kette and blown fuse interact genetically during the second fusion step of myogenesis in Drosophila

Fusion of myofibre branches is a physiological feature of healthy human skeletal muscle regeneration

BACKGROUND

The occurrence of hyperplasia, through myofibre splitting, remains a widely debated phenomenon. Structural alterations and fibre typing of skeletal muscle fibres, as seen during regeneration and in certain muscle diseases, can be challenging to interpret. Neuromuscular electrical stimulation can induce myofibre necrosis followed by changes in spatial and temporal cellular processes. Thirty days following electrical stimulation, remnants of regeneration can be seen in the myofibre and its basement membrane as the presence of small myofibres and encroachment of sarcolemma and basement membrane (suggestive of myofibre branching/splitting). The purpose of this study was to investigate myofibre branching and fibre type in a systematic manner in human skeletal muscle undergoing adult regenerative myogenesis.

METHODS

Electrical stimulation was used to induce myofibre necrosis to the vastus lateralis muscle of one leg in 5 young healthy males. Muscle tissue samples were collected from the stimulated leg 30 days later and from the control leg for comparison. Biopsies were sectioned and stained for dystrophin and laminin to label the sarcolemma and basement membrane, respectively, as well as ATPase, and antibodies against types I and II myosin, and embryonic and neonatal myosin. Myofibre branches were followed through 22 serial Sects. (264 μm). Single fibres and tissue blocks were examined by confocal and electron microscopy, respectively.

RESULTS

Regular branching of small myofibre segments was observed (median length 144 μm), most of which were observed to fuse further along the parent fibre. Central nuclei were frequently observed at the point of branching/fusion. The branch commonly presented with a more immature profile (nestin + , neonatal myosin + , disorganised myofilaments) than the parent myofibre, together suggesting fusion of the branch, rather than splitting. Of the 210 regenerating muscle fibres evaluated, 99.5% were type II fibres, indicating preferential damage to type II fibres with our protocol. Furthermore, these fibres demonstrated 7 different stages of "fibre-type" profiles.

CONCLUSIONS

By studying the regenerating tissue 30 days later with a range of microscopy techniques, we find that so-called myofibre branching or splitting is more likely to be fusion of myotubes and is therefore explained by incomplete regeneration after a necrosis-inducing event.

Amphioxus muscle transcriptomes reveal vertebrate-like myoblast fusion genes and a highly conserved role of insulin signalling in the metabolism of muscle

BACKGROUND

The formation and functioning of muscles are fundamental aspects of animal biology, and the evolution of 'muscle genes' is central to our understanding of this tissue. Feeding-fasting-refeeding experiments have been widely used to assess muscle cellular and metabolic responses to nutrition. Though these studies have focused on vertebrate models and only a few invertebrate systems, they have found similar processes are involved in muscle degradation and maintenance. Motivation for these studies stems from interest in diseases whose pathologies involve muscle atrophy, a symptom also triggered by fasting, as well as commercial interest in the muscle mass of animals kept for consumption. Experimentally modelling atrophy by manipulating nutritional state causes muscle mass to be depleted during starvation and replenished with refeeding so that the genetic mechanisms controlling muscle growth and degradation can be understood.

RESULTS

Using amphioxus, the earliest branching chordate lineage, we address the gap in previous work stemming from comparisons between distantly related vertebrate and invertebrate models. Our amphioxus feeding-fasting-refeeding muscle transcriptomes reveal a highly conserved myogenic program and that the pro-orthologues of many vertebrate myoblast fusion genes were present in the ancestral chordate, despite these invertebrate chordates having unfused mononucleate myocytes. We found that genes differentially expressed between fed and fasted amphioxus were orthologous to the genes that respond to nutritional state in vertebrates. This response is driven in a large part by the highly conserved IGF/Akt/FOXO pathway, where depleted nutrient levels result in activation of FOXO, a transcription factor with many autophagy-related gene targets.

CONCLUSION

Reconstruction of these gene networks and pathways in amphioxus muscle provides a key point of comparison between the distantly related groups assessed thus far, significantly refining the reconstruction of the ancestral state for chordate myoblast fusion genes and identifying the extensive role of duplicated genes in the IGF/Akt/FOXO pathway across animals. Our study elucidates the evolutionary trajectory of muscle genes as they relate to the increased complexity of vertebrate muscles and muscle development.

Drosophila Myoblast Fusion: Invasion and Resistance for the Ultimate Union

Cell-cell fusion is indispensable for creating life and building syncytial tissues and organs. Ever since the discovery of cell-cell fusion, how cells join together to form zygotes and multinucleated syncytia has remained a fundamental question in cell and developmental biology. In the past two decades, Drosophila myoblast fusion has been used as a powerful genetic model to unravel mechanisms underlying cell-cell fusion in vivo. Many evolutionarily conserved fusion-promoting factors have been identified and so has a surprising and conserved cellular mechanism. In this review, we revisit key findings in Drosophila myoblast fusion and highlight the critical roles of cellular invasion and resistance in driving cell membrane fusion.

The histone chaperone NAP1L3 is required for haematopoietic stem cell maintenance and differentiation

Nucleosome assembly proteins (NAPs) are histone chaperones with an important role in chromatin structure and epigenetic regulation of gene expression. We find that high gene expression levels of mouse Nap1l3 are restricted to haematopoietic stem cells (HSCs) in mice. Importantly, with shRNA or CRISPR-Cas9 mediated loss of function of mouse Nap1l3 and with overexpression of the gene, the number of colony-forming cells and myeloid progenitor cells in vitro are reduced. This manifests as a striking decrease in the number of HSCs, which reduces their reconstituting activities in vivo. Downregulation of human NAP1L3 in umbilical cord blood (UCB) HSCs impairs the maintenance and proliferation of HSCs both in vitro and in vivo. NAP1L3 downregulation in UCB HSCs causes an arrest in the G0 phase of cell cycle progression and induces gene expression signatures that significantly correlate with downregulation of gene sets involved in cell cycle regulation, including E2F and MYC target genes. Moreover, we demonstrate that HOXA3 and HOXA5 genes are markedly upregulated when NAP1L3 is suppressed in UCB HSCs. Taken together, our findings establish an important role for NAP1L3 in HSC homeostasis and haematopoietic differentiation.

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.

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.

Surface apposition and multiple cell contacts promote myoblast fusion in Drosophila flight muscles

Transmission EM methods reveal that cell–cell fusion of individual myoblasts with growing Drosophila flight muscles is a stepwise process in which the cell adhesion and branched actin machineries mediate tight apposition and formation of multiple contacts and pores between the surfaces of the fusing cells.

Fusion of individual myoblasts to form multinucleated myofibers constitutes a widely conserved program for growth of the somatic musculature. We have used electron microscopy methods to study this key form of cell–cell fusion during development of the indirect flight muscles (IFMs) of Drosophila melanogaster. We find that IFM myoblast–myotube fusion proceeds in a stepwise fashion and is governed by apparent cross talk between transmembrane and cytoskeletal elements. Our analysis suggests that cell adhesion is necessary for bringing myoblasts to within a minimal distance from the myotubes. The branched actin polymerization machinery acts subsequently to promote tight apposition between the surfaces of the two cell types and formation of multiple sites of cell–cell contact, giving rise to nascent fusion pores whose expansion establishes full cytoplasmic continuity. Given the conserved features of IFM myogenesis, this sequence of cell interactions and membrane events and the mechanistic significance of cell adhesion elements and the actin-based cytoskeleton are likely to represent general principles of the myoblast fusion process.

Drosophila comes of age as a model system for understanding the function of cytoskeletal proteins in cells, tissues, and organisms

For the last 100 years, Drosophila melanogaster has been a powerhouse genetic system for understanding mechanisms of inheritance, development, and behavior in animals. In recent years, advances in imaging and genetic tools have led to Drosophila becoming one of the most effective systems for unlocking the subcellular functions of proteins (and particularly cytoskeletal proteins) in complex developmental settings. In this review, written for non-Drosophila experts, we will discuss critical technical advances that have enabled these cell biological insights, highlighting three examples of cytoskeletal discoveries that have arisen as a result: (1) regulation of Arp2/3 complex in myoblast fusion, (2) cooperation of the actin filament nucleators Spire and Cappuccino in establishment of oocyte polarity, and (3) coordination of supracellular myosin cables. These specific examples illustrate the unique power of Drosophila both to uncover new cytoskeletal structures and functions, and to place these discoveries in a broader in vivo context, providing insights that would have been impossible in a cell culture model or in vitro. Many of the cellular structures identified in Drosophila have clear counterparts in mammalian cells and tissues, and therefore elucidating cytoskeletal functions in Drosophila will be broadly applicable to other organisms.

Mechanisms of myoblast fusion during muscle development

The development and regeneration of skeletal muscle require the fusion of mononucleated muscle cells to form multinucleated, contractile muscle fibers. Studies using a simple genetic model, Drosophila melanogaster, have discovered many evolutionarily conserved fusion-promoting factors in vivo. Recent work in zebrafish and mouse also identified several vertebrate-specific factors required for myoblast fusion. Here, we integrate progress in multiple in vivo systems and highlight conceptual advance in understanding how muscle cell membranes are brought together for fusion. We focus on the molecular machinery at the fusogenic synapse and present a three-step model to describe the molecular and cellular events leading to fusion pore formation.

Identification of singles bar as a direct transcriptional target of Drosophila Myocyte enhancer factor-2 and a regulator of adult myoblast fusion

In Drosophila, myoblast fusion is a conserved process in which founder cells (FCs) and fusion competent myoblasts (FCMs) fuse to form a syncytial muscle fiber. Mutants for the myogenic regulator Myocyte enhancer factor-2 (MEF2) show a failure of myoblast fusion, indicating that MEF2 regulates the fusion process. Indeed, chromatin immunoprecipitation studies show that several genes involved in myoblast fusion are bound by MEF2 during embryogenesis. Of these, the MARVEL domain gene singles bar (sing), is down-regulated in MEF2 knockdown pupae, and has five consensus MEF2 binding sites within a 9000-bp region. To determine if MEF2 is an essential and direct regulator of sing during pupal muscle development, we identified a 315-bp myoblast enhancer of sing. This enhancer was active during myoblast fusion, and mutation of two MEF2 sites significantly decreased enhancer activity. We show that lack of sing expression resulted in adult lethality and muscle loss, due to a failure of fusion during the pupal stage. Additionally, we sought to determine if sing was required in either FCs or FCMs to support fusion. Interestingly, knockdown of sing in either population did not significantly affect fusion, however, knockdown in both FCs and FCMs resulted in muscles with significantly reduced nuclei numbers, provisionally indicating that sing function is required in either cell type, but not both. Finally, we found that MEF2 regulated sing expression at the embryonic stage through the same 315-bp enhancer, indicating that sing is a MEF2 target at both critical stages of myoblast fusion. Our studies define for the first time how MEF2 directly controls fusion at multiple stages of the life cycle, and provide further evidence that the mechanisms of fusion characterized in Drosophila embryos is also used in the formation of the more complex adult muscles.

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.

GRAF1 promotes ferlin-dependent myoblast fusion

Myoblast fusion (a critical process by which muscles grow) occurs in a multi-step fashion that requires actin and membrane remodeling; but important questions remain regarding the spatial/temporal regulation of and interrelationship between these processes. We recently reported that the Rho-GAP, GRAF1, was particularly abundant in muscles undergoing fusion to form multinucleated fibers and that enforced expression of GRAF1 in cultured myoblasts induced robust fusion by a process that required GAP-dependent actin remodeling and BAR domain-dependent membrane sculpting. Herein we developed a novel line of GRAF1-deficient mice to explore a role for this protein in the formation/maturation of myotubes in vivo. Post-natal muscles from GRAF1-depleted mice exhibited a significant and persistent reduction in cross-sectional area, impaired regenerative capacity and a significant decrease in force production indicative of lack of efficient myoblast fusion. A significant fusion defect was recapitulated in isolated myoblasts depleted of GRAF1 or its closely related family member GRAF2. Mechanistically, we show that GRAF1 and 2 facilitate myoblast fusion, at least in part, by promoting vesicle-mediated translocation of fusogenic ferlin proteins to the plasma membrane.

Distinct genetic programs guide Drosophila circular and longitudinal visceral myoblast fusion

BACKGROUND

The visceral musculature of Drosophila larvae comprises circular visceral muscles tightly interwoven with longitudinal visceral muscles. During myogenesis, the circular muscles arise by one-to-one fusion of a circular visceral founder cell (FC) with a visceral fusion-competent myoblast (FCM) from the trunk visceral mesoderm, and longitudinal muscles arise from FCs of the caudal visceral mesoderm. Longitudinal FCs migrate anteriorly under guidance of fibroblast growth factors during embryogenesis; it is proposed that they fuse with FCMs from the trunk visceral mesoderm to give rise to syncytia containing up to six nuclei.

RESULTS

Using fluorescence in situ hybridization and immunochemical analyses, we investigated whether these fusion events during migration use the same molecular repertoire and cellular components as fusion-restricted myogenic adhesive structure (FuRMAS), the adhesive signaling center that mediates myoblast fusion in the somatic mesoderm. Longitudinal muscles were formed by the fusion of one FC with Sns-positive FCMs, and defects in FCM specification led to defects in longitudinal muscle formation. At the fusion sites, Duf/Kirre and the adaptor protein Rols7 accumulated in longitudinal FCs, and Blow and F-actin accumulated in FCMs. The accumulation of these four proteins at the fusion sites argues for FuRMAS-like adhesion and signaling centers. Longitudinal fusion was disturbed in rols and blow single, and scar wip double mutants. Mutants of wasp or its interaction partner wip had no defects in longitudinal fusion.

CONCLUSIONS

Our results indicated that all embryonic fusion events depend on the same cell-adhesion molecules, but that the need for Rols7 and regulators of F-actin distinctly differs. Rols7 was required for longitudinal visceral and somatic myoblast fusion but not for circular visceral fusion. Importantly, longitudinal fusion depended on Kette and SCAR/Wave but was independent of WASp-dependent Arp2/3 activation. Thus, the complexity of the players involved in muscle formation increases from binucleated circular muscles to longitudinal visceral muscles to somatic muscles.

G-protein coupled receptor BAI3 promotes myoblast fusion in vertebrates

Muscle fibers form as a result of myoblast fusion, yet the cell surface receptors regulating this process are unknown in vertebrates. In Drosophila, myoblast fusion involves the activation of the Rac pathway by the guanine nucleotide exchange factor Myoblast City and its scaffolding protein ELMO, downstream of cell-surface cell-adhesion receptors. We previously showed that the mammalian ortholog of Myoblast City, DOCK1, functions in an evolutionarily conserved manner to promote myoblast fusion in mice. In search for regulators of myoblast fusion, we identified the G-protein coupled receptor brain-specific angiogenesis inhibitor (BAI3) as a cell surface protein that interacts with ELMO. In cultured cells, BAI3 or ELMO1/2 loss of function severely impaired myoblast fusion without affecting differentiation and cannot be rescued by reexpression of BAI3 mutants deficient in ELMO binding. The related BAI protein family member, BAI1, is functionally distinct from BAI3, because it cannot rescue the myoblast fusion defects caused by the loss of BAI3 function. Finally, embryonic muscle precursor expression of a BAI3 mutant unable to bind ELMO was sufficient to block myoblast fusion in vivo. Collectively, our findings provide a role for BAI3 in the relay of extracellular fusion signals to their intracellular effectors, identifying it as an essential transmembrane protein for embryonic vertebrate myoblast fusion.

SCAR/WAVE-mediated processing of engulfed apoptotic corpses is essential for effective macrophage migration in Drosophila

In vitro studies have shown that SCAR/WAVE activates the Arp2/3 complex to generate actin filaments, which in many cell types are organised into lamellipodia that are thought to have an important role in cell migration. Here we demonstrate that SCAR is utilised by Drosophila macrophages to drive their developmental and inflammatory migrations and that it is regulated via the Hem/Kette/Nap1-containing SCAR/WAVE complex. SCAR is also important in protecting against bacterial pathogens and in wound repair as SCAR mutant embryos succumb more readily to both sterile and infected wounds. However, in addition to driving the formation of lamellipodia in macrophages, SCAR is required cell autonomously for the correct processing of phagocytosed apoptotic corpses by these professional phagocytes. Removal of this phagocytic burden by preventing apoptosis rescues macrophage lamellipodia formation and partially restores motility. Our results show that efficient processing of phagosomes is critical for effective macrophage migration in vivo. These findings have important implications for the resolution of macrophages from chronic wounds and the behaviour of those associated with tumours, because phagocytosis of debris may serve to prolong the presence of these cells at these sites of pathology.

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.

The actin regulator N-WASp is required for muscle-cell fusion in mice

A fundamental aspect of skeletal myogenesis involves extensive rounds of cell fusion, in which individual myoblasts are incorporated into growing muscle fibers. Here we demonstrate that N-WASp, a ubiquitous nucleation-promoting factor of branched microfilament arrays, is an essential contributor to skeletal muscle-cell fusion in developing mouse embryos. Analysis both in vivo and in primary satellite-cell cultures, shows that disruption of N-WASp function does not interfere with the program of skeletal myogenic differentiation, and does not affect myoblast motility, morphogenesis and attachment capacity. N-WASp-deficient myoblasts, however, fail to fuse. Furthermore, our analysis suggests that myoblast fusion requires N-WASp activity in both partners of a fusing myoblast pair. These findings reveal a specific role for N-WASp during mammalian myogenesis. WASp-family elements appear therefore to act as universal mediators of the myogenic cell-cell fusion mechanism underlying formation of functional muscle fibers, in both vertebrate and invertebrate species.

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.

Analysis of skeletal muscle development in Drosophila

The Drosophila system has been invaluable in providing important insights into mesoderm specification, muscle specification, myoblast fusion, muscle differentiation, and myofibril assembly. Here, we present a series of Drosophila protocols that enable the researcher to visualize muscle precursors and differentiated muscles, at all stages of development. In doing so, we also highlight the variety of techniques that are used to create these findings. These protocols are directly used for the Drosophila system, and are provided with explanatory detail to enable the researcher to apply them to other systems.

The Drosophila Hem/Kette/Nap1 protein regulates asymmetric division of neural precursor cells by regulating localization of Inscuteable and Numb

The Hem/Kette/Nap1 protein is involved in many biological processes. We have recently reported that Hem is required for the normal migration of neurons in the Drosophila embryo. In this paper, we report that Hem regulates the asymmetric division of neural precursor cells. We find that a well-studied Hem/Kette mutant allele produces at least two main, but possibly more, phenotypic classes of mutant embryos, and these phenotypes correlate with variable levels of maternal wild type Hem protein in the developing embryo. While the weaker class exhibits weak axon guidance defect and the mis-migration of neurons, the stronger class causes severe axon guidance defects, mis-migration of neurons and symmetric division of ganglion mother cells (GMC) of the RP2/sib lineage. We also show that the basis for the loss of asymmetric division is due to non-localization of Inscuteable and Numb in GMC-1. A non-asymmetric Numb segregates to both daughter cells of GMC-1, which then prevents Notch signaling from specifying a sib fate. This causes both cells to adopt an RP2 fate. Furthermore, loss of function for Abelson tyrosine kinase also causes loss of asymmetric localization of Inscuteable and Numb and symmetric division of GMC-1, the loss of function for WAVE has a very weakly penetrant loss of asymmetry defect. These results define another role for Hem/Kette/Nap1 in a neural precursor cell during neurogenesis.

Drosophila Swiprosin-1/EFHD2 accumulates at the prefusion complex stage during Drosophila myoblast fusion

In the Drosophila embryo, transient cell adhesion during myoblast fusion is known to lead to the formation of fusion-restricted myogenic-adhesive structures (FuRMASs). Here, we report that within these FuRMASs, a Drosophila homologue of human and mouse swiprosins (EF-hand-domain-containing proteins) is expressed, which we named Drosophila Swiprosin-1 (Drosophila Swip-1). Drosophila Swip-1 is highly conserved and is closely related to the calcium-binding proteins swiprosin-1 and swiprosin-2 that have a role in the immune system in humans and mice. Our study shows that Drosophila Swip-1 is also expressed in corresponding cells of the Drosophila immune system. During myoblast fusion, Drosophila Swip-1 accumulates transiently in the foci of fusion-competent myoblasts (FCMs). Both the EF-hand and the coiled-coil domain of Drosophila Swip-1 are required to localise the protein to these foci. The formation of Drosophila Swip-1 foci requires successful cell adhesion between FCMs and founder cells (FCs) or growing myotubes. Moreover, Drosophila Swip-1 foci were found to increase in number in sing(22) mutants, which arrest myoblast fusion after prefusion complex formation. By contrast, Drosophila Swip-1 foci are not significantly enriched in blow(2) and kette(J4-48) mutants, which stop myogenesis beyond the prefusion complex stage but before plasma membrane merging. Therefore, we hypothesise that Drosophila Swip-1 participates in the breakdown of the prefusion complex during the progression of myoblast fusion.

The actin nucleator WASp is required for myoblast fusion during adult Drosophila myogenesis

Myoblast fusion provides a fundamental, conserved mechanism for muscle fiber growth. We demonstrate here that the functional contribution of Wsp, the Drosophila homolog of the conserved actin nucleation-promoting factor (NPF) WASp, is essential for myoblast fusion during the formation of muscles of the adult fly. Disruption of Wsp function results in complete arrest of myoblast fusion in all muscles examined. Wsp activity during adult Drosophila myogenesis is specifically required for muscle cell fusion and is crucial both for the formation of new muscle fibers and for the growth of muscles derived from persistent larval templates. Although Wsp is expressed both in fibers and individual myoblasts, its activity in either one of these cell types is sufficient. SCAR, a second major Arp2/3 NPF, is also required during adult myoblast fusion. Formation of fusion-associated actin 'foci' is dependent on Arp2/3 complex function, but appears to rely on a distinct, unknown nucleator. The comprehensive nature of these requirements identifies Arp2/3-based branched actin polymerization as a universal mechanism underlying myoblast fusion.

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.

The Hem protein mediates neuronal migration by inhibiting WAVE degradation and functions opposite of Abelson tyrosine kinase

In the nervous system, neurons form in different regions, then they migrate and occupy specific positions. We have previously shown that RP2/sib, a well-studied neuronal pair in the Drosophila ventral nerve cord (VNC), has a complex migration route. Here, we show that the Hem protein, via the WAVE complex, regulates migration of GMC-1 and its progeny RP2 neuron. In Hem or WAVE mutants, RP2 neuron either abnormally migrates, crossing the midline from one hemisegment to the contralateral hemisegment, or does not migrate at al and fail to send out its axon projection. We report that Hem regulates neuronal migration through stabilizing WAVE. Since Hem and WAVE normally form a complex, our data argues that in the absence of Hem, WAVE, which is presumably no longer in a complex, becomes susceptible to degradation. We also find that Abelson tyrosine kinase affects RP2 migration in a similar manner as Hem and WAVE, and appears to operate via WAVE. However, while Abl negatively regulates the levels of WAVE, it regulates migration via regulating the activity of WAVE. Our results also show that during the degradation of WAVE, Hem function is opposite to that of and downstream of Abl.

Skeletal muscle differentiation and fusion are regulated by the BAR-containing Rho-GTPase-activating protein (Rho-GAP), GRAF1

Although RhoA activity is necessary for promoting myogenic mesenchymal stem cell fates, recent studies in cultured cells suggest that down-regulation of RhoA activity in specified myoblasts is required for subsequent differentiation and myotube formation. However, whether this phenomenon occurs in vivo and which Rho modifiers control these later events remain unclear. We found that expression of the Rho-GTPase-activating protein, GRAF1, was transiently up-regulated during myogenesis, and studies in C2C12 cells revealed that GRAF1 is necessary and sufficient for mediating RhoA down-regulation and inducing muscle differentiation. Moreover, forced expression of GRAF1 in pre-differentiated myoblasts drives robust muscle fusion by a process that requires GTPase-activating protein-dependent actin remodeling and BAR-dependent membrane binding or sculpting. Moreover, morpholino-based knockdown studies in Xenopus laevis determined that GRAF1 expression is critical for muscle development. GRAF1-depleted embryos exhibited elevated RhoA activity and defective myofibrillogenesis that resulted in progressive muscle degeneration, defective motility, and embryonic lethality. Our results are the first to identify a GTPase-activating protein that regulates muscle maturation and to highlight the functional importance of BAR domains in myotube formation.

Characterization of early steps in muscle morphogenesis in a Drosophila primary culture system

Myogenesis in Drosophila embryos requires fusion between Founder cells (FCs) and Fusion Competent myoblasts (FCMs) to form multinucleate myotubes. Myoblast fusion is well characterized in embryos, and many factors required for this process have been identified; however, a number of questions pertaining to the mechanisms of fusion remain and are challenging to answer in the embryo. We have developed a modified primary cell culture protocol to address these questions in vitro. Using this system, we determined the optimal time for examining fusion in culture and confirmed that known fusion proteins are expressed and localized as in embryos. Importantly, we disrupted the actin and microtubule networks with the drugs latrunculin B and nocodazole, respectively, confirming that actin is required for myoblast fusion and showing for the first time that microtubules are also required for this process in Drosophila. Finally, we show that myotubes in culture adopt and maintain specific muscle identities.

Born to run: creating the muscle fiber

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

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.

Molecular mechanisms of myoblast fusion across species

Skeletal muscle development, growth and regeneration depend on the ability of progenitor myoblasts to fuse to one another in a series of ordered steps. Whereas the cellular steps leading to the formation of a multinucleated myofiber are conserved in several model organisms, the molecular regulatory factors may vary. Understanding the common and divergent mechanisms regulating myoblast fusion in Drosophila melanogaster (fruit fly), Danio rerio (zebrafish) and Mus musculus (mouse) provides a better insight into the process of myoblast fusion than any of these models could provide alone. Deciphering the mechanisms of myoblast fusion from simpler to more complex organisms is of fundamental interest to skeletal muscle biology and may provide therapeutic avenues for various diseases that affect muscle.

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.

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.

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.

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.

A novel function of heparan sulfate in the regulation of cell-cell fusion

Despite the important contribution of cell-cell fusion in the development and physiology of eukaryotes, little is known about the mechanisms that regulate this process. Our study shows that glycosaminoglycans and more specifically heparan sulfate (HS) expressed on the cell surface and extracellular matrix may act as negative regulator of cell-cell fusion. Using herpes simplex virus type-1 as a tool to enhance cell-cell fusion, we demonstrate that the absence of HS expression on the cell surface results in a significant increase in cell-cell fusion. An identical phenomenon was observed when other viruses or polyethylene glycol was used as fusion enhancer. Cells deficient in HS biosynthesis showed increased activity of two Rho GTPases, RhoA and Cdc42, both of which showed a correlation between increased activity and increased cell-cell fusion. This could serve as a possible explanation as to why HS-deficient cells showed significantly enhanced cell-cell fusion and suggests that HS could regulate fusion via fine tuning of RhoA and Cdc42 activities.

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.

Nap1-mediated actin remodeling is essential for mammalian myoblast fusion

Myoblast fusion is crucial for the formation, growth, maintenance and regeneration of healthy skeletal muscle. Unfortunately, the molecular machinery, cell behaviors, and membrane and cytoskeletal remodeling events that govern fusion and myofiber formation remain poorly understood. Using time-lapse imaging approaches on mouse C2C12 myoblasts, we identify discrete and specific molecular events at myoblast membranes during fusion and myotube formation. These events include rearrangement of cell shape from fibroblast to spindle-like morphologies, changes in lamellipodial and filopodial extensions during different periods of differentiation, and changes in membrane alignment and organization during fusion. We find that actin-cytoskeleton remodeling is crucial for these events: pharmacological inhibition of F-actin polymerization leads to decreased lamellipodial and filopodial extensions and to reduced myoblast fusion. Additionally, shRNA-mediated inhibition of Nap1, a member of the WAVE actin-remodeling complex, results in accumulations of F-actin structures at the plasma membrane that are concomitant with a decrease in myoblast fusion. Our data highlight distinct and essential roles for actin cytoskeleton remodeling during mammalian myoblast fusion, provide a platform for cellular and molecular dissection of the fusion process, and suggest a functional conservation of Nap1-regulated actin-cytoskeleton remodeling during myoblast fusion between mammals and Drosophila.

The SCAR and WASp nucleation-promoting factors act sequentially to mediate Drosophila myoblast fusion

The actin nucleation-promoting factors SCAR/WAVE and WASp, together with associated elements, mediate the formation of muscle fibres through myoblast fusion during Drosophila embryogenesis. Our phenotypic analysis, following the disruption of these two pathways, suggests that they function in a sequential manner. Suppressor of cyclic AMP receptor (SCAR) activity is required before the formation of pores in the membranes of fusing cells, whereas Wiskott-Aldrich syndrome protein (WASp) promotes the expansion of nascent pores and completion of the fusion process. Genetic epistasis experiments are consistent with this step-wise temporal progression. Our observations further imply a separate, Rac-dependent role for the SCAR complex in promoting myoblast migration. In keeping with the sequential utilization of the two systems, we observe abnormal accumulations of filamentous actin at the fusion sites when both pathways are disrupted, resembling those present when only SCAR-complex function is impaired. This observation further suggests that actin-filament accumulation at the fusion sites might not depend on Arp2/3 activity altogether.

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.

The small G-proteins Rac1 and Cdc42 are essential for myoblast fusion in the mouse

Rac1 and Cdc42 are small G-proteins that regulate actin dynamics and affect plasma membrane protrusion and vesicle traffic. We used conditional mutagenesis in mice to demonstrate that Rac1 and Cdc42 are essential for myoblast fusion in vivo and in vitro. The deficit in fusion of Rac1 or Cdc42 mutant myoblasts correlates with a deficit in the recruitment of actin fibers and vinculin to myoblast contact sites. Comparison of the changes observed in mutant myogenic cells indicates that Rac1 and Cdc42 function in a nonredundant and not completely overlapping manner during the fusion process. Our genetic analysis demonstrates thus that the function of Rac in myoblast fusion is evolutionarily conserved from insects to mammals and that Cdc42, a molecule hitherto not implicated in myoblast fusion, is essential for the fusion of murine myoblasts.

Actin regulators take the reins in Drosophila myoblast fusion

Skeletal muscle formation, growth and repair depend on myoblast fusion events. Therefore, in-depth understanding of the underlying molecular mechanisms controlling these events that ultimately lead to skeletal muscle formation may be fundamental for developing new therapies for tissue repair. To this end, the greatest advances in furthering understanding myoblast fusion has been made in Drosophila. Recent studies have shown that transient F-actin structures, so-called actin plugs or foci, are known to form at the site of contacting myoblasts. Indeed, actin regulators of the WASP family that control the activation of the Arp2/3 complex and thereby branched F-actin formation have been demonstrated to be crucial for myoblast fusion. Myoblast-specific cell adhesion molecules seem to be involved in the recruitment of WASP family members to the site of myoblast fusion and form a Fusion-Restricted Myogenic-Adhesive Structure (FuRMAS). Currently, the exact role of the FuRMAS is not completely understood. However, recent studies indicate that WASP-dependent F-actin regulation is required for fusion pore formation as well as for the correct integration of fusing myoblasts into the growing muscle. In this review, I discuss latest cellular studies, and recent genetic and biochemical analyses on actin regulation during myoblast fusion.

The WAVE/SCAR complex promotes polarized cell movements and actin enrichment in epithelia during C. elegans embryogenesis

The WAVE/SCAR complex promotes actin nucleation through the Arp2/3 complex, in response to Rac signaling. We show that loss of WVE-1/GEX-1, the only C. elegans WAVE/SCAR homolog, by genetic mutation or by RNAi, has the same phenotype as loss of GEX-2/Sra1/p140/PIR121, GEX-3/NAP1/HEM2/KETTE, or ABI-1/ABI, the three other components of the C. elegans WAVE/SCAR complex. We find that the entire WAVE/SCAR complex promotes actin-dependent events at different times and in different tissues during development. During C. elegans embryogenesis loss of CED-10/Rac1, WAVE/SCAR complex components, or Arp2/3 blocks epidermal cell migrations despite correct epidermal cell differentiation. 4D movies show that this failure occurs due to decreased membrane dynamics in specific epidermal cells. Unlike myoblasts in Drosophila, epidermal cell fusions in C. elegans can occur in the absence of WAVE/SCAR or Arp2/3. Instead we find that subcellular enrichment of F-actin in epithelial tissues requires the Rac-WAVE/SCAR-Arp2/3 pathway. Intriguingly, we find that at the same stage of development both F-actin and WAVE/SCAR proteins are enriched apically in one epithelial tissue and basolaterally in another. We propose that temporally and spatially regulated actin nucleation by the Rac-WAVE/SCAR-Arp2/3 pathway is required for epithelial cell organization and movements during morphogenesis.

WASP and SCAR have distinct roles in activating the Arp2/3 complex during myoblast fusion

Myoblast fusion takes place in two steps in mammals and in Drosophila. First, founder cells (FCs) and fusion-competent myoblasts (FCMs) fuse to form a trinucleated precursor, which then recruits further FCMs. This process depends on the formation of the fusion-restricted myogenic-adhesive structure (FuRMAS), which contains filamentous actin (F-actin) plugs at the sites of cell contact. Fusion relies on the HEM2 (NAP1) homolog Kette, as well as Blow and WASP, a member of the Wiskott-Aldrich-syndrome protein family. Here, we show the identification and characterization of schwächling--a new Arp3-null allele. Ultrastructural analyses demonstrate that Arp3 schwächling mutants can form a fusion pore, but fail to integrate the fusing FCM. Double-mutant experiments revealed that fusion is blocked completely in Arp3 and wasp double mutants, suggesting the involvement of a further F-actin regulator. Indeed, double-mutant analyses with scar/WAVE and with the WASP-interacting partner vrp1 (sltr, wip)/WIP show that the F-actin regulator scar also controls F-actin formation during myoblast fusion. Furthermore, the synergistic phenotype observed in Arp3 wasp and in scar vrp1 double mutants suggests that WASP and SCAR have distinct roles in controlling F-actin formation. From these findings we derived a new model for actin regulation during myoblast fusion.

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.