Analysis of the cell adhesion molecule sticks-and-stones reveals multiple redundant functional domains, protein-interaction motifs and phosphorylated tyrosines that direct myoblast fusion in Drosophila melanogaster

Inter-organ steroid hormone signaling promotes myoblast fusion via direct transcriptional regulation of a single key effector gene

Steroid hormones regulate tissue development and physiology by modulating the transcription of a broad spectrum of genes. In insects, the principal steroid hormones, ecdysteroids, trigger the expression of thousands of genes through a cascade of transcription factors (TFs) to coordinate developmental transitions such as larval molting and metamorphosis. However, whether ecdysteroid signaling can bypass transcriptional hierarchies to exert its function in individual developmental processes is unclear. Here, we report that a single non-TF effector gene mediates the transcriptional output of ecdysteroid signaling in Drosophila myoblast fusion, a critical step in muscle development and differentiation. Specifically, we show that the 20-hydroxyecdysone (commonly referred to as "ecdysone") secreted from an extraembryonic tissue, amnioserosa, acts on embryonic muscle cells to directly activate the expression of antisocial (ants), which encodes an essential scaffold protein enriched at the fusogenic synapse. Not only is ants transcription directly regulated by the heterodimeric ecdysone receptor complex composed of ecdysone receptor (EcR) and ultraspiracle (USP) via ecdysone-response elements but also more strikingly, expression of ants alone is sufficient to rescue the myoblast fusion defect in ecdysone signaling-deficient mutants. We further show that EcR/USP and a muscle-specific TF Twist synergistically activate ants expression in vitro and in vivo. Taken together, our study provides the first example of a steroid hormone directly activating the expression of a single key non-TF effector gene to regulate a developmental process via inter-organ signaling and provides a new paradigm for understanding steroid hormone signaling in other developmental and physiological processes.

Prediction of interactions between cell surface proteins by machine learning

Cells detect changes in their external environments or communicate with each other through proteins on their surfaces. These cell surface proteins form a complicated network of interactions in order to fulfill their functions. The interactions between cell surface proteins are highly dynamic and, thus, challenging to detect using traditional experimental techniques. Here, we tackle this challenge using a computational framework. The primary focus of the framework is to develop new tools to identify interactions between domains in the immunoglobulin (Ig) fold, which is the most abundant domain family in cell surface proteins. These interactions could be formed between ligands and receptors from different cells or between proteins on the same cell surface. In practice, we collected all structural data on Ig domain interactions and transformed them into an interface fragment pair library. A high-dimensional profile can then be constructed from the library for a given pair of query protein sequences. Multiple machine learning models were used to read this profile so that the probability of interaction between the query proteins could be predicted. We tested our models on an experimentally derived dataset that contains 564 cell surface proteins in humans. The cross-validation results show that we can achieve higher than 70% accuracy in identifying the PPIs within this dataset. We then applied this method to a group of 46 cell surface proteins in Caenorhabditis elegans. We screened every possible interaction between these proteins. Many interactions recognized by our machine learning classifiers have been experimentally confirmed in the literature. In conclusion, our computational platform serves as a useful tool to help identify potential new interactions between cell surface proteins in addition to current state-of-the-art experimental techniques. The tool is freely accessible for use by the scientific community. Moreover, the general framework of the machine learning classification can also be extended to study the interactions of proteins in other domain superfamilies.

Germline and Somatic Cell Syncytia in Insects

Syncytia are common in the animal and plant kingdoms both under normal and pathological conditions. They form through cell fusion or division of a founder cell without cytokinesis. A particular type of syncytia occurs in invertebrate and vertebrate gametogenesis when the founder cell divides several times with partial cytokinesis producing a cyst (nest) of germ line cells connected by cytoplasmic bridges. The ultimate destiny of the cyst's cells differs between animal groups. Either all cells of the cyst become the gametes or some cells endoreplicate or polyploidize to become the nurse cells (trophocytes). Although many types of syncytia are permanent, the germ cell syncytium is temporary, and eventually, it separates into individual gametes. In this chapter, we give an overview of syncytium types and focus on the germline and somatic cell syncytia in various groups of insects. We also describe the multinuclear giant cells, which form through repetitive nuclear divisions and cytoplasm hypertrophy, but without cell fusion, and the accessory nuclei, which bud off the oocyte nucleus, migrate to its cortex and become included in the early embryonic syncytium.

Prediction of Interactions between Cell Surface Proteins by Machine Learning

Cells detect changes of external environments or communicate with each other through proteins on their surfaces. These cell surface proteins form a complicated network of interactions in order to fulfill their functions. The interactions between cell surface proteins are highly dynamic and thus challenging to detect using traditional experimental techniques. Here we tackle this challenge by a computational framework. The primary focus of the framework is to develop new tools to identify interactions between domains in immunoglobulin (Ig) fold, which is the most abundant domain family in cell surface proteins. These interactions could be formed between ligands and receptors from different cells, or between proteins on the same cell surface. In practice, we collected all structural data of Ig domain interactions and transformed them into an interface fragment pair library. A high dimensional profile can be then constructed from the library for a given pair of query protein sequences. Multiple machine learning models were used to read this profile, so that the probability of interaction between the query proteins can be predicted. We tested our models to an experimentally derived dataset which contains 564 cell surface proteins in human. The cross-validation results show that we can achieve higher than 70% accuracy in identifying the PPIs within this dataset. We then applied this method to a group of 46 cell surface proteins in C elegans. We screened every possible interaction between these proteins. Many interactions recognized by our machine learning classifiers have been experimentally confirmed in the literatures. In conclusion, our computational platform serves a useful tool to help identifying potential new interactions between cell surface proteins in addition to current state-of-the-art experimental techniques. The tool is freely accessible for use by the scientific community. Moreover, the general framework of the machine learning classification can also be extended to study interactions of proteins in other domain superfamilies.

Phosphorylation of the Myogenic Factor Myocyte Enhancer Factor-2 Impacts Myogenesis In Vivo

Activity of the myogenic regulatory protein myocyte enhancer factor-2 (MEF2) is modulated by post-translational modification. We investigated the in vivo phosphorylation of Drosophila MEF2, and identified serine 98 (S98) as a phosphorylated residue. Phospho-mimetic (S98E) and phospho-null (S98A) isoforms of MEF2 did not differ from wild-type in their activity in vitro, so we used CRISPR/Cas9 to generate an S98A allele of the endogenous gene. In mutant larvae we observed phenotypes characteristic of reduced MEF2 function, including reduced body wall muscle size and reduced expression of myofibrillar protein genes; conversely,S98A homozygotes showed enhanced MEF2 function through muscle differentiation within the adult myoblasts associated with the wing imaginal disc. In adults, S98A homozygotes were viable with normal mobility, yet showed patterning defects in muscles that were enhanced when the S98A allele was combined with a Mef2 null allele. Overall our data indicate that blocking MEF2 S98 phosphorylation in myoblasts enhances its myogenic capability, whereas blocking S98 phosphorylation in differentiating muscles attenuates MEF2 function. Our studies are among the first to assess the functional significance of MEF2 phosphorylation sites in the intact animal, and suggest that the same modification can have profoundly different effects upon MEF2 function depending upon the developmental context.

Modeling of ACTN4-Based Podocytopathy Using Drosophila Nephrocytes

Introduction

Genetic disorders are among the most prevalent causes leading to progressive glomerular disease and, ultimately, end-stage renal disease (ESRD) in children and adolescents. Identification of underlying genetic causes is indispensable for targeted treatment strategies and counseling of affected patients and their families.

Methods

Here, we report on a boy who presented at 4 years of age with proteinuria and biopsy-proven focal segmental glomerulosclerosis (FSGS) that was temporarily responsive to treatment with ciclosporin A. Molecular genetic testing identified a novel mutation in alpha-actinin-4 (p.M240T). We describe a feasible and efficient experimental approach to test its pathogenicity by combining in silico, in vitro, and in vivo analyses.

Results

The de novo p.M240T mutation led to decreased alpha-actinin-4 stability as well as protein mislocalization and actin cytoskeleton rearrangements. Transgenic expression of wild-type human alpha-actinin-4 in Drosophila melanogaster nephrocytes was able to ameliorate phenotypes associated with the knockdown of endogenous actinin. In contrast, p.M240T, as well as other established disease variants p.W59R and p.K255E, failed to rescue these phenotypes, underlining the pathogenicity of the novel alpha-actinin-4 variant.

Conclusion

Our data highlight that the newly identified alpha-actinin-4 mutation indeed encodes for a disease-causing variant of the protein and promote the Drosophila model as a simple and convenient tool to study monogenic kidney disease in vivo.

Sticks and Stones, a conserved cell surface ligand for the Type IIa RPTP Lar, regulates neural circuit wiring in Drosophila

Type IIa receptor-like protein tyrosine phosphatases (RPTPs) are essential for neural development. They have cell adhesion molecule (CAM)-like extracellular domains that interact with cell-surface ligands and coreceptors. We identified the immunoglobulin superfamily CAM Sticks and Stones (Sns) as a new partner for the Drosophila Type IIa RPTP Lar. Lar and Sns bind to each other in embryos and in vitro, and the human Sns ortholog, Nephrin, binds to human Type IIa RPTPs. Genetic analysis shows that Lar and Sns function together to regulate larval neuromuscular junction development, axon guidance in the mushroom body (MB), and innervation of the optic lobe (OL) medulla by R7 photoreceptors. In the neuromuscular system, Lar and Sns are both required in motor neurons, and may function as coreceptors. In the MB and OL, however, the relevant Lar-Sns interactions are in trans (between neurons), so Sns functions as a Lar ligand in these systems.

Rabphilin silencing causes dilated cardiomyopathy in a Drosophila model of nephrocyte damage

Heart failure (HF) and the development of chronic kidney disease (CKD) have a direct association. Both can be cause and consequence of the other. Many factors are known, such as diabetes or hypertension, which can lead to the appearance and/or development of these two conditions. However, it is suspected that other factors, namely genetic ones, may explain the differences in the manifestation and progression of HF and CKD among patients. One candidate factor is Rph, a gene expressed in the nervous and excretory system in mammals and Drosophila, encoding a Rab small GTPase family effector protein implicated in vesicular trafficking. We found that Rph is expressed in the Drosophila heart, and the silencing of Rph gene expression in this organ had a strong impact in the organization of fibers and functional cardiac parameters. Specifically, we observed a significant increase in diastolic and systolic diameters of the heart tube, which is a phenotype that resembles dilated cardiomyopathy in humans. Importantly, we also show that silencing of Rabphilin (Rph) expression exclusively in the pericardial nephrocytes, which are part of the flies' excretory system, brings about a non-cell-autonomous effect on the Drosophila cardiac system. In summary, in this work, we demonstrate the importance of Rph in the fly cardiac system and how silencing Rph expression in nephrocytes affects the Drosophila cardiac system.

Reverse-engineering forces responsible for dynamic clustering and spreading of multiple nuclei in developing muscle cells

How cells position their organelles is a fundamental biological question. During Drosophila embryonic muscle development, multiple nuclei transition from being clustered together to splitting into two smaller clusters to spreading along the myotube’s length. Perturbations of microtubules and motor proteins disrupt this sequence of events. These perturbations do not allow intuiting which molecular forces govern the nuclear positioning; we therefore used computational screening to reverse-engineer and identify these forces. The screen reveals three models. Two suggest that the initial clustering is due to nuclear repulsion from the cell poles, while the third, most robust, model poses that this clustering is due to a short-ranged internuclear attraction. All three models suggest that the nuclear spreading is due to long-ranged internuclear repulsion. We test the robust model quantitatively by comparing it with data from perturbed muscle cells. We also test the model using agent-based simulations with elastic dynamic microtubules and molecular motors. The model predicts that, in longer mammalian myotubes with a large number of nuclei, the spreading stage would be preceded by segregation of the nuclei into a large number of clusters, proportional to the myotube length, with a small average number of nuclei per cluster.

Dynamin regulates the dynamics and mechanical strength of the actin cytoskeleton as a multifilament actin-bundling protein

The dynamin GTPase is known to bundle actin filaments, but the underlying molecular mechanism and physiological relevance remain unclear. Our genetic analyses revealed a function of dynamin in propelling invasive membrane protrusions during myoblast fusion in vivo. Using biochemistry, total internal reflection fluorescence microscopy, electron microscopy and cryo-electron tomography, we show that dynamin bundles actin while forming a helical structure. At its full capacity, each dynamin helix captures 12-16 actin filaments on the outer rim of the helix. GTP hydrolysis by dynamin triggers disassembly of fully assembled dynamin helices, releasing free dynamin dimers/tetramers and facilitating Arp2/3-mediated branched actin polymerization. The assembly/disassembly cycles of dynamin promote continuous actin bundling to generate mechanically stiff actin super-bundles. Super-resolution and immunogold platinum replica electron microscopy revealed dynamin along actin bundles at the fusogenic synapse. These findings implicate dynamin as a unique multifilament actin-bundling protein that regulates the dynamics and mechanical strength of the actin cytoskeletal network.

Drosophila melanogaster and its nephrocytes: A versatile model for glomerular research

Glomerular disorders are a predominant cause of chronic kidney diseases and end-stage renal failure. Especially podocytes, epithelial cells which represent the outermost part of the filtration barrier, are affected by disease and experience a gradual loss of function. Despite recent advances in identifying potential pathways underlying podocyte injury, treatment remains challenging. It is therefore desirable to employ suitable model organisms in order to study glomerular disease and elucidate affected pathways. Due to its diverse ways of genetic manipulation and high genomic conservation, Drosophila melanogaster is a powerful model organism for biomedical research. The fly was recently used to assess podocytopathies by exploiting the nephrocyte system. Nephrocytes are spherical cells within the body cavity of the fly responsible for detoxification and clearance of unwanted substances. More importantly, they share many characteristics with mammalian podocytes. Here, we summarize how to use Drosophila as a model organism for podocyte research. We discuss examples of techniques that can be used to genetically manipulate nephrocytes and provide protocols for nephrocyte isolation and for morphological as well as functional analysis.

RNA Interference Screening for Genes Regulating Drosophila Muscle Morphogenesis

RNA interference (RNAi) is the method of choice to systematically test for gene function in an intact organism. The model organism Drosophila has the advantage that RNAi is cell autonomous, meaning it does not spread from one cell to the next. Hence, RNAi can be performed in a tissue-specific manner by expressing short or long inverted repeat constructs (hairpins) designed to target mRNAs from one specific target gene. This achieves tissue-specific knock-down of a target gene of choice. Here, we detail the methodology to test gene function in Drosophila muscle tissue by expressing hairpins in a muscle-specific manner using the GAL4-UAS system. We further discuss the systematic RNAi resource collections available which also permit large scale screens in a muscle-specific manner. The full power of such screens is revealed by combination of high-throughput assays followed by detailed morphological assays. Together, this chapter should be a practical guide to enable the reader to either test a few candidate genes, or large gene sets for particular functions in Drosophila muscle tissue and provide first insights into the biological process the gene might be important for in muscle.

Spectrin is a mechanoresponsive protein shaping fusogenic synapse architecture during myoblast fusion

Spectrin is a membrane skeletal protein best known for its structural role in maintaining cell shape and protecting cells from mechanical damage. Here, we report that α/βH-spectrin (βH is also called karst) dynamically accumulates and dissolves at the fusogenic synapse between fusing Drosophila muscle cells, where an attacking fusion partner invades its receiving partner with actin-propelled protrusions to promote cell fusion. Using genetics, cell biology, biophysics and mathematical modelling, we demonstrate that spectrin exhibits a mechanosensitive accumulation in response to shear deformation, which is highly elevated at the fusogenic synapse. The transiently accumulated spectrin network functions as a cellular fence to restrict the diffusion of cell-adhesion molecules and a cellular sieve to constrict the invasive protrusions, thereby increasing the mechanical tension of the fusogenic synapse to promote cell membrane fusion. Our study reveals a function of spectrin as a mechanoresponsive protein and has general implications for understanding spectrin function in dynamic cellular processes.

Structural Characterization and Function Prediction of Immunoglobulin-like Fold in Cell Adhesion and Cell Signaling

Domains that belong to an immunoglobulin (Ig) fold are extremely abundant in cell surface receptors, which play significant roles in cell-cell adhesion and signaling. Although the structures of domains in an Ig fold share common topology of β-barrels, functions of receptors in adhesion and signaling are regulated by the very heterogeneous binding between these domains. Additionally, only a small number of domains are directly involved in the binding between two multidomain receptors. It is challenging and time consuming to experimentally detect the binding partners of a given receptor and further determine which specific domains in this receptor are responsible for binding. Therefore, current knowledge in the binding mechanism of Ig-fold domains and their impacts on cell adhesion and signaling is very limited. A bioinformatics study can shed light on this topic from a systematic point of view. However, there is so far no computational analysis on the structural and functional characteristics of the entire Ig fold. We constructed nonredundant structural data sets for all domains in Ig fold, depending on their functions in cell adhesion and signaling. We found that data sets of domains in adhesion receptors show different binding preference from domains in signaling receptors. Using structural alignment, we further built a common structural template for each group of a domain data set. By mapping the protein-protein binding interface of each domain in a group onto the surface of its structural template, we found binding interfaces are highly overlapped within each specific group. These overlapped interfaces, we called consensus binding interfaces, are distinguishable among different data sets of domains. Finally, the residue compositions on the consensus interfaces were used as indicators for multiple machine learning algorithms to predict if they can form homotypic interactions with each other. The overall performance of the cross-validation tests shows that our prediction accuracies ranged between 0.6 and 0.8.

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.

Understanding the Functional Roles of Multiple Extracellular Domains in Cell Adhesion Molecules with a Coarse-Grained Model

Intercellular contacts in multicellular organisms are maintained by membrane receptors called cell adhesion molecules (CAMs), which are expressed on cell surfaces. One interesting feature of CAMs is that almost all of their extracellular regions contain repeating copies of structural domains. It is not clear why so many extracellular domains need to be evolved through natural selection. We tackled this problem by computational modeling. A generic model of CAMs was constructed based on the domain organization of neuronal CAM, which is engaged in maintaining neuron-neuron adhesion in central nervous system. By placing these models on a cell-cell interface, we developed a Monte-Carlo simulation algorithm that incorporates both molecular factors including conformational changes of CAMs and cellular factor including fluctuations of plasma membranes to approach the physical process of CAM-mediated adhesion. We found that the presence of multiple domains at the extracellular region of a CAM plays a positive role in regulating its trans-interaction with other CAMs from the opposite side of cell surfaces. The trans-interaction can further be facilitated by the intramolecular contacts between different extracellular domains of a CAM. Finally, if more than one CAM is introduced on each side of cell surfaces, the lateral binding (cis-interactions) between these CAMs will positively correlate with their trans-interactions only within a small energetic range, suggesting that cell adhesion is an elaborately designed process in which both trans- and cis-interactions are fine-tuned collectively by natural selection. In short, this study deepens our general understanding of the molecular mechanisms of cell adhesion.

Identification of the essential protein domains for Mib2 function during the development of the Drosophila larval musculature and adult flight muscles

The proper differentiation and maintenance of myofibers is fundamental to a functional musculature. Disruption of numerous mostly structural factors leads to perturbations of these processes. Among the limited number of known regulatory factors for these processes is Mind bomb2 (Mib2), a muscle-associated E3 ubiquitin ligase, which was previously established to be required for maintaining the integrity of larval muscles. In this study, we have examined the mechanistic aspects of Mib2 function by performing a detailed functional dissection of the Mib2 protein. We show that the ankyrin repeats, in its entirety, and the hitherto uncharacterized Mib-specific domains (MIB), are important for the major function of Mib2 in skeletal and visceral muscles in the Drosophila embryo. Furthermore, we characterize novel mib2 alleles that have arisen from a forward genetic screen aimed at identifying regulators of myogenesis. Two of these alleles are viable, but flightless hypomorphic mib2 mutants, and harbor missense mutations in the MIB domain and RING finger, respectively. Functional analysis of these new alleles, including in vivo imaging, demonstrates that Mib2 plays an additional important role in the development of adult thorax muscles, particularly in maintaining the larval templates for the dorsal longitudinal indirect flight muscles during metamorphosis.

Cardiomyocyte Regulation of Systemic Lipid Metabolism by the Apolipoprotein B-Containing Lipoproteins in Drosophila

The heart has emerged as an important organ in the regulation of systemic lipid homeostasis; however, the underlying mechanism remains poorly understood. Here, we show that Drosophila cardiomyocytes regulate systemic lipid metabolism by producing apolipoprotein B-containing lipoproteins (apoB-lipoproteins), essential lipid carriers that are so far known to be generated only in the fat body. In a Drosophila genetic screen, we discovered that when haplo-insufficient, microsomal triglyceride transfer protein (mtp), required for the biosynthesis of apoB-lipoproteins, suppressed the development of diet-induced obesity. Tissue-specific inhibition of Mtp revealed that whereas knockdown of mtp only in the fat body decreases systemic triglyceride (TG) content on normal food diet (NFD) as expected, knockdown of mtp only in the cardiomyocytes also equally decreases systemic TG content on NFD, suggesting that the cardiomyocyte- and fat body-derived apoB-lipoproteins serve similarly important roles in regulating whole-body lipid metabolism. Unexpectedly, on high fat diet (HFD), knockdown of mtp in the cardiomyocytes, but not in fat body, protects against the gain in systemic TG levels. We further showed that inhibition of the Drosophila apoB homologue, apolipophorin or apoLpp, another gene essential for apoB-lipoprotein biosynthesis, affects systemic TG levels similarly to that of Mtp inhibition in the cardiomyocytes on NFD or HFD. Finally, we determined that HFD differentially alters Mtp and apoLpp expression in the cardiomyocytes versus the fat body, culminating in higher Mtp and apoLpp levels in the cardiomyocytes than in fat body and possibly underlying the predominant role of cardiomyocyte-derived apoB-lipoproteins in lipid metabolic regulation. Our findings reveal a novel and significant function of heart-mediated apoB-lipoproteins in controlling lipid homeostasis.

The heart is increasingly recognized to serve an important role in the regulation of whole-body lipid homeostasis; however, the underlying mechanisms remained poorly understood. Here, our study in Drosophila reveals that cardiomyocytes regulate systemic lipid metabolism by producing apolipoprotein B-containing lipoproteins (apoB-lipoproteins), essential lipid carriers that are so far known to be generated only in the fat body (insect liver and adipose tissue). We found that apoB-lipoproteins generated by the Drosophila cardiomyocytes serve an equally significant role as their fat body-derived counterparts in maintaining systemic lipid homeostasis on normal food diet. Importantly, on high fat diet (HFD), the cardiomyocyte-derived apoB-lipoproteins are the major determinants of whole-body lipid metabolism, a role which could be attributed to the HFD-induced up-regulation of apoB-lipoprotein biosynthesis genes in the cardiomyocytes and their down-regulation in the fat body. Taken together, our results reveal that apoB-lipoproteins are new players in mediating the heart control of lipid metabolism, and provide first evidence supporting the notion that HFD-induced differential regulation of apoB-lipoprotein biosynthesis genes could alter the input of different tissue-derived apoB-lipoproteins in systemic lipid metabolic control.

Using the Drosophila Nephrocyte to Model Podocyte Function and Disease

Glomerular disorders are a major cause of end-stage renal disease and effective therapies are often lacking. Nephrocytes are considered to be part of the Drosophila excretory system and form slit diaphragms across cellular membrane invaginations. Nehphrocytes have been shown to share functional, morphological, and molecular features with podocytes, which form the glomerular filter in vertebrates. Here, we report the progress and the evolving tool-set of this model system. Combining a functional, accessible slit diaphragm with the power of the genetic tool-kit in Drosophila, the nephrocyte has the potential to greatly advance our understanding of the glomerular filtration barrier in health and disease.

KirrelL, a member of the Ig-domain superfamily of adhesion proteins, is essential for fusion of primary mesenchyme cells in the sea urchin embryo

In the sea urchin embryo, primary mesenchyme cells (PMCs) adhere to one another and fuse via filopodia, forming cable-like structures within which skeletal rods are deposited. Although this process was first described more than a century ago, molecules that participate in PMC adhesion and fusion have not been identified. Here we show that KirrelL, a PMC-specific, Ig domain-containing transmembrane protein, is essential for PMC fusion, probably by mediating filopodial adhesions that are a pre-requisite for subsequent membrane fusion. We show that KirrelL is not required for PMC specification, migration, or for direct filopodial contacts between PMCs. In the absence of KirrelL, however, filopodial contacts do not result in fusion. kirrelL is a member of a family of closely related, intronless genes that likely arose through an echinoid-specific gene expansion, possibly via retrotransposition. Our findings are significant in that they establish a direct linkage between the transcriptional network deployed in the PMC lineage and an effector molecule required for a critically important PMC morphogenetic process. In addition, our results point to a conserved role for Ig domain-containing adhesion proteins in facilitating cell fusion in both muscle and non-muscle cell lineages during animal development.

Drosophila FoxL1 non-autonomously coordinates organ placement during embryonic development

Determining how organs attain precise positioning within an organism is a crucial facet of developmental biology. The Fox family winged-helix transcription factors are known to play key roles in development of multiple organs. Drosophila FoxL1 (aka Fd64A) is dynamically expressed in embryos but its function is completely uncharacterized. FoxL1 is expressed in a single group of body wall - muscles in the 2nd and 3rd thoracic segments, in homologous abdominal muscles at earlier stages, and in the hindgut mesoderm from early through late embryogenesis. We show that FoxL1 expression in T2 and T3 is in VIS5, which is not a single muscle spanning the entire thorax, as previously published, but is, instead, three individual muscles, each spanning a single thoracic segment. We generate mutations in foxL1 and show that, surprisingly, none of the tissues that express FoxL1 are affected by its loss. Instead, loss of foxL1 results in defects in salivary gland positioning and morphology, as well as defects in the migration of hemocytes, germ cells and Malpighian tubules. We also show that FoxL1-dependent expression of secreted Sema2a in T3 VIS5 is required for normal salivary gland positioning. Altogether, these findings suggest that Drosophila FoxL1 functions like its mammalian counterpart in non-autonomously orchestrating the behaviors of surrounding tissues.

Drosophila Kette coordinates myoblast junction dissolution and the ratio of Scar-to-WASp during myoblast fusion

The fusion of founder cells and fusion-competent myoblasts (FCMs) is crucial for muscle formation in Drosophila. Characteristic events of myoblast fusion include the recognition and adhesion of myoblasts, and the formation of branched F-actin by the Arp2/3 complex at the site of cell–cell contact. At the ultrastructural level, these events are reflected by the appearance of finger-like protrusions and electron-dense plaques that appear prior to fusion. Severe defects in myoblast fusion are caused by the loss of Kette (a homolog of Nap1 and Hem-2, also known as NCKAP1 and NCKAP1L, respectively), a member of the regulatory complex formed by Scar or WAVE proteins (represented by the single protein, Scar, in flies). kette mutants form finger-like protrusions, but the electron-dense plaques are extended. Here, we show that the electron-dense plaques in wild-type and kette mutant myoblasts resemble other electron-dense structures that are known to function as cellular junctions. Furthermore, analysis of double mutants and attempts to rescue the kette mutant phenotype with N-cadherin, wasp and genes of members of the regulatory Scar complex revealed that Kette has two functions during myoblast fusion. First, Kette controls the dissolution of electron-dense plaques. Second, Kette controls the ratio of the Arp2/3 activators Scar and WASp in FCMs.

Summary: The Drosophila protein Kette is essential for myoblast fusion. It controls the dissolution of electron-dense plaques and the ratio of Scar and WASp proteins in fusion-competent myoblasts during fusion pore formation.

Imaging Approaches to Investigate Myonuclear Positioning in Drosophila

In the skeletal muscle, nuclei are positioned at the periphery of each myofiber and are evenly distributed along its length. Improper positioning of myonuclei has been correlated with muscle disease and decreased muscle function. Several mechanisms required for regulating nuclear position have been identified using the fruit fly, Drosophila melanogaster. The conservation of the myofiber between the fly and vertebrates, the availability of advanced genetic tools, and the ability to visualize dynamic processes using fluorescent proteins in vivo makes the fly an excellent system to study myonuclear positioning. This chapter describes time-lapse and fixed imaging methodologies using both the Drosophila embryo and the larva to investigate mechanisms of myonuclear positioning.

Live imaging provides new insights on dynamic F-actin filopodia and differential endocytosis during myoblast fusion in Drosophila

The process of myogenesis includes the recognition, adhesion, and fusion of committed myoblasts into multinucleate syncytia. In the larval body wall muscles of Drosophila, this elaborate process is initiated by Founder Cells and Fusion-Competent Myoblasts (FCMs), and cell adhesion molecules Kin-of-IrreC (Kirre) and Sticks-and-stones (Sns) on their respective surfaces. The FCMs appear to provide the driving force for fusion, via the assembly of protrusions associated with branched F-actin and the WASp, SCAR and Arp2/3 pathways. In the present study, we utilize the dorsal pharyngeal musculature that forms in the Drosophila embryo as a model to explore myoblast fusion and visualize the fusion process in live embryos. These muscles rely on the same cell types and genes as the body wall muscles, but are amenable to live imaging since they do not undergo extensive morphogenetic movement during formation. Time-lapse imaging with F-actin and membrane markers revealed dynamic FCM-associated actin-enriched protrusions that rapidly extend and retract into the myotube from different sites within the actin focus. Ultrastructural analysis of this actin-enriched area showed that they have two morphologically distinct structures: wider invasions and/or narrow filopodia that contain long linear filaments. Consistent with this, formin Diaphanous (Dia) and branched actin nucleator, Arp3, are found decorating the filopodia or enriched at the actin focus, respectively, indicating that linear actin is present along with branched actin at sites of fusion in the FCM. Gain-of-function Dia and loss-of-function Arp3 both lead to fusion defects, a decrease of F-actin foci and prominent filopodia from the FCMs. We also observed differential endocytosis of cell surface components at sites of fusion, with actin reorganizing factors, WASp and SCAR, and Kirre remaining on the myotube surface and Sns preferentially taken up with other membrane proteins into early endosomes and lysosomes in the myotube.

Expression pattern of immunoglobulin superfamily members in the silkworm, Bombyx mori

Immunoglobulin superfamily (IgSF) proteins are involved in cell adhesion, cell communication and immune functions. In this study, 152 IgSF genes containing at least one immunoglobulin (Ig) domain were predicted in the Bombyx mori silkworm genome. Of these, 145 were distributed on 25 chromosomes with no genes on chromosomes 16, 18 and 26. Multiple sequence alignments and phylogenetic evolution analysis indicated that IgSFs evolved rapidly. Gene ontology (GO) annotation indicated that IgSF members functioned as cellular components and in molecular functions and biological processes. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis suggested that IgSF proteins were involved in signal transduction, signaling molecules and interaction, and cell communication. Microarray-based expression data showed tissue expression for 136 genes in anterior silkgland, middle silkgland, posterior silkgland, testis, ovary, fat body, midgut, integument, hemocyte, malpighian tubule and head. Expression pattern of IgSF genes in the silkworm ovary and midgut was analyzed by RNA-Seq. Expression of 105 genes was detected in the ovary in strain Dazao. Expression in the midgut was detected for 74 genes in strain Lan5 and 75 genes in strain Ou17. Expression of 34 IgSF genes in the midgut relative to the actin A3 gene was significantly different between strains Lan5 and Ou17. Furthermore, 1 IgSF gene was upregulated and 1 IgSF gene was downregulated in strain Lan5, and 4 IgSF genes were upregulated and 2 IgSF genes were downregulated in strain Ou17 after silkworms were challenged with B. mori cypovirus (BmCPV), indicating potential involvement in the response to BmCPV-infection. These results provide an overview of IgSF family members in silkworms, and lay the foundation for further functional studies.

A guide to study Drosophila muscle biology

The development and molecular composition of muscle tissue is evolutionarily conserved. Drosophila is a powerful in vivo model system to investigate muscle morphogenesis and function. Here, we provide a short and comprehensive overview of the important developmental steps to build Drosophila body muscle in embryos, larvae and pupae. We describe key methods, including muscle histology, live imaging and genetics, to study these steps at various developmental stages and include simple behavioural assays to assess muscle function in larvae and adults. We list valuable antibodies and fly strains that can be used for these different methods. This overview should guide the reader to choose the best marker or the appropriate method to obtain high quality muscle morphogenesis data in Drosophila.

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.

Bidirectional Notch activation represses fusion competence in swarming adult Drosophila myoblasts

A major aspect of indirect flight muscle formation during adult Drosophila myogenesis involves transition of a semi-differentiated and proliferating pool of myoblasts to a mature myoblast population, capable of fusing with nascent myotubes and generating mature muscle fibers. Here we examine the molecular genetic programs underlying these two phases of myoblast differentiation. We show that the cell adhesion proteins Dumbfounded (Duf) and Sticks and stones (Sns), together with their paralogs Roughest (Rst) and Hibris (Hbs), respectively, are required for adhesion of migrating myoblasts to myotubes and initiation of myoblast-myotube fusion. As myoblasts approach their myotube targets, they are maintained in a semi-differentiated state by continuous Notch activation, where each myoblast provides the ligand Delta to its neighbors. This unique form of bidirectional Notch activation is achieved by finely tuning the levels of the ligand and receptor. Activation of Notch signaling in myoblasts represses expression of key fusion elements such as Sns. Only upon reaching the vicinity of the myotubes does Notch signaling decay, leading to terminal differentiation of the myoblasts. The ensuing induction of proteins required for fusion enables myoblasts to fuse with the myotubes and give rise to subsequent muscle fiber growth.

Group I PAKs function downstream of Rac to promote podosome invasion during myoblast fusion in vivo

Group I p21-activated kinases organize actin filaments in myoblasts into dense foci, which promote podosome invasion and subsequent myoblast fusion.

The p21-activated kinases (PAKs) play essential roles in diverse cellular processes and are required for cell proliferation, apoptosis, polarity establishment, migration, and cell shape changes. Here, we have identified a novel function for the group I PAKs in cell–cell fusion. We show that the two Drosophila group I PAKs, DPak3 and DPak1, have partially redundant functions in myoblast fusion in vivo, with DPak3 playing a major role. DPak3 is enriched at the site of fusion colocalizing with the F-actin focus within a podosome-like structure (PLS), and promotes actin filament assembly during PLS invasion. Although the small GTPase Rac is involved in DPak3 activation and recruitment to the PLS, the kinase activity of DPak3 is required for effective PLS invasion. We propose a model whereby group I PAKs act downstream of Rac to organize the actin filaments within the PLS into a dense focus, which in turn promotes PLS invasion and fusion pore initiation during myoblast fusion.

Dock mediates Scar- and WASp-dependent actin polymerization through interaction with cell adhesion molecules in founder cells and fusion-competent myoblasts

The formation of the larval body wall musculature of Drosophila depends on the asymmetric fusion of two myoblast types, founder cells (FCs) and fusion-competent myoblasts (FCMs). Recent studies have established an essential function of Arp2/3-based actin polymerization during myoblast fusion, formation of a dense actin focus at the site of fusion in FCMs, and a thin sheath of actin in FCs and/or growing muscles. The formation of these actin structures depends on recognition and adhesion of myoblasts that is mediated by cell surface receptors of the immunoglobulin superfamily. However, the connection of the cell surface receptors with Arp2/3-based actin polymerization is poorly understood. To date only the SH2-SH3 adaptor protein Crk has been suggested to link cell adhesion with Arp2/3-based actin polymerization in FCMs. Here, we propose that the SH2-SH3 adaptor protein Dock, like Crk, links cell adhesion with actin polymerization. We show that Dock is expressed in FCs and FCMs and colocalizes with the cell adhesion proteins Sns and Duf at cell-cell contact points. Biochemical data in this study indicate that different domains of Dock are involved in binding the cell adhesion molecules Duf, Rst, Sns and Hbs. We emphasize the importance of these interactions by quantifying the enhanced myoblast fusion defects in duf dock, sns dock and hbs dock double mutants. Additionally, we show that Dock interacts biochemically and genetically with Drosophila Scar, Vrp1 and WASp. Based on these data, we propose that Dock links cell adhesion in FCs and FCMs with either Scar- or Vrp1-WASp-dependent Arp2/3 activation.

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.

The Drosophila nephrocyte

PURPOSE OF REVIEW

The functioning kidney requires proper organization in multiple cell types that mediate filtration and removal of wastes. Interest has increasingly focused on the podocyte as an important mediator of kidney function; defects in podocyte function likely mediate a broad palate of kidney dysfunctions. Here I explore recent work that establishes the Drosophila nephrocyte as a useful model for podocyte function and dysfunction.

RECENT FINDINGS

Although described many decades in the past, recent evidence has emphasized important similarities in the molecules that construct the 'nephrocyte diaphragm' and its vertebrate cousin the 'podocyte diaphragm'. For example, loss of Nephrin and its associated proteins lead to collapse of these structures and loss of proper filtration.

SUMMARY

These data emphasize both differences between the podocyte and nephrocyte and also useful similarities. These similarities provide the promise of bringing Drosophila genetics--strongly successful in other disciplines--to the complex problem of how podocyte dysfunction leads to disease. To further explore this point I discuss work on Nephrin in a better understood tissue, the Drosophila eye, in which the role of Nephrin and its connection to actin dynamics is under intense study.

Recent advances in imaging embryonic myoblast fusion in Drosophila

Myoblast fusion in the Drosophila embryos is a complex process that includes changes in cell movement, morphology and behavior over time. The advent of fluorescent proteins (FPs) has made it possible to track and image live cells, to capture the process of myoblast fusion, and to carry out quantitative analysis of myoblasts in real time. By tagging proteins with FPs, it is also possible to monitor the subcellular events that accompany the fusion process. Herein, we discuss the recent progress that has been made in imaging myoblast fusion in Drosophila, reagents that are now available, and microscopy conditions to consider. Using an Actin-FP fusion protein along with a membrane marker to outline the cells, we show the dynamic formation and breakdown of F-actin foci at sites of fusion. We also describe the methods used successfully to show that these foci are primarily if not wholly present in the fusion-competent myoblasts.

Competition between Blown fuse and WASP for WIP binding regulates the dynamics of WASP-dependent actin polymerization in vivo

Dynamic rearrangements of the actin cytoskeleton play a key role in numerous cellular processes. In Drosophila, fusion between a muscle founder cell and a fusion competent myoblast (FCM) is mediated by an invasive, F-actin-enriched podosome-like structure (PLS). Here, we show that the dynamics of the PLS is controlled by Blown fuse (Blow), a cytoplasmic protein required for myoblast fusion but whose molecular function has been elusive. We demonstrate that Blow is an FCM-specific protein that colocalizes with WASP, WIP/Solitary, and the actin focus within the PLS. Biochemically, Blow modulates the stability of the WASP-WIP complex by competing with WASP for WIP binding, leading to a rapid exchange of WASP, WIP and G-actin within the PLS, which, in turn, actively invades the adjacent founder cell to promote fusion pore formation. These studies identify a regulatory protein that modulates the actin cytoskeletal dynamics by controlling the stability of the WASP-WIP complex.

Cell death and selective adhesion reorganize the dorsoventral boundary for zigzag patterning of Drosophila wing margin hairs

Animal tissues and organs are comprised of several types of cells, which are often arranged in a well-ordered pattern. The posterior part of the Drosophila wing margin is covered with a double row of long hairs, which are equally and alternately derived from the dorsal and ventral sides of the wing, exhibiting a zigzag pattern in the lateral view. How this geometrically regular pattern is formed has not been fully understood. In this study, we show that this zigzag pattern is created by rearrangement of wing margin cells along the dorsoventral boundary flanked by the double row of hair cells during metamorphosis. This cell rearrangement is induced by selective apoptosis of wing margin cells that are spatially separated from hair cells. As a result of apoptosis, the remaining wing margin cells are rearranged in a well-ordered manner, which shapes corrugated lateral sides of both dorsal and ventral edges to interlock them for zigzag patterning. We further show that the corrugated topology of the wing edges is achieved by cell-type specific expression and localization of four kinds of NEPH1/nephrin family proteins through heterophilic adhesion between wing margin cells and hair cells. Homophilic E-cadherin adhesion is also required for attachment of the corrugated dorsoventral edges. Taken together, our results demonstrate that sequential coordination of apoptosis and epithelial architecture with selective adhesion creates the zigzag hair alignment. This may be a common mechanism for geometrically ordered repetitive packing of several types of cells in similarly patterned developmental fields such as the mammalian organ of Corti.

Asymmetric Mbc, active Rac1 and F-actin foci in the fusion-competent myoblasts during myoblast fusion in Drosophila

Myoblast fusion is an intricate process that is initiated by cell recognition and adhesion, and culminates in cell membrane breakdown and formation of multinucleate syncytia. In the Drosophila embryo, this process occurs asymmetrically between founder cells that pattern the musculature and fusion-competent myoblasts (FCMs) that account for the bulk of the myoblasts. The present studies clarify and amplify current models of myoblast fusion in several important ways. We demonstrate that the non-conventional guanine nucleotide exchange factor (GEF) Mbc plays a fundamental role in the FCMs, where it functions to activate Rac1, but is not required in the founder cells for fusion. Mbc, active Rac1 and F-actin foci are highly enriched in the FCMs, where they localize to the Sns:Kirre junction. Furthermore, Mbc is crucial for the integrity of the F-actin foci and the FCM cytoskeleton, presumably via its activation of Rac1 in these cells. Finally, the local asymmetric distribution of these proteins at adhesion sites is reminiscent of invasive podosomes and, consistent with this model, they are enriched at sites of membrane deformation, where the FCM protrudes into the founder cell/myotube. These data are consistent with models promoting actin polymerization as the driving force for myoblast fusion.

An invasive podosome-like structure promotes fusion pore formation during myoblast fusion

Recent studies in Drosophila have implicated actin cytoskeletal remodeling in myoblast fusion, but the cellular mechanisms underlying this process remain poorly understood. Here we show that actin polymerization occurs in an asymmetric and cell type-specific manner between a muscle founder cell and a fusion-competent myoblast (FCM). In the FCM, a dense F-actin-enriched focus forms at the site of fusion, whereas a thin sheath of F-actin is induced along the apposing founder cell membrane. The FCM-specific actin focus invades the apposing founder cell with multiple finger-like protrusions, leading to the formation of a single-channel macro fusion pore between the two muscle cells. Two actin nucleation-promoting factors of the Arp2/3 complex, WASP and Scar, are required for the formation of the F-actin foci, whereas WASP but not Scar promotes efficient foci invasion. Our studies uncover a novel invasive podosome-like structure (PLS) in a developing tissue and reveal a previously unrecognized function of PLSs in facilitating cell membrane juxtaposition and fusion.

Downstream of identity genes: muscle-type-specific regulation of the fusion process

In all metazoan organisms, the diversification of cell types involves determination of cell fates and subsequent execution of specific differentiation programs. During Drosophila myogenesis, identity genes specify the fates of founder myoblasts, from which derive all individual larval muscles. Here, to understand how cell fate information residing within founders is translated during differentiation, we focus on three identity genes, eve, lb, and slou, and how they control the size of individual muscles by regulating the number of fusion events. They achieve this by setting expression levels of Mp20, Pax, and mspo, three genes that regulate actin dynamics and cell adhesion and, as we show here, modulate the fusion process in a muscle-specific manner. Thus, these data show how the identity information implemented by transcription factors is translated via target genes into cell-type-specific programs of differentiation.

Myoblast fusion in Drosophila

The body wall musculature of a Drosophila larva is composed of an intricate pattern of 30 segmentally repeated muscle fibers in each abdominal hemisegment. Each muscle fiber has unique spatial and behavioral characteristics that include its location, orientation, epidermal attachment, size and pattern of innervation. Many, if not all, of these properties are dictated by founder cells, which determine the muscle pattern and seed the fusion process. Myofibers are then derived from fusion between a specific founder cell and several fusion competent myoblasts (FCMs) fusing with as few as 3-5 FCMs in the small muscles on the most ventral side of the embryo and as many as 30 FCMs in the larger muscles on the dorsal side of the embryo. The focus of the present review is the formation of the larval muscles in the developing embryo, summarizing the major issues and players in this process. We have attempted to emphasize experimentally-validated details of the mechanism of myoblast fusion and distinguish these from the theoretically possible details that have not yet been confirmed experimentally. We also direct the interested reader to other recent reviews that discuss myoblast fusion in Drosophila, each with their own perspective on the process [1-4]. With apologies, we use gene nomenclature as specified by Flybase (http://flybase.org) but provide Table 1 with alternative names and references.

The intracellular domain of Dumbfounded affects myoblast fusion efficiency and interacts with Rolling pebbles and Loner

Drosophila body wall muscles are multinucleated syncytia formed by successive fusions between a founder myoblast and several fusion competent myoblasts. Initial fusion gives rise to a bi/trinucleate precursor followed by more fusion cycles forming a mature muscle. This process requires the functions of various molecules including the transmembrane myoblast attractants Dumbfounded (Duf) and its paralogue Roughest (Rst), a scaffold protein Rolling pebbles (Rols) and a guanine nucleotide exchange factor Loner. Fusion completely fails in a duf, rst mutant, and is blocked at the bi/trinucleate stage in rols and loner single mutants. We analysed the transmembrane and intracellular domains of Duf, by mutating conserved putative signaling sites and serially deleting the intracellular domain. These were tested for their ability to translocate and interact with Rols and Loner and to rescue the fusion defect in duf, rst mutant embryos. Studying combinations of double mutants, further tested the function of Rols, Loner and other fusion molecules. Here we show that serial truncations of the Duf intracellular domain successively compromise its function to translocate and interact with Rols and Loner in addition to affecting myoblast fusion efficiency in embryos. Putative phosphorylation sites function additively while the extreme C terminus including a PDZ binding domain is dispensable for its function. We also show that fusion is completely blocked in a rols, loner double mutant and is compromised in other double mutants. These results suggest an additive function of the intracellular domain of Duf and an early function of Rols and Loner which is independent of Duf.

Multi-step control of muscle diversity by Hox proteins in the Drosophila embryo

Hox transcription factors control many aspects of animal morphogenetic diversity. The segmental pattern of Drosophila larval muscles shows stereotyped variations along the anteroposterior body axis. Each muscle is seeded by a founder cell and the properties specific to each muscle reflect the expression by each founder cell of a specific combination of 'identity' transcription factors. Founder cells originate from asymmetric division of progenitor cells specified at fixed positions. Using the dorsal DA3 muscle lineage as a paradigm, we show here that Hox proteins play a decisive role in establishing the pattern of Drosophila muscles by controlling the expression of identity transcription factors, such as Nautilus and Collier (Col), at the progenitor stage. High-resolution analysis, using newly designed intron-containing reporter genes to detect primary transcripts, shows that the progenitor stage is the key step at which segment-specific information carried by Hox proteins is superimposed on intrasegmental positional information. Differential control of col transcription by the Antennapedia and Ultrabithorax/Abdominal-A paralogs is mediated by separate cis-regulatory modules (CRMs). Hox proteins also control the segment-specific number of myoblasts allocated to the DA3 muscle. We conclude that Hox proteins both regulate and contribute to the combinatorial code of transcription factors that specify muscle identity and act at several steps during the muscle-specification process to generate muscle diversity.

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.

The immunoglobulin superfamily member Hbs functions redundantly with Sns in interactions between founder and fusion-competent myoblasts

The body wall muscle of a Drosophila larva is generated by fusion between founder cells and fusion-competent myoblasts (FCMs). Initially, a founder cell recognizes and fuses with one or two FCMs to form a muscle precursor, then the developing syncitia fuses with additional FCMs to form a muscle fiber. These interactions require members of the immunoglobulin superfamily (IgSF), with Kin-of-IrreC (Kirre) and Roughest (Rst) functioning redundantly in the founder cell and Sticks-and-stones (Sns) serving as their ligand in the FCMs. Previous studies have not resolved the role of Hibris (Hbs), a paralog of Sns, suggesting that it functions as a positive regulator of myoblast fusion and as a negative regulator that antagonizes the activity of Sns. The results herein resolve this issue, demonstrating that sns and hbs function redundantly in the formation of several muscle precursors, and that loss of one copy of sns enhances the myoblast fusion phenotype of hbs mutants. We further show that excess Hbs rescues some fusion in sns mutant embryos beyond precursor formation, consistent with its ability to drive myoblast fusion, but show using chimeric molecules that Hbs functions less efficiently than Sns. In conjunction with a physical association between Hbs and SNS in cis, these data account for the previously observed UAS-hbs overexpression phenotypes. Lastly, we demonstrate that either an Hbs or Sns cytodomain is essential for muscle precursor formation, and signaling from IgSF members found exclusively in the founder cells is not sufficient to direct precursor formation.