 is sufficient to confer tumor-propagating potential to RMS cells and caused tumors to initiate earlier and have higher penetrance. Analysis of human RMS revealed that MYF5 and MYOD are mutually-exclusively expressed and each is required for sustained tumor growth. ChIP-seq and mechanistic studies in human RMS uncovered that MYF5 and MYOD bind common DNA regulatory elements to alter transcription of genes that regulate muscle development and cell cycle progression. Our data support unappreciated and dominant oncogenic roles for MYF5 and MYOD convergence on common transcriptional targets to regulate human RMS growth.

DOI:http://dx.doi.org/10.7554/eLife.19214.001

Differentiated muscles are mandatory for gas-filling of the Drosophila airway system

At the end of development, organs acquire functionality, thereby ensuring autonomy of an organism when it separates from its mother or a protective egg. In insects, respiratory competence starts when the tracheal system fills with gas just before hatching of the juvenile animal. Cellular and molecular mechanisms of this process are not fully understood. Analyses of the phenotype of Drosophila embryos with malformed muscles revealed that they fail to gas-fill their tracheal system. Indeed, we show that major regulators of muscle formation like Lame duck and Blown fuse are important, while factors involved in the development of subsets of muscles including cardiac and visceral muscles are dispensable for this process, suggesting that somatic muscles (or parts of them) are essential to enable tracheal terminal differentiation. Based on our phenotypic data, we assume that somatic muscle defect severity correlates with the penetrance of the gas-filling phenotype. This argues that a limiting molecular or mechanical muscle-borne signal tunes tracheal differentiation. We think that in analogy to the function of smooth muscles in vertebrate lungs, a balance of physical forces between muscles and the elasticity of tracheal walls may be decisive for tracheal terminal differentiation in Drosophila.

Summary: During embryogenesis in Drosophila melanogaster, without involving the nervous system, somatic muscles control terminal differentiation of the airway system by stimulating gas-filling before hatching.

Muscle cell fate choice requires the T-box transcription factor midline in Drosophila

Drosophila Midline (Mid) is an ortholog of vertebrate Tbx20, which plays roles in the developing heart, migrating cranial motor neurons, and endothelial cells. Mid functions in cell-fate specification and differentiation of tissues that include the ectoderm, cardioblasts, neuroblasts, and egg chambers; however, a role in the somatic musculature has not been described. We identified mid in genetic and molecular screens for factors contributing to somatic muscle morphogenesis. Mid is expressed in founder cells (FCs) for several muscle fibers, and functions cooperatively with the T-box protein H15 in lateral oblique muscle 1 and the segment border muscle. Mid is particularly important for the specification and development of the lateral transverse (LT) muscles LT3 and LT4, which arise by asymmetric division of a single muscle progenitor. Mid is expressed in this progenitor and its two sibling FCs, but is maintained only in the LT4 FC. Both muscles were frequently missing in mid mutant embryos, and LT4-associated expression of the transcription factor Krüppel (Kr) was lost. When present, LT4 adopted an LT3-like morphology. Coordinately, mid misexpression caused LT3 to adopt an LT4-like morphology and was associated with ectopic Kr expression. From these data, we concluded that mid functions first in the progenitor to direct development of LT3 and LT4, and later in the FCs to influence whichever of these differentiation profiles is selected. Mid is the first T-box factor shown to influence LT3 and LT4 muscle identity and, along with the T-box protein Optomotor-blind-related-gene 1 (Org-1), is representative of a new class of transcription factors in muscle specification.

Myogenesis in the thoracic limbs of the American lobster

Newly hatched lobster larvae have biramous thoracic limbs composed of an endopodite, which is used for walking in the adult, and an exopodite used for swimming. Several behavioural and physiological aspects of larval locomotion as well the ontogeny of the neuromuscular system have been examined in developing decapod crustaceans. Nevertheless, the cellular basis of embryonic muscle formation in these animals is poorly understood. Therefore, the present report analyses muscle formation in embryos of the American lobster Homarus americanus Milne Edwards, 1837 (Malacostraca, Eucarida, Decapoda, Homarida) using the monoclonal antibody 016C6 that recognizes an isoform of myosin heavy chain. 016C6 labelling at 25% of embryonic development (E25%) revealed that syncytial muscle precursor cells establish the muscles in the endopodites. During subsequent embryogenesis, these muscle precursors subdivide into several distinct units thereby giving rise to pairs of antagonistic primordial muscles in each of the successive podomeres, the layout of which at E45% already resembles the arrangement in the adult thoracopods. The pattern of primordial muscles was also mapped in the exopodites of thoracic limbs three to eight. Immunohistochemistry against acetylated α-tubulin and against presynaptic vesicle-associated phosphoproteins at E45% demonstrated the existence of characteristic neural tracts within the developing limbs as well as putative neuromuscular synapses in both the embryonic exo- and endopodites. The results are compared to muscle development in other Crustacea.

Muscle precursor cells in the developing limbs of two isopods (Crustacea, Peracarida): an immunohistochemical study using a novel monoclonal antibody against myosin heavy chain

In the hot debate on arthropod relationships, Crustaceans and the morphology of their appendages play a pivotal role. To gain new insights into how arthropod appendages evolved, developmental biologists recently have begun to examine the expression and function of Drosophila appendage genes in Crustaceans. However, cellular aspects of Crustacean limb development such as myogenesis are poorly understood in Crustaceans so that the interpretative context in which to analyse gene functions is still fragmentary. The goal of the present project was to analyse muscle development in Crustacean appendages, and to that end, monoclonal antibodies against arthropod muscle proteins were generated. One of these antibodies recognises certain isoforms of myosin heavy chain and strongly binds to muscle precursor cells in malacostracan Crustacea. We used this antibody to study myogenesis in two isopods, Porcellio scaber and Idotea balthica (Crustacea, Malacostraca, Peracarida), by immunohistochemistry. In these animals, muscles in the limbs originate from single muscle precursor cells, which subsequently grow to form multinucleated muscle precursors. The pattern of primordial muscles in the thoracic limbs was mapped, and results compared to muscle development in other Crustaceans and in insects.

Stereotypic founder cell patterning and embryonic muscle formation in Drosophila require nautilus (MyoD) gene function

nautilus is the only MyoD-related gene in Drosophila. Nautilus expression begins around stage 9 at full germ-band extension in a subset of mesodermal cells organized in a stereotypic pattern in each hemisegment. The muscle founder cell marker Duf-LacZ, produced by the enhancer trap line rP298LacZ, is coexpressed in numerous Nautilus-positive cells when founders first appear. Founders entrain muscle identity through the restricted expression of transcription factors such as S59, eve, and Kr, all of which are observed in subsets of the nautilus expressing founders. We inactivated the nautilus gene using homology-directed gene targeting and Gal4/UAS regulated RNAi to determine whether loss of nautilus gene activity affected founder cell function. Both methods produced a range of defects that included embryonic muscle disruption, reduced viability and female sterility, which could be rescued by hsp70-nautilus cDNA transgenes. Our results demonstrate Nautilus expression marks early founders that give rise to diverse muscle groups in the embryo, and that nautilus gene activity is required to seed the correct founder myoblast pattern that prefigures the muscle fiber arrangement during embryonic development.

MAP kinase converts MyoD into an instructive muscle differentiation factor in Xenopus

In amphibian development, muscle is specified in the dorsal lateral marginal zone (DLMZ) of the gastrula embryo. Two critical events specify the formation of skeletal muscle: the expression of the myogenic transcription factor, XMyoD, and the secretion of bone morphogenetic protein (BMP) antagonists by the adjacent Spemann organizer. Inhibition of BMP signaling during early gastrula stages converts XMyoD protein into an instructive differentiation factor in the DLMZ. Yet, the intracellular signaling factors connecting BMP antagonism and activation of XMyoD remain unknown. Our data show that BMP antagonism induces the activity of mitogen-activated protein kinase (MAPK), and that the activity of MAPK is necessary for muscle-specific differentiation. Treatment of gastrula-stage DLMZ explants with MAPK pathway inhibitors ventralized mesoderm and prevented muscle differentiation. Expression of XMyoD in ventral mesoderm weakly induced muscle formation; however, the coexpression of a constitutively active MEK1 with XMyoD efficiently induced muscle differentiation. Activation of the MAPK pathway did not induce the transcription of XMyoD, but increased its protein levels and transcriptional activity. Thus, MAPK activation is subsequent to BMP antagonism, and participates in the dorsalization of mesoderm by converting the XMyoD protein into a potent differentiation factor.

Homeotic Complex (Hox) gene regulation and homeosis in the mesoderm of the Drosophila melanogaster embryo: the roles of signal transduction and cell autonomous regulation

In this paper we evaluate homeosis and Homeotic Complex (Hox) regulatory hierarchies in the somatic and visceral mesoderm. We demonstrate that both Hox control of signal transduction and cell autonomous regulation are critical for establishing normal Hox expression patterns and the specification of segmental identity and morphology. We present data identifying novel regulatory interactions associated with the segmental register shift in Hox expression domains between the epidermis/somatic mesoderm and visceral mesoderm. A proposed mechanism for the gap between the expression domains of Sex combs reduced (Scr) and Antennapedia (Antp) in the visceral mesoderm is provided. Previously, Hox gene interactions have been shown to occur on multiple levels: direct cross-regulation, competition for binding sites at downstream targets and through indirect feedback involving signal transduction. We find that extrinsic specification of cell fate by signaling can be overridden by Hox protein expression in mesodermal cells and propose the term autonomic dominance for this phenomenon.

Control of Drosophila paramyosin/miniparamyosin gene expression. Differential regulatory mechanisms for muscle-specific transcription

To define the transcriptional mechanisms contributing to stage- and tissue-specific expression of muscle genes, we performed transgenic analysis of Drosophila paramyosin gene regulation. This gene has two promoters, one for paramyosin and one for miniparamyosin, which are active in partially overlapping domains. Regions between -0.9 and -1.7 kilobases upstream of each initiation site contribute to the temporal and spatial expression patterns. By comparing the Drosophila melanogaster and Drosophila virilis promoters, conserved binding sites were found for known myogenic factors, including one MEF2 site and three E boxes. In contrast with previous data, our experiments with the paramyosin promoter indicate that the MEF2 site is essential but not sufficient for proper paramyosin gene transcription. Mutations in the three E boxes, on the other hand, do not produce any effect in embryonic/larval muscles. Thus MEF2 site- and E box-binding proteins can play different roles in the regulation of different muscle-specific genes. For the miniparamyosin promoters, several conserved sequences were shown to correspond to functionally important regions. Our data further show that the two promoters work independently. Even when both promoters are active in the same muscle fiber, the transcription driven by one of the promoters is not affected by transcription driven by the other.

Tinman regulates the transcription of the beta3 tubulin gene (betaTub60D) in the dorsal vessel of Drosophila

During Drosophila embryogenesis, the beta3 tubulin gene is expressed in the visceral and somatic mesoderm as well as in the dorsal vessel. Transcription of the gene is limited to four pairs of cardioblasts per segment. Here we show that its expression in the dorsal vessel (dv) is mediated by a 333-bp enhancer located upstream of the gene. The homeodomain protein Tinman is also expressed in these cardioblasts, implying that Tinman might be a key regulator of the beta3 tubulin gene. Gel retardation and footprint assays indeed revealed two Tinman binding sites within the dv-specific enhancer. We analyzed the relevance of the Tinman binding sites in a transgenic fly assay and observed distinct functions for both sites. The BS(Tin-1460) site is absolutely required for expression in cardioblasts, while BS(Tin-1425) is needed for high-level expression. Thus, these two Tinman binding sites act in concert to drive beta3 tubulin gene expression during heart development. Tinman initially functions in the specification of visceral mesoderm and heart progenitors, but remains expressed in cardioblasts until dorsal closure. Overall, our data demonstrate a late function for Tinman in the regulation of beta3 tubulin gene expression in the forming heart of Drosophila.

Essential genes for myoblast fusion in Drosophila embryogenesis

In Drosophila, as in vertebrates, each muscle is a syncytium and arises from mesodermal cells by successive fusion. This requires cell-cell recognition, alignment, formation of prefusion complexes, followed by electron-dense plaques and membrane breakdown. Because muscle development in Drosophila is rapid and well-documented, it has been possible to identify several genes essential for fusion. Molecular analysis of two of these genes revealed the importance of cytoplasmic components. One of these, Myoblast city, is expressed in several tissues and is homologous to the mammalian protein DOCK180. Myoblast city is presumably involved in cell recognition and cell adhesion. Blown fuse, the second cytoplasmic component, is selectively expressed in the mesoderm and essential in order to proceed from the prefusion complex to electron-dense plaques at opposed membranes between adjacent myoblasts. The rolling stone gene is transiently expressed during myoblast fusion. The Rost protein is located in the membrane and thus might be a key component for cell recognition. The molecular characterization of further genes relevant for fusion such as singles bar and sticks and stones will help to elucidate the mechanism of myoblast fusion in Drosophila.

Targeted disruption of gene function in Drosophila by RNA interference (RNA-i): a role for nautilus in embryonic somatic muscle formation

The expression of the MyoD gene homolog, nautilus (nau), in the Drosophila embryo defines a subset of mesodermal cells known as the muscle "pioneer" or "founder" cells. These cells are thought to establish the future muscle pattern in each hemisegment. Founders appear to recruit fusion-competent mesodermal cells to establish a particular muscle fiber type. In support of this concept every somatic muscle in the embryo is associated with one or more nautilus-positive cells. However, because of the lack of known (isolated) nautilus mutations, no direct test of the founder cell hypothesis has been possible. We now have utilized toxin ablation and genetic interference by double-stranded RNA (RNA interference or RNA-i) to determine both the role of the nautilus-expressing cells and the nautilus gene, respectively, in embryonic muscle formation. In the absence of nautilus-expressing cells muscle formation is severely disrupted or absent. A similar phenotype is observed with the elimination of the nautilus gene product by genetic interference upon injection of nautilus double-stranded RNA. These results define a crucial role for nautilus in embryonic muscle formation. The application of RNA interference to a variety of known Drosophila mutations as controls gave phenotypes essentially indistinguishable from the original mutation. RNA-i provides a powerful approach for the targeted disruption of a given genetic function in Drosophila.

Seeking muscle stem cells

Skeletal muscle development requires the formation of myoblasts that can fuse with each other to form multinucleate myofibers. Distinct primary and secondary, slow and fast, populations of myofibers form by the time of birth. At embryonic, fetal, and perinatal stages of development, temporally distinct lineages of myogenic cells arise and contribute to the formation of these multiple types of myofibers. In addition, spatially distinct lineages of myogenic cells arise and form the anterior head muscles, limb (hypaxial) muscles, and dorsal (epaxial) muscles. There is strong evidence that myoblasts are produced from muscle stem cells, which are self-renewing cells that do not themselves terminally differentiate but produce progeny that are capable of becoming myoblasts and myofibers. Muscle stem cells, which may be multipotent, appear to be distinguishable from myoblasts by a number of indirect and direct criteria. Muscle stem cells arise either in unsegmented paraxial mesoderm (anterior head muscle progenitors) or in segmented mesoderm of the somites (epaxial and hypaxial muscle progenitors). These initial stages of myogenesis are regulated by positive and negative signals, including Wnt, BMP, and Shh family members, from nearby notochord, neural tube, ectoderm, and lateral mesoderm tissues. The formation of skeletal muscles, therefore, depends on the generation of spatially and temporally distinct lineages of myogenic cells. Myogenic cell lineages begin with muscle stem cells which produce the myoblasts that fuse to form myofibers.