myoblasts incompetent encodes a zinc finger transcription factor required to specify fusion-competent myoblasts in Drosophila

Agent-based model demonstrates the impact of nonlinear, complex interactions between cytokinces on muscle regeneration

Muscle regeneration is a complex process due to dynamic and multiscale biochemical and cellular interactions, making it difficult to identify microenvironmental conditions that are beneficial to muscle recovery from injury using experimental approaches alone. To understand the degree to which individual cellular behaviors impact endogenous mechanisms of muscle recovery, we developed an agent-based model (ABM) using the Cellular-Potts framework to simulate the dynamic microenvironment of a cross-section of murine skeletal muscle tissue. We referenced more than 100 published studies to define over 100 parameters and rules that dictate the behavior of muscle fibers, satellite stem cells (SSCs), fibroblasts, neutrophils, macrophages, microvessels, and lymphatic vessels, as well as their interactions with each other and the microenvironment. We utilized parameter density estimation to calibrate the model to temporal biological datasets describing cross-sectional area (CSA) recovery, SSC, and fibroblast cell counts at multiple timepoints following injury. The calibrated model was validated by comparison of other model outputs (macrophage, neutrophil, and capillaries counts) to experimental observations. Predictions for eight model perturbations that varied cell or cytokine input conditions were compared to published experimental studies to validate model predictive capabilities. We used Latin hypercube sampling and partial rank correlation coefficient to identify in silico perturbations of cytokine diffusion coefficients and decay rates to enhance CSA recovery. This analysis suggests that combined alterations of specific cytokine decay and diffusion parameters result in greater fibroblast and SSC proliferation compared to individual perturbations with a 13% increase in CSA recovery compared to unaltered regeneration at 28 days. These results enable guided development of therapeutic strategies that similarly alter muscle physiology (i.e. converting extracellular matrix [ECM]-bound cytokines into freely diffusible forms as studied in cancer therapeutics or delivery of exogenous cytokines) during regeneration to enhance muscle recovery after injury.

Agent-based model demonstrates the impact of nonlinear, complex interactions between cytokines on muscle regeneration

Muscle regeneration is a complex process due to dynamic and multiscale biochemical and cellular interactions, making it difficult to identify microenvironmental conditions that are beneficial to muscle recovery from injury using experimental approaches alone. To understand the degree to which individual cellular behaviors impact endogenous mechanisms of muscle recovery, we developed an agent-based model (ABM) using the Cellular Potts framework to simulate the dynamic microenvironment of a cross-section of murine skeletal muscle tissue. We referenced more than 100 published studies to define over 100 parameters and rules that dictate the behavior of muscle fibers, satellite stem cells (SSC), fibroblasts, neutrophils, macrophages, microvessels, and lymphatic vessels, as well as their interactions with each other and the microenvironment. We utilized parameter density estimation to calibrate the model to temporal biological datasets describing cross-sectional area (CSA) recovery, SSC, and fibroblast cell counts at multiple time points following injury. The calibrated model was validated by comparison of other model outputs (macrophage, neutrophil, and capillaries counts) to experimental observations. Predictions for eight model perturbations that varied cell or cytokine input conditions were compared to published experimental studies to validate model predictive capabilities. We used Latin hypercube sampling and partial rank correlation coefficient to identify in silico perturbations of cytokine diffusion coefficients and decay rates to enhance CSA recovery. This analysis suggests that combined alterations of specific cytokine decay and diffusion parameters result in greater fibroblast and SSC proliferation compared to individual perturbations with a 13% increase in CSA recovery compared to unaltered regeneration at 28 days. These results enable guided development of therapeutic strategies that similarly alter muscle physiology (i.e. converting ECM-bound cytokines into freely diffusible forms as studied in cancer therapeutics or delivery of exogenous cytokines) during regeneration to enhance muscle recovery after injury.

Methylation status and expression patterns of myomaker gene play important roles in postnatal development in the Japanese flounder (Paralichthys olivaceus)

Myomaker is a membrane protein that plays a crucial role in the fusion of myoblasts during muscle growth. DNA methylation, a significant factor, regulates gene expression. The aim of this study was to examine the methylation and mRNA expression patterns of the myomaker gene during 8 different postnatal developmental stages in the Japanese flounder (L: 7 days post hatch (dph); M1: 21 dph; M2: 28 dph; M3: 35 dph; J1: 90 dph; J2: 180 dph; A1: 24 months; A2: 36 months). Muscle tissue samples were taken from Japanese flounder at different postnatal development stages to measure the extent of DNA methylation and gene expression. Methylation level in the promoter and exon 1 of myomaker was measured using bisulfite sequencing, and the relative expression of myomaker during each developmental stage was measured by quantitative PCR. The relative expression levels of myomaker were up-regulated from stages L to M2, M3 to J2, and methylation of myomaker was negatively correlated with mRNA expression. Furthermore, the CpG site located at -26 bp in the promoter was the lowest methylated region in all developmental stages. These results offer a basis for understanding the mechanism by which myomaker regulates muscle formation during postnatal development.

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.

Distinct roles and requirements for Ras pathway signaling in visceral versus somatic muscle founder specification

Pleiotropic signaling pathways must somehow engender specific cellular responses. In the Drosophila mesoderm, Ras pathway signaling specifies muscle founder cells from among the broader population of myoblasts. For somatic muscles, this is an inductive process mediated by the ETS-domain downstream Ras effectors Pointed and Aop (Yan). We demonstrate here that for the circular visceral muscles, despite superficial similarities, a significantly different specification mechanism is at work. Not only is visceral founder cell specification not dependent on Pointed or Aop, but Ras pathway signaling in its entirety can be bypassed. Our results show that de-repression, not activation, is the predominant role of Ras signaling in the visceral mesoderm and that, accordingly, Ras signaling is not required in the absence of repression. The key repressor acts downstream of the transcription factor Lame duck and is likely a member of the ETS transcription factor family. Our findings fit with a growing body of data that point to a complex interplay between the Ras pathway, ETS transcription factors, and enhancer binding as a crucial mechanism for determining unique responses to Ras signaling.

APC/CFzr regulates cardiac and myoblast cell numbers and plays a crucial role during myoblast fusion

Somatic muscles are formed by the iterative fusion of myoblasts into muscle fibres. This process is driven by the recurrent recruitment of proteins to the cell membrane to induce F-actin nucleation at the fusion site. Although various proteins involved in myoblast fusion have been identified, knowledge about their sub-cellular regulation is rather elusive. We identified the anaphase-promoting complex (APC/C) adaptor Fizzy related (Fzr) as an essential regulator of heart and muscle development. We show that APC/CFzr regulates the fusion of myoblasts as well as mitotic exit of pericardial cells, cardioblasts and myoblasts. Surprisingly, over-proliferation is not causative for the observed fusion defects. Instead, fzr mutants exhibit smaller F-actin foci at the fusion site, and display reduced membrane breakdown between adjacent myoblasts. We show that lack of APC/CFzr causes the accumulation and mislocalisation of Rols and Duf, two proteins involved in the fusion process. Duf seems to serve as direct substrate of the APC/CFzr, and its destruction depends on the presence of distinct degron sequences. These novel findings indicate that protein destruction and turnover constitute major events during myoblast fusion.

Fear-of-intimacy mediated zinc transport controls the function of Zn-finger transcription factors involved in myogenesis

Zinc is a component of one tenth of all human proteins. Its cellular concentration is tightly regulated because its dyshomeostasis has catastrophic health consequences. Two families of zinc transporters control zinc homeostasis in organisms, but there is little information about their specific developmental roles. We show that the ZIP transporter fear-of-intimacy (foi) is necessary for the formation of Drosophila muscles. In foi mutants, myoblasts segregate normally, but their specification is affected, leading to the formation of a misshapen muscle pattern and distorted midgut. The observed phenotypes could be ascribed to the inactivation of specific zing-finger transcription factors (ZFTFs), supporting the hypothesis that they a consequence of a zinc intracellular depletion. Accordingly, foi phenotypes can be rescued by mesodermal expression of other ZIP members with similar subcellular localization. We propose that Foi acts mostly as a transporter to regulate zinc intracellular homeostasis, thereby impacting on the activity of ZFTFs that control specific developmental processes. Our results additionally suggest a possible explanation for the presence of large numbers of zinc transporters in organisms based on differences in ion transport specificity and/or degrees of activity among transporters.

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.

Myomaker, Regulated by MYOD, MYOG and miR-140-3p, Promotes Chicken Myoblast Fusion

The fusion of myoblasts is an important step during skeletal muscle differentiation. A recent study in mice found that a transmembrane protein called Myomaker, which is specifically expressed in muscle, is critical for myoblast fusion. However, the cellular mechanism of its roles and the regulatory mechanism of its expression remain unclear. Chicken not only plays an important role in meat production but is also an ideal model organism for muscle development research. Here, we report that Myomaker is also essential for chicken myoblast fusion. Forced expression of Myomaker in chicken primary myoblasts promotes myoblast fusion, whereas knockdown of Myomaker by siRNA inhibits myoblast fusion. MYOD and MYOG, which belong to the family of myogenic regulatory factors, can bind to a conserved E-box located proximal to the Myomaker transcription start site and induce Myomaker transcription. Additionally, miR-140-3p can inhibit Myomaker expression and myoblast fusion, at least in part, by binding to the 3ʹ UTR of Myomaker in vitro. These findings confirm the essential roles of Myomaker in avian myoblast fusion and show that MYOD, MYOG and miR-140-3p can regulate Myomaker expression.

Specification of the somatic musculature in Drosophila

The somatic muscle system formed during Drosophila embryogenesis is required for larvae to hatch, feed, and crawl. This system is replaced in the pupa by a new adult muscle set, responsible for activities such as feeding, walking, and flight. Both the larval and adult muscle systems are comprised of distinct muscle fibers to serve these specific motor functions. In this way, the Drosophila musculature is a valuable model for patterning within a single tissue: while all muscle cells share properties such as the contractile apparatus, properties such as size, position, and number of nuclei are unique for a particular muscle. In the embryo, diversification of muscle fibers relies first on signaling cascades that pattern the mesoderm. Subsequently, the combinatorial expression of specific transcription factors leads muscle fibers to adopt particular sizes, shapes, and orientations. Adult muscle precursors (AMPs), set aside during embryonic development, proliferate during the larval phases and seed the formation of the abdominal, leg, and flight muscles in the adult fly. Adult muscle fibers may either be formed de novo from the fusion of the AMPs, or are created by the binding of AMPs to an existing larval muscle. While less is known about adult muscle specification compared to the larva, expression of specific transcription factors is also important for its diversification. Increasingly, the mechanisms required for the diversification of fly muscle have found parallels in vertebrate systems and mark Drosophila as a robust model system to examine questions about how diverse cell types are generated within an organism.

Coordinated repression and activation of two transcriptional programs stabilizes cell fate during myogenesis

Molecular models of cell fate specification typically focus on the activation of specific lineage programs. However, the concurrent repression of unwanted transcriptional networks is also essential to stabilize certain cellular identities, as shown in a number of diverse systems and phyla. Here, we demonstrate that this dual requirement also holds true in the context of Drosophila myogenesis. By integrating genetics and genomics, we identified a new role for the pleiotropic transcriptional repressor Tramtrack69 in myoblast specification. Drosophila muscles are formed through the fusion of two discrete cell types: founder cells (FCs) and fusion-competent myoblasts (FCMs). When tramtrack69 is removed, FCMs appear to adopt an alternative muscle FC-like fate. Conversely, ectopic expression of this repressor phenocopies muscle defects seen in loss-of-function lame duck mutants, a transcription factor specific to FCMs. This occurs through Tramtrack69-mediated repression in FCMs, whereas Lame duck activates a largely distinct transcriptional program in the same cells. Lineage-specific factors are therefore not sufficient to maintain FCM identity. Instead, their identity appears more plastic, requiring the combination of instructive repressive and activating programs to stabilize cell fate.

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

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

Integrative analysis of the zinc finger transcription factor Lame duck in the Drosophila myogenic gene regulatory network

Contemporary high-throughput technologies permit the rapid identification of transcription factor (TF) target genes on a genome-wide scale, yet the functional significance of TFs requires knowledge of target gene expression patterns, cooperating TFs, and cis-regulatory element (CRE) structures. Here we investigated the myogenic regulatory network downstream of the Drosophila zinc finger TF Lame duck (Lmd) by combining both previously published and newly performed genomic data sets, including ChIP sequencing (ChIP-seq), genome-wide mRNA profiling, cell-specific expression patterns of putative transcriptional targets, analysis of histone mark signatures, studies of TF cooccupancy by additional mesodermal regulators, TF binding site determination using protein binding microarrays (PBMs), and machine learning of candidate CRE motif compositions. Our findings suggest that Lmd orchestrates an extensive myogenic regulatory network, a conclusion supported by the identification of Lmd-dependent genes, histone signatures of Lmd-bound genomic regions, and the relationship of these features to cell-specific gene expression patterns. The heterogeneous cooccupancy of Lmd-bound regions with additional mesodermal regulators revealed that different transcriptional inputs are used to mediate similar myogenic gene expression patterns. Machine learning further demonstrated diverse combinatorial motif patterns within tissue-specific Lmd-bound regions. PBM analysis established the complete spectrum of Lmd DNA binding specificities, and site-directed mutagenesis of Lmd and additional newly discovered motifs in known enhancers demonstrated the critical role of these TF binding sites in supporting full enhancer activity. Collectively, these findings provide insights into the transcriptional codes regulating muscle gene expression and offer a generalizable approach for similar studies in other systems.

Myoblast fusion: lessons from flies and mice

The fusion of myoblasts into multinucleate syncytia plays a fundamental role in muscle function, as it supports the formation of extended sarcomeric arrays, or myofibrils, within a large volume of cytoplasm. Principles learned from the study of myoblast fusion not only enhance our understanding of myogenesis, but also contribute to our perspectives on membrane fusion and cell-cell fusion in a wide array of model organisms and experimental systems. Recent studies have advanced our views of the cell biological processes and crucial proteins that drive myoblast fusion. Here, we provide an overview of myoblast fusion in three model systems that have contributed much to our understanding of these events: the Drosophila embryo; developing and regenerating mouse muscle; and cultured rodent muscle cells.

Differential regulation of mesodermal gene expression by Drosophila cell type-specific Forkhead transcription factors

A common theme in developmental biology is the repeated use of the same gene in diverse spatial and temporal domains, a process that generally involves transcriptional regulation mediated by multiple separate enhancers, each with its own arrangement of transcription factor (TF)-binding sites and associated activities. Here, by contrast, we show that the expression of the Drosophila Nidogen (Ndg) gene at different embryonic stages and in four mesodermal cell types is governed by the binding of multiple cell-specific Forkhead (Fkh) TFs – including Biniou (Bin), Checkpoint suppressor homologue (CHES-1-like) and Jumeau (Jumu) – to three functionally distinguishable Fkh-binding sites in the same enhancer. Whereas Bin activates the Ndg enhancer in the late visceral musculature, CHES-1-like cooperates with Jumu to repress this enhancer in the heart. CHES-1-like also represses the Ndg enhancer in a subset of somatic myoblasts prior to their fusion to form multinucleated myotubes. Moreover, different combinations of Fkh sites, corresponding to two different sequence specificities, mediate the particular functions of each TF. A genome-wide scan for the occurrence of both classes of Fkh domain recognition sites in association with binding sites for known cardiac TFs showed an enrichment of combinations containing the two Fkh motifs in putative enhancers found within the noncoding regions of genes having heart expression. Collectively, our results establish that different cell-specific members of a TF family regulate the activity of a single enhancer in distinct spatiotemporal domains, and demonstrate how individual binding motifs for a TF class can differentially influence gene expression.

Molecular and cellular mechanisms of mammalian cell fusion

The fusion of one cell with another occurs in development, injury and disease. Despite the diversity of fusion events, five steps in sequence appear common. These steps include programming fusion-competent status, chemotaxis, membrane adhesion, membrane fusion, and post-fusion resetting. Recent advances in the field start to reveal the molecules involved in each step. This review focuses on some key molecules and cellular events of cell fusion in mammals. Increasing evidence demonstrates that membrane lipid rafts, adhesion proteins and actin rearrangement are critical in the final step of membrane fusion. Here we propose a new model for the formation and expansion of membrane fusion pores based on recent observations on myotube formation. In this model, membrane lipid rafts first recruit adhesion molecules and align with opposing membranes, with the help of a cortical actin "wall" as a rigid supportive platform. Second, the membrane adhesion proteins interact with each other and trigger actin rearrangement, which leads to rapid dispersion of lipid rafts and flow of a highly fluidic phospholipid bilayer into the site. Finally, the opposing phospholipid bilayers are then pushed into direct contact leading to the formation of fusion pores by the force generated through actin polymerization. The actin polymerization generated force also drives the expansion of the fusion pores. However, several key questions about the process of cell fusion still remain to be explored. The understanding of the mechanisms of cell fusion may provide new opportunities in correcting development disorders or regenerating damaged tissues by inhibiting or promoting molecular events associated with fusion.

Drosophila araucan and caupolican integrate intrinsic and signalling inputs for the acquisition by muscle progenitors of the lateral transverse fate

A central issue of myogenesis is the acquisition of identity by individual muscles. In Drosophila, at the time muscle progenitors are singled out, they already express unique combinations of muscle identity genes. This muscle code results from the integration of positional and temporal signalling inputs. Here we identify, by means of loss-of-function and ectopic expression approaches, the Iroquois Complex homeobox genes araucan and caupolican as novel muscle identity genes that confer lateral transverse muscle identity. The acquisition of this fate requires that Araucan/Caupolican repress other muscle identity genes such as slouch and vestigial. In addition, we show that Caupolican-dependent slouch expression depends on the activation state of the Ras/Mitogen Activated Protein Kinase cascade. This provides a comprehensive insight into the way Iroquois genes integrate in muscle progenitors, signalling inputs that modulate gene expression and protein activity.

In Drosophila, as in vertebrates, the muscular system consists of different types of muscles that must act in coordination with the nervous system to control the adequate release of contraction power required for the proper functioning of the organism. Therefore, the acquisition of specific identities by individual muscles is a key step in the generation of the muscular system. In Drosophila, muscle progenitors (specific myoblasts that seed the formation of mature muscles) integrate positional and temporal signalling inputs, resulting in the expression of unique combinations of muscle identity genes, which confer on them specific fates. Up to now, very little was known of how this integration takes place at a molecular level and how a particular code is translated into a specific muscle fate. Here we show that the acquisition of the lateral transverse muscle fate requires the repression mediated by Araucan and Caupolican, two homeoproteins of the Iroquois Complex, of other muscle identity genes, like slouch and vestigial. The repressor or activator function of the Iroquois proteins depends on the activity of the Ras signalling pathway. Therefore, our work places Iroquois genes at a nodal point that integrates signalling inputs and regulates protein activity and cell fate determination.

Characterisation of the role of Vrp1 in cell fusion during the development of visceral muscle of Drosophila melanogaster

Background

In Drosophila muscle cell fusion takes place both during the formation of the somatic mesoderm and the visceral mesoderm, giving rise to the skeletal muscles and the gut musculature respectively. The core process of myoblast fusion is believed to be similar for both organs. The actin cytoskeleton regulator Verprolin acts by binding to WASP, which in turn binds to the Arp2/3 complex and thus activates actin polymerization. While Verprolin has been shown to be important for somatic muscle cell fusion, the function of this protein in visceral muscle fusion has not been determined.

Results

Verprolin is specifically expressed in the fusion competent myoblasts of the visceral mesoderm, suggesting a role in visceral mesoderm fusion. We here describe a novel Verprolin mutant allele which displays subtle visceral mesoderm fusion defects in the form of mislocalization of the immunoglobulin superfamily molecule Duf/Kirre, which is required on the myoblast cell surface to facilitate attachment between cells that are about to fuse, indicating a function for Verprolin in visceral mesoderm fusion. We further show that Verprolin mutant cells are capable of both migrating and fusing and that the WASP-binding domain of Verprolin is required for rescue of the Verprolin mutant phenotype.

Conclusions

Verprolin is expressed in the visceral mesoderm and plays a role in visceral muscle fusion as shown by mislocalization of Duf/Kirre in the Verprolin mutant, however it is not absolutely required for myoblast fusion in either the visceral or the somatic mesoderm.

Combinatorial binding leads to diverse regulatory responses: Lmd is a tissue-specific modulator of Mef2 activity

Understanding how complex patterns of temporal and spatial expression are regulated is central to deciphering genetic programs that drive development. Gene expression is initiated through the action of transcription factors and their cofactors converging on enhancer elements leading to a defined activity. Specific constellations of combinatorial occupancy are therefore often conceptualized as rigid binding codes that give rise to a common output of spatio-temporal expression. Here, we assessed this assumption using the regulatory input of two essential transcription factors within the Drosophila myogenic network. Mutations in either Myocyte enhancing factor 2 (Mef2) or the zinc-finger transcription factor lame duck (lmd) lead to very similar defects in myoblast fusion, yet the underlying molecular mechanism for this shared phenotype is not understood. Using a combination of ChIP-on-chip analysis and expression profiling of loss-of-function mutants, we obtained a global view of the regulatory input of both factors during development. The majority of Lmd-bound enhancers are co-bound by Mef2, representing a subset of Mef2's transcriptional input during these stages of development. Systematic analyses of the regulatory contribution of both factors demonstrate diverse regulatory roles, despite their co-occupancy of shared enhancer elements. These results indicate that Lmd is a tissue-specific modulator of Mef2 activity, acting as both a transcriptional activator and repressor, which has important implications for myogenesis. More generally, this study demonstrates considerable flexibility in the regulatory output of two factors, leading to additive, cooperative, and repressive modes of co-regulation.

While genetic studies are essential to reveal the phenotypic relationships between genes, it is often very difficult to disentangle the molecular mechanism of two genes that phenocopy each other. In this study, we used global scale and single gene analysis to investigate the relationship between two transcription factors whose mutant embryos have a similar defect in myogenesis. In Drosophila, Mef2 mutant embryos display a block in myoblast fusion, which is very similar to what is observed in mutant embryos for lmd, a zinc-finger transcription factor. To understand the underlying nature of these defects we used ChIP-on-chip analysis to obtain a global view of their co-regulated enhancers, and we used expression profiling of mutant embryos to reveal their downstream transcriptional response. The results indicate that Lmd acts as a tissue specific modulator of Mef2 activity. Using in vivo and in vitro reporter assays, we show that co-binding to the same enhancer element can lead to diverse regulatory responses. The presence of Lmd has an additive, cooperative, or repressive effect on Mef2 activity, demonstrating that it acts as a molecular switch for gene expression during muscle differentiation. More broadly, our results highlight the difficulty in translating information on combinatorial binding data into a functional regulatory response.

Myoblast fusion: when it takes more to make one

Cell-cell fusion is a crucial and highly regulated event in the genesis of both form and function of many tissues. One particular type of cell fusion, myoblast fusion, is a key cellular process that shapes the formation and repair of muscle. Despite its importance for human health, the mechanisms underlying this process are still not well understood. The purpose of this review is to highlight the recent literature pertaining to myoblast fusion and to focus on a comparison of these studies across several model systems, particularly the fly, zebrafish and mouse. Advances in technical analysis and imaging have allowed identification of new fusion genes and propelled further characterization of previously identified genes in each of these systems. Among the cellular steps identified as critical for myoblast fusion are migration, recognition, adhesion, membrane alignment and membrane pore formation and resolution. Importantly, striking new evidence indicates that orthologous genes govern several of these steps across these species. Taken together, comparisons across three model systems are illuminating a once elusive process, providing exciting new insights and a useful framework of genes and mechanisms.

FuRMAS: triggering myoblast fusion in Drosophila

In Drosophila, as in mammals, myoblast fusion is fundamental for development. This fusion process has two distinct phases that share common ultrastructural features and at least some molecular players between Drosophila and vertebrates. Here, we integrate the latest data on the key molecular players and ultrastructural features found during myoblast fusion into a new working model to explain this fundamental cellular process. At cell-cell contact sites, a protein complex (FuRMAS) serves as a signalling centre and might restrict the area of membrane fusion. The FuRMAS consists of a ring of cell adhesion molecules, signalling proteins, and F-actin. Regulated F-actin branching plays a pivotal role in myoblast fusion with regard to vesicle transport, fusion pore formation, and expansion as well as the integration of the fusion-competent myoblast into the growing myotube. Interestingly, local F-actin accumulation is a typical feature of other transient adhesive structures such as the immunological synapse, podosomes, and invadopodia. Developmental Dynamics 238:1513-1525, 2009. (c) 2009 Wiley-Liss, Inc.

A genomic approach to myoblast fusion in Drosophila

We have developed an integrated genetic, genomic, and computational approach to identify and characterize genes involved in myoblast fusion in Drosophila. We first used fluorescence-activated cell sorting to purify mesodermal cells both from wild-type embryos and from 12 variant genotypes in which muscle development is perturbed in known ways. Then, we obtained gene expression profiles for the purified cells by hybridizing isolated mesodermal RNA to Affymetrix GeneChip arrays. These data were subsequently compounded into a statistical metaanalysis that predicts myoblast subtype-specific gene expression signatures that were later validated by in situ hybridization experiments. Finally, we analyzed the myogenic functions of a subset of these myoblast genes using a double-stranded RNA interference assay in living embryos expressing green fluorescent protein under control of a muscle-specific promoter. This experimental strategy led to the identification of several previously uncharacterized genes required for myoblast fusion in Drosophila.

Antagonistic function of Lmd and Zfh1 fine tunes cell fate decisions in the Twi and Tin positive mesoderm of Drosophila melanogaster

In this study we show that cell fate decisions in the dorsal and lateral mesoderm of Drosophila melanogaster depend on the antagonistic action of the Gli-like transcription factor Lame duck (Lmd) and the zinc finger homeodomain factor Zfh1. Lmd expression leads to the reduction of Zfh1 positive cell types, thereby restricting the number of Odd-skipped (Odd) positive and Tinman (Tin) positive pericardial cells in the dorsal mesoderm. In more lateral regions, ectopic activation of Zfh1 or loss of Lmd leads to an excess of adult muscle precursor (AMP) like cells. We also observed that Lmd is co-expressed with Tin in the early dorsal mesoderm and leads to a reduction of Tin expression in cells destined to become dorsal fusion competent myoblasts (FCMs). In the absence of Lmd function, these cells remain Tin positive and develop as Tin positive pericardial cells although they do not express Zfh1. We show further that Tin repression and pericardial restriction in the dorsal mesoderm facilitated by Lmd is instructed by a late Decapentaplegic (Dpp) signal that is abolished in embryos carrying the disk region mutation dpp(d6).

Visualizing new dimensions in Drosophila myoblast fusion

Over several years, genetic studies in the model system, Drosophila melanogastor, have uncovered genes that when mutated, lead to a block in myoblast fusion. Analyses of these gene products have suggested that Arp2/3-mediated regulation of the actin cytoskeleton is crucial to myoblast fusion in the fly. Recent advances in imaging in Drosophila embryos, both in fixed and live preparations, have led to a new appreciation of both the three-dimensional organization of the somatic mesoderm and the cell biology underlying myoblast fusion.

Myoblast fusion in fly and vertebrates: new genes, new processes and new perspectives

Muscle formation and repair depends critically on the fusion of myoblasts. Despite the importance of this process, little is known about the cellular and molecular mechanisms regulating fusion. Forward genetic screens in Drosophila melanogaster have uncovered genes that, when mutated, prevent myoblast fusion. Analyses of these gene products have indicated that the actin cytoskeleton and its regulation play a central role in the fusion process. In this review, we discuss recent advances in the field, including new imaging approaches to analyze fusion as well as a description of novel genes required for fusion. In particular, we highlight what has been learned about the requirement of a specific actin structure at the site of fusion. We also place these findings from Drosophila within the context of myoblast fusion in vertebrates.

Myoblast fusion in Drosophila

Myogenic differentiation in Drosophila melanogaster, as in many other organisms, involves the generation of multinucleate muscle fibers through the fusion of myoblasts. Prior to fusion, the myoblasts become specified as one of two distinct cell types. They then become competent to fuse and express genes associated with cell recognition and adhesion. Initially, cell-type- specific adhesion molecules mediate recognition and fusion between these two distinct populations of myoblasts. Intracellular proteins that are essential for the fusion process are then recruited to points of cell-cell contact at the membrane, where the cell surface molecules have become localized. Many of these cytosolic proteins contribute to reorganization of the cytoskeleton through activation of small guanosine triphosphatases and recruitment of actin nucleating proteins. Following the initial fusion event, the ultimate size of the syncytia is achieved through multiple rounds of fusion between the developing syncytia and mononucleate myoblasts. Ultrastructural changes associated with cell fusion include recruitment of electron-dense vesicles to points of cell-cell contact, resolution of these vesicles into fusion plaques, fusion pore formation, and membrane vesiculation. This chapter reviews our current understanding of the genes, pathways, and ultrastructural events associated with fusion in the Drosophila embryo, giving rise to multinucleate syncytia that will be used throughout larval life.

Expression and functional analysis of a novel Fusion Competent Myoblast specific GAL4 driver

In the Drosophila embryo, body wall muscles are formed by the fusion of two cell types, Founder Cells (FCs) and Fusion Competent Myoblasts (FCMs). Using an enhancer derived from the Dmef2 gene ([C/D]( *)), we report the first GAL4 driver specifically expressed in FCMs. We have determined that this GAL4 driver causes expression in a subset of FCMs and, upon fusion, in developing myotubes from stage 14 onwards. In addition, we have shown that using this Dmef2-5x[C/D]( *)-GAL4 driver to express dominant negative Rac in only FCMs causes a partial fusion block. This novel GAL4 driver will provide a useful reagent to study Drosophila myoblast fusion and muscle differentiation.

3D analysis of founder cell and fusion competent myoblast arrangements outlines a new model of myoblast fusion

Formation of the Drosophila larval body wall muscles requires the specification, coordinated cellular behaviors and fusion of two cell types: Founder Cells (FCs) that control the identity of the individual muscle and Fusion Competent Myoblasts (FCMs) that provide mass. These two cell types come together to control the final size, shape and attachment of individual muscles. However, the spatial arrangement of these cells over time, the sequence of fusion events and the contribution of these cellular relationships to the fusion process have not been addressed. We analyzed the three-dimensional arrangements of FCs and FCMs over the course of myoblast fusion and assayed whether these issues impact the process of myoblast fusion. We examined the timing of the fusion process by analyzing the fusion profile of individual muscles in wild type and fusion mutants. We showed that there are two temporal phases of myoblast fusion in wild type embryos. Limited fusion events occur during the first 3 h of fusion, while the majority of fusion events occur in the remaining 2.5 h. Altogether, our data have led us to propose a new model of myoblast fusion where the frequency of myoblast fusion events may be influenced by the spatial arrangements of FCs and FCMs.

WIP/WASp-based actin-polymerization machinery is essential for myoblast fusion in Drosophila

Formation of syncytial muscle fibers involves repeated rounds of cell fusion between growing myotubes and neighboring myoblasts. We have established that Wsp, the Drosophila homolog of the WASp family of microfilament nucleation-promoting factors, is an essential facilitator of myoblast fusion in Drosophila embryos. D-WIP, a homolog of the conserved Verprolin/WASp Interacting Protein family of WASp-binding proteins, performs a key mediating role in this context. D-WIP, which is expressed specifically in myoblasts, associates with both the WASp-Arp2/3 system and with the myoblast adhesion molecules Dumbfounded and Sticks and Stones, thereby recruiting the actin-polymerization machinery to sites of myoblast attachment and fusion. Our analysis demonstrates that this recruitment is normally required late in the fusion process, for enlargement of nascent fusion pores and breakdown of the apposed cell membranes. These observations identify cellular and developmental roles for the WASp-Arp2/3 pathway, and provide a link between force-generating actin polymerization and cell fusion.

A critical function for the actin cytoskeleton in targeted exocytosis of prefusion vesicles during myoblast fusion

Myoblast fusion is an essential step during muscle differentiation. Previous studies in Drosophila have revealed a signaling pathway that relays the fusion signal from the plasma membrane to the actin cytoskeleton. However, the function for the actin cytoskeleton in myoblast fusion remains unclear. Here we describe the characterization of solitary (sltr), a component of the myoblast fusion signaling cascade. sltr encodes the Drosophila ortholog of the mammalian WASP-interacting protein. Sltr is recruited to sites of fusion by the fusion-competent cell-specific receptor Sns and acts as a positive regulator for actin polymerization at these sites. Electron microscopy analysis suggests that formation of F-actin-enriched foci at sites of fusion is involved in the proper targeting and coating of prefusion vesicles. These studies reveal a surprising cell-type specificity of Sltr-mediated actin polymerization in myoblast fusion, and demonstrate that targeted exocytosis of prefusion vesicles is a critical step prior to plasma membrane fusion.

Hand is a direct target of the forkhead transcription factor Biniou during Drosophila visceral mesoderm differentiation

BACKGROUND

The visceral trunk mesoderm in Drosophila melanogaster develops under inductive signals from the ectoderm. This leads to the activation of the key regulators Tinman, Bagpipe and Biniou that are crucial for specification of the circular visceral muscles. How further differentiation is regulated is widely unknown, therefore it seems to be essential to identify downstream target genes of the early key regulators. In our report we focus on the analysis of the transcriptional control of the highly conserved transcription factor Hand in circular visceral muscle cells, providing evidence that the hand gene is a direct target of Biniou.

RESULTS

Herein we describe the identification of a regulatory region in the hand gene essential and sufficient for the expression in the visceral mesoderm during embryogenesis. We found that hand expression in the circular visceral mesoderm is abolished in embryos mutant for the FoxF domain containing transcription factor Biniou. Furthermore we demonstrate that Biniou regulates hand expression by direct binding to a 300 bp sequence element, located within the 3rd intron of the hand gene. This regulatory element is highly conserved in different Drosophila species. In addition, we provide evidence that Hand is dispensable for the initial differentiation of the embryonic visceral mesoderm.

CONCLUSION

In the present report we show that cross species sequence comparison of non-coding sequences between orthologous genes is a powerful tool to identify conserved regulatory elements. Combining functional dissection experiments in vivo and protein/DNA binding studies we identified hand as a direct target of Biniou in the circular visceral muscles.

Defective decapentaplegic signaling results in heart overgrowth and reduced cardiac output in Drosophila

During germ-band extension, Decapentaplegic (Dpp) signals from the dorsal ectoderm to maintain Tinman (Tin) expression in the underlying mesoderm. This signal specifies the cardiac field, and homologous genes (BMP2/4 and Nkx2.5) perform this function in mammals. We showed previously that a second Dpp signal from the dorsal ectoderm restricts the number of pericardial cells expressing the transcription factor Zfh1. Here we report that, via Zfh1, the second Dpp signal restricts the number of Odd-skipped-expressing and the number of Tin-expressing pericardial cells. Dpp also represses Tin expression independently of Zfh1, implicating a feed-forward mechanism in the regulation of Tin pericardial cell number. In the adjacent dorsal muscles, Dpp has the opposite effect. Dpp maintains Krüppel and Even-skipped expression required for muscle development. Our data show that Dpp refines the cardiac field by limiting the number of pericardial cells. This maintains the boundary between pericardial and dorsal muscle cells and defines the size of the heart. In the absence of the second Dpp signal, pericardial cells overgrow and this significantly reduces larval cardiac output. Our study suggests the existence of a second round of BMP signaling in mammalian heart development and that perhaps defects in this signal play a role in congenital heart defects.

The MARVEL domain protein, Singles Bar, is required for progression past the pre-fusion complex stage of myoblast fusion

Multinucleated myotubes develop by the sequential fusion of individual myoblasts. Using a convergence of genomic and classical genetic approaches, we have discovered a novel gene, singles bar (sing), that is essential for myoblast fusion. sing encodes a small multipass transmembrane protein containing a MARVEL domain, which is found in vertebrate proteins involved in processes such as tight junction formation and vesicle trafficking where--as in myoblast fusion--membrane apposition occurs. sing is expressed in both founder cells and fusion competent myoblasts preceding and during myoblast fusion. Examination of embryos injected with double-stranded sing RNA or embryos homozygous for ethane methyl sulfonate-induced sing alleles revealed an identical phenotype: replacement of multinucleated myofibers by groups of single, myosin-expressing myoblasts at a stage when formation of the mature muscle pattern is complete in wild-type embryos. Unfused sing mutant myoblasts form clusters, suggesting that early recognition and adhesion of these cells are unimpaired. To further investigate this phenotype, we undertook electron microscopic ultrastructural studies of fusing myoblasts in both sing and wild-type embryos. These experiments revealed that more sing mutant myoblasts than wild-type contain pre-fusion complexes, which are characterized by electron-dense vesicles paired on either side of the fusing plasma membranes. In contrast, embryos mutant for another muscle fusion gene, blown fuse (blow), have a normal number of such complexes. Together, these results lead to the hypothesis that sing acts at a step distinct from that of blow, and that sing is required on both founder cell and fusion-competent myoblast membranes to allow progression past the pre-fusion complex stage of myoblast fusion, possibly by mediating fusion of the electron-dense vesicles to the plasma membrane.

Cell-cell fusion

Cell-cell fusion is a highly regulated and dramatic cellular event that is required for development and homeostasis. Fusion may also play a role in the development of cancer and in tissue repair by stem cells. While virus-cell fusion and the fusion of intracellular membranes have been the subject of intense investigation during the past decade, cell-cell fusion remains poorly understood. Given the importance of this cell-biological phenomenon, a number of investigators have begun analyses of the molecular mechanisms that mediate the specialized fusion events of a variety of cell types and species. We discuss recent genetic and biochemical studies that are beginning to yield exciting insights into the fusion mechanisms of Saccharomyces cerevisiae mating pairs, Caenorhabditis elegans epithelial cells and gametes, Drosophila melanogaster and mammalian myoblasts, and mammalian macrophages.

RNA interference screen to identify genes required for Drosophila embryonic nervous system development

RNA interference (RNAi) has been shown to be a powerful method to study the function of genes in vivo by silencing endogenous mRNA with double-stranded (ds) RNA. Previously, we performed in vivo RNAi screening and identified 43 Drosophila genes, including 18 novel genes required for the development of the embryonic nervous system. In the present study, 22 additional genes affecting embryonic nervous system development were found. Novel RNAi-induced phenotypes affecting nervous system development were found for 16 of the 22 genes. Seven of the genes have unknown functions. Other genes found encode transcription factors, a chromatin-remodeling protein, membrane receptors, signaling molecules, and proteins involved in cell adhesion, RNA binding, and ion transport. Human orthologs were identified for proteins encoded by 16 of the genes. The total number of dsRNAs that we have tested for an RNAi-induced phenotype affecting the embryonic nervous system, including our previous study, is 7,312, which corresponds to approximately 50% of the genes in the Drosophila genome.

Parcas, a regulator of non-receptor tyrosine kinase signaling, acts during anterior-posterior patterning and somatic muscle development in Drosophila melanogaster

We have isolated parcas (pcs) in a screen to identify novel regulators of muscle morphogenesis. Pcs is expressed in the ovary and oocyte during oogenesis and again in the embryo, specifically in the developing mesoderm, throughout muscle development. pcs is first required in the ovary during oogenesis for patterning and segmentation of the early Drosophila embryo due primarily to its role in the regulation of Oskar (Osk) levels. In addition to the general patterning defects observed in embryos lacking maternal contribution of pcs, these embryos show defects in Wingless (Wg) expression, causing losses of Wg-dependent cell types within the affected segment. pcs activity is required again later during embryogenesis in the developing mesoderm for muscle development. Loss and gain of function studies demonstrate that pcs is necessary at distinct times for muscle specification and morphogenesis. Pcs is predicted to be a novel regulator of non-receptor tyrosine kinase (NRTK) signaling. We have identified one target of Pcs regulation, the Drosophila Tec kinase Btk29A. While Btk29A appears to be regulated by Pcs during its early role in patterning and segmentation, it does not appear to be a major target of Pcs regulation during muscle development. We propose that Pcs fulfils its distinct roles during development by the regulation of multiple NRTKs.

Distinct posttranscriptional mechanisms regulate the activity of the Zn finger transcription factor lame duck during Drosophila myogenesis

Skeletal muscle formation in Drosophila melanogaster requires two types of myoblasts, muscle founders and fusion-competent myoblasts. Lame duck (Lmd), a member of the Gli superfamily of transcription factors, is essential for the specification and differentiation of fusion-competent myoblasts. We report herein that appropriate levels of active Lmd protein are attained by a combination of posttranscriptional mechanisms. We provide evidence that two different regions of the Lmd protein are critical for modulating the balance between its nuclear translocation and its retention within the cytoplasm. Activation of the Lmd protein is also tempered by posttranslational modifications of the protein that do not detectably change its subcellular localization. We further show that overexpression of Lmd protein derivatives that are constitutively nuclear or hyperactive results in severe muscle defects. These findings underscore the importance of regulated Lmd protein activity in maintaining proper activation of downstream target genes, such as Mef2, within fusion-competent myoblasts.

An integrated strategy for analyzing the unique developmental programs of different myoblast subtypes

An important but largely unmet challenge in understanding the mechanisms that govern the formation of specific organs is to decipher the complex and dynamic genetic programs exhibited by the diversity of cell types within the tissue of interest. Here, we use an integrated genetic, genomic, and computational strategy to comprehensively determine the molecular identities of distinct myoblast subpopulations within the Drosophila embryonic mesoderm at the time that cell fates are initially specified. A compendium of gene expression profiles was generated for primary mesodermal cells purified by flow cytometry from appropriately staged wild-type embryos and from 12 genotypes in which myogenesis was selectively and predictably perturbed. A statistical meta-analysis of these pooled datasets--based on expected trends in gene expression and on the relative contribution of each genotype to the detection of known muscle genes--provisionally assigned hundreds of differentially expressed genes to particular myoblast subtypes. Whole embryo in situ hybridizations were then used to validate the majority of these predictions, thereby enabling true-positive detection rates to be estimated for the microarray data. This combined analysis reveals that myoblasts exhibit much greater gene expression heterogeneity and overall complexity than was previously appreciated. Moreover, it implicates the involvement of large numbers of uncharacterized, differentially expressed genes in myogenic specification and subsequent morphogenesis. These findings also underscore a requirement for considerable regulatory specificity for generating diverse myoblast identities. Finally, to illustrate how the developmental functions of newly identified myoblast genes can be efficiently surveyed, a rapid RNA interference assay that can be scored in living embryos was developed and applied to selected genes. This integrated strategy for examining embryonic gene expression and function provides a substantially expanded framework for further studies of this model developmental system.

Single-minded, Dmef2, Pointed, and Su(H) act on identified regulatory sequences of the roughest gene in Drosophila melanogaster

Roughest (Rst) is a cell adhesion molecule of the immunoglobulin superfamily that has multiple and diverse functions during the development of Drosophila melanogaster. The pleiotropic action of Rst is reflected by its complex and dynamic expression during the development of Drosophila. By an enhancer detection screen, we previously identified several cis-regulatory modules that mediate specific expression of the roughest gene in Drosophila developmental processes. To identify trans-regulators of rst expression, we used the Gal4/UAS system to screen for factors that were sufficient to activate Rst expression when ectopically expressed. By this method we identified the transcription factors Single-minded, Pointed.P1, and Su(H)-VP16. Furthermore, we showed that these factors and, in addition, Dmef2 are able to ectopically activate rst expression via the previously described rst cis-regulatory modules. This fact and the use of mutant analysis allocates the action of the transcription factors to specific developmental contexts. In the case of Sim, we could show that it regulates rst expression in the embryonic midline, but not in the optic lobes. Mutagenesis of Sim consensus binding sites in the regulatory module required for rst expression in the embryonic midline, abolished rst expression; indicating that the regulation of rst by Sim is direct.

A positive feedback loop between Dumbfounded and Rolling pebbles leads to myotube enlargement in Drosophila

In Drosophila, myoblasts are subdivided into founders and fusion-competent myoblasts (fcm) with myotubes forming through fusion of one founder and several fcm. Duf and rolling pebbles 7 (Rols7; also known as antisocial) are expressed in founders, whereas sticks and stones (SNS) is present in fcm. Duf attracts fcm toward founders and also causes translocation of Rols7 from the cytoplasm to the fusion site. We show that Duf is a type 1 transmembrane protein that induces Rols7 translocation specifically when present intact and engaged in homophilic or Duf–SNS adhesion. Although its membrane-anchored extracellular domain functions as an attractant and is sufficient for the initial round of fusion, subsequent fusions require replenishment of Duf through cotranslocation with Rols7 tetratricopeptide repeat/coiled-coil domain-containing vesicles to the founder/myotube surface, causing both Duf and Rols7 to be at fusion sites between founders/myotubes and fcm. This implicates the Duf–Rols7 positive feedback loop to the occurrence of fusion at specific sites along the membrane and provides a mechanism by which the rate of fusion is controlled.

Towards a molecular pathway for myoblast fusion in Drosophila

Intercellular fusion among myoblasts is required for the generation of multinucleated muscle fibers during skeletal muscle development. Recent studies in Drosophila have shed light on the molecular mechanisms that underlie this process, and a signaling pathway that relays fusion signals from the cell membrane to the cytoskeleton has emerged. In this article, we review these recent advances and discuss how Drosophila offers a powerful model system to study myoblast fusion in vivo.

Transcription of Drosophila troponin I gene is regulated by two conserved, functionally identical, synergistic elements

The Drosophila wings-up A gene encodes Troponin I. Two regions, located upstream of the transcription initiation site (upstream regulatory element) and in the first intron (intron regulatory element), regulate gene expression in specific developmental and muscle type domains. Based on LacZ reporter expression in transgenic lines, upstream regulatory element and intron regulatory element yield identical expression patterns. Both elements are required for full expression levels in vivo as indicated by quantitative reverse transcription-polymerase chain reaction assays. Three myocyte enhancer factor-2 binding sites have been functionally characterized in each regulatory element. Using exon specific probes, we show that transvection is based on transcriptional changes in the homologous chromosome and that Zeste and Suppressor of Zeste 3 gene products act as repressors for wings-up A. Critical regions for transvection and for Zeste effects are defined near the transcription initiation site. After in silico analysis in insects (Anopheles and Drosophila pseudoobscura) and vertebrates (Ratus and Coturnix), the regulatory organization of Drosophila seems to be conserved. Troponin I (TnI) is expressed before muscle progenitors begin to fuse, and sarcomere morphogenesis is affected by TnI depletion as Z discs fail to form, revealing a novel developmental role for the protein or its transcripts. Also, abnormal stoichiometry among TnI isoforms, rather than their absolute levels, seems to cause the functional muscle defects.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Myoblast fusion in Drosophila

Somatic muscle formation is an unusual process as it requires the cells involved, the myoblasts, to relinquish their individual state and fuse with one another to form a syncitial muscle fiber. The potential use of myoblast fusion therapies to rebuild damaged muscles has generated continuing interest in elucidating the molecular basis of the fusion process. Yet, until recently, few of the molecular players involved in this process had been identified. Now, however, it has been possible to couple a detailed understanding of the cellular basis of the fusion process with powerful classical and molecular genetic strategies in the Drosophila embryo. We review the cellular studies, and the recent genetic and biochemical analyses that uncovered interacting extracellular molecules present on fusing myoblasts and the intracellular effectors that facilitate fusion. With the conservation of proteins and protein functions across species, it is likely that these findings in Drosophila will benefit understanding of the myoblast fusion process in higher organisms.

Muscle differentiation: how two cells become one

A key feature of myogenesis is the fusion of myoblasts to form multinucleate myotubes. Recent work in Drosophila has uncovered a collection of genes that operate at different stages of this process. Some interactions between them have been described that begin to define links from outside the cell via the plasma membrane to the cytoskeleton. Future studies will establish the extent to which the molecular mechanisms of myoblast fusion are conserved between Drosophila and other animals, as found in other aspects of myogenesis.