A role for the Myoblast city homologues Dock1 and Dock5 and the adaptor proteins Crk and Crk-like in zebrafish myoblast fusion

Fine-Tuning of the Actin Cytoskeleton and Cell Adhesion During Drosophila Development by the Unconventional Guanine Nucleotide Exchange Factors Myoblast City and Sponge

The evolutionarily conserved Dock proteins function as unconventional guanine nucleotide exchange factors (GEFs). Upon binding to engulfment and cell motility (ELMO) proteins, Dock-ELMO complexes activate the Rho family of small GTPases to mediate a diverse array of biological processes, including cell motility, apoptotic cell clearance, and axon guidance. Overlapping expression patterns and functional redundancy among the 11 vertebrate Dock family members, which are subdivided into four families (Dock A, B, C, and D), complicate genetic analysis. In both vertebrate and invertebrate systems, the actin dynamics regulator, Rac, is the target GTPase of the Dock-A subfamily. However, it remains unclear whether Rac or Rap1 are the in vivo downstream GTPases of the Dock-B subfamily. Drosophila melanogaster is an excellent genetic model organism for understanding Dock protein function as its genome encodes one ortholog per subfamily: Myoblast city (Mbc; Dock A) and Sponge (Spg; Dock B). Here we show that the roles of Spg and Mbc are not redundant in the Drosophila somatic muscle or the dorsal vessel. Moreover, we confirm the in vivo role of Mbc upstream of Rac and provide evidence that Spg functions in concert with Rap1, possibly to regulate aspects of cell adhesion. Together these data show that Mbc and Spg can have different downstream GTPase targets. Our findings predict that the ability to regulate downstream GTPases is dependent on cellular context and allows for the fine-tuning of actin cytoskeletal or cell adhesion events in biological processes that undergo cell morphogenesis.

The Rac1 regulator ELMO controls basal body migration and docking in multiciliated cells through interaction with Ezrin

Cilia are microtubule-based organelles that are present on most cells and are required for normal tissue development and function. Defective cilia cause complex syndromes with multiple organ manifestations termed ciliopathies. A crucial step during ciliogenesis in multiciliated cells (MCCs) is the association of future basal bodies with the apical plasma membrane, followed by their correct spacing and planar orientation. Here, we report a novel role for ELMO-DOCK1, which is a bipartite guanine nucleotide exchange factor complex for the small GTPase Rac1, and for the membrane-cytoskeletal linker Ezrin, in regulating centriole/basal body migration, docking and spacing. Downregulation of each component results in ciliopathy-related phenotypes in zebrafish and disrupted ciliogenesis in Xenopus epidermal MCCs. Subcellular analysis revealed a striking impairment of basal body docking and spacing, which is likely to account for the observed phenotypes. These results are substantiated by showing a genetic interaction between elmo1 and ezrin b. Finally, we provide biochemical evidence that the ELMO-DOCK1-Rac1 complex influences Ezrin phosphorylation and thereby probably serves as an important molecular switch. Collectively, we demonstrate that the ELMO-Ezrin complex orchestrates ciliary basal body migration, docking and positioning in vivo.

Akap12 is essential for the morphogenesis of muscles involved in zebrafish locomotion

Swimming behavior in fish is driven by coordinated contractions of muscle fibers. In zebrafish, slow muscle cell migration is crucial for the formation of the muscle network; slow myoblasts, which arise from medial adaxial cells, migrate radially to the lateral surface of the trunk and tail during embryogenesis. This study found that the zebrafish A-kinase anchoring protein (akap)12 isoforms akap12α and akap12β are required for muscle morphogenesis and locomotor activity. Embryos deficient in akap12 exhibited reduced spontaneous coiling, touch response, and free swimming. Akap12-depleted slow but not fast muscle cells were misaligned, suggesting that the behavioral abnormalities resulted from specific defects in slow muscle patterning; indeed, slow muscle cells and muscle pioneers in these embryos showed abnormal migration in a cell-autonomous manner. Taken together, these results suggest that akap12 plays a critical role in the development of zebrafish locomotion by regulating the normal morphogenesis of muscles.

Translational profiling through biotinylation of tagged ribosomes in zebrafish

Heterogeneity within a population of cells of the same type is a common theme in metazoan biology. Dissecting complex developmental and physiological processes crucially relies on our ability to probe the expression profile of these cell subpopulations. Current strategies rely on cell enrichment based on sequential or simultaneous use of multiple intersecting markers starting from a heterogeneous cell suspension. The extensive tissue manipulations required to generate single-cell suspensions, as well as the complexity of the required equipment, inherently complicate these approaches. Here, we propose an alternative methodology based on a genetically encoded system in the model organism Danio rerio (zebrafish). In transgenic fish, we take advantage of the combinatorial biotin transfer system, where polysome-associated mRNAs are selectively recovered from cells expressing both a tagged ribosomal subunit, Rpl10a, and the bacterial biotin ligase BirA. We have applied this technique to skeletal muscle development and identified new genes with interesting temporal expression patterns. Through this work we have thus developed additional tools for highly specific gene expression profiling.

A dynamic analysis of muscle fusion in the chick embryo

Skeletal muscle development, growth and regeneration depend upon the ability of muscle cells to fuse into multinucleated fibers. Surprisingly little is known about the cellular events that underlie fusion during amniote development. Here, we have developed novel molecular tools to characterize muscle cell fusion during chick embryo development. We show that all cell populations arising from somites fuse, but each with unique characteristics. Fusion in the trunk is slow and independent of fiber length. By contrast, the addition of nuclei in limb muscles is three times more rapid than in trunk and is tightly associated with fiber growth. A complex interaction takes place in the trunk, where primary myotome cells from the medial somite border rarely fuse to one another, but readily do so with anterior and posterior border cells. Conversely, resident muscle progenitors actively fuse with one another, but poorly with the primary myotome. In summary, this study unveils an unexpected variety of fusion behaviors in distinct embryonic domains that is likely to reflect a tight molecular control of muscle fusion in vertebrates.

Insights into the biological functions of Dock family guanine nucleotide exchange factors

Rho GTPases regulate many essential processes during development, yet the full impact of their upstream regulation through guanine nucleotide exchange factors (GEFs) is only beginning to be appreciated. In this review, Laurin and Côté focus on emerging biological functions of the mammalian Dock family of GEFs in development and disease and discuss how recent discoveries might be exploited for novel therapeutic strategies.

Rho GTPases play key regulatory roles in many aspects of embryonic development, regulating processes such as differentiation, proliferation, morphogenesis, and migration. Two families of guanine nucleotide exchange factors (GEFs) found in metazoans, Dbl and Dock, are responsible for the spatiotemporal activation of Rac and Cdc42 proteins and their downstream signaling pathways. This review focuses on the emerging roles of the mammalian DOCK family in development and disease. We also discuss, when possible, how recent discoveries concerning the biological functions of these GEFs might be exploited for the development of novel therapeutic strategies.

Sample drift correction following 4D confocal time-lapse imaging

The generation of four-dimensional (4D) confocal datasets; consisting of 3D image sequences over time; provides an excellent methodology to capture cellular behaviors involved in developmental processes.  The ability to track and follow cell movements is limited by sample movements that occur due to drift of the sample or, in some cases, growth during image acquisition. Tracking cells in datasets affected by drift and/or growth will incorporate these movements into any analysis of cell position. This may result in the apparent movement of static structures within the sample. Therefore prior to cell tracking, any sample drift should be corrected. Using the open source Fiji distribution (1)  of ImageJ (2,3) and the incorporated LOCI tools (4), we developed the Correct 3D drift plug-in to remove erroneous sample movement in confocal datasets. This protocol effectively compensates for sample translation or alterations in focal position by utilizing phase correlation to register each time-point of a four-dimensional confocal datasets while maintaining the ability to visualize and measure cell movements over extended time-lapse experiments.

Identification and expression analysis of the zebrafish homologs of the ceramide synthase gene family

BACKGROUND

Sphingolipids represent a major class of lipids which both serve as structural components of membranes and as bioactive molecules involved in lipid signaling. Ceramide synthases (cers) reside in the center of sphingolipid metabolism by producing ceramide through de novo synthesis or degradative pathways. While the six mammalian cers family members have been extensively studied in cell culture and in adult tissues, a systematic analysis of cers expression and function during embryogenesis is still lacking.

RESULTS

Using bioinformatic and phylogenetic analysis, we identified nine highly conserved homologs of the vertebrate cers gene family in the zebrafish genome. A systematic expression analysis throughout five developmental stages indicates that, whereas until 48 hours post fertilization most zebrafish cers homologs are expressed in distinct patterns, e.g., in the intermediate cell mass and the pronephric duct, they show a highly overlapping expression during later stages of embryonic development, mostprominently in the developing brain.

CONCLUSIONS

In this study, the expression of the cers gene homologs is comprehensively analyzed for the first time during vertebrate embryogenesis. Our data indicate that each embryonic tissue has a unique profile of cers expression during zebrafish embryogenesis suggesting tissue-specific profiles of ceramides and their derivatives.

Control of muscle fibre-type diversity during embryonic development: the zebrafish paradigm

Vertebrate skeletal muscle is composed of distinct types of fibre that are functionally adapted through differences in their physiological and metabolic properties. An understanding of the molecular basis of fibre-type specification is of relevance to human health and fitness. The zebrafish provides an attractive model for investigating fibre type specification; not only are their rapidly developing embryos optically transparent, but in contrast to amniotes, the embryonic myotome shows a discrete temporal and spatial separation of fibre type ontogeny that simplifies its analysis. Here we review the current state of understanding of muscle fibre type specification and differentiation during embryonic development of the zebrafish, with a particular focus on the roles of the Prdm1a and Sox6 transcription factors, and consider the relevance of these findings to higher vertebrate muscle biology.

Phosphatidylserine receptor BAI1 and apoptotic cells as new promoters of myoblast fusion

Skeletal muscle arises from the fusion of precursor myoblasts into multinucleated myofibers^1,2. While conserved transcription factors and signaling proteins involved in myogenesis have been identified, upstream regulators are less well understood. Here, we report an unexpected discovery that the membrane protein BAI1, previously linked to recognition of apoptotic cells by phagocytes³, promotes myoblast fusion. Endogenous BAI1 expression increased during myoblast fusion, and BAI1 overexpression enhanced myoblast fusion via signaling through ELMO/Dock180/Rac1 proteins⁴. During myoblast fusion, a fraction of myoblasts underwent apoptosis and exposed phosphatidylserine (PtdSer), an established ligand for BAI1³. Blocking apoptosis potently impaired myoblast fusion, and adding back apoptotic myoblasts restored fusion. Furthermore, primary human myoblasts could be induced to form myotubes by adding apoptotic myoblasts, even under normal growth conditions. In vivo, myofibers from Bai1^−/− mice are smaller than wild-type littermates. Muscle regeneration after injury was also impaired in Bai1^−/− mice, highlighting a role for BAI1 in mammalian myogenesis. Collectively, these data identify signaling via the phosphatidylserine receptor BAI1 and apoptotic cells as novel promoters of myoblast fusion, with significant implications for muscle development and repair.

Signaling mechanisms in mammalian myoblast fusion

Myoblast fusion is a critical process that contributes to the growth of muscle during development and to the regeneration of myofibers upon injury. Myoblasts fuse with each other as well as with multinucleated myotubes to enlarge the myofiber. Initial studies demonstrated that myoblast fusion requires extracellular calcium and changes in cell membrane topography and cytoskeletal organization. More recent studies have identified several cell-surface and intracellular proteins that mediate myoblast fusion. Furthermore, emerging evidence suggests that myoblast fusion is also regulated by the activation of specific cell-signaling pathways that lead to the expression of genes whose products are essential for the fusion process and for modulating the activity of molecules that are involved in cytoskeletal rearrangement. Here, we review the roles of the major signaling pathways in mammalian myoblast fusion.

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.

Starfish ApDOCK protein essentially functions in larval defense system operated by mesenchyme cells

In larvae of the starfish, Asterina pectinifera, mesenchyme cells operate in the defense system through various behaviors. We have investigated mesenchyme cell dynamics during the immune response by identifying ApDOCK, a new member of the DOCK180 superfamily protein. In 4-day-old bipinnaria larvae processed for morpholino oligonucleotide-mediated knockdown of ApDOCK, injection of inorganic foreign substances revealed that (1) mesenchyme cells fail to undergo either directed migration toward a large oil-droplet or persistent spreading on the oil-droplet after contact; (2) neither uptake of micro-beads nor cell-to-cell fusion on the large oil-droplet differed from that of mesenchyme cells from control larvae. Similar behaviors were also recorded in experiments where bacteria were injected. Under culture conditions, the expression level of ApDOCK mRNA was significantly associated with the immunological behavior of mesenchyme cells. Apparently, the mesenchyme cells from ApDOCK loss-of-function larvae exhibited insufficient lamellipodium formation via lack of fibrous form of actin organization at the leading edge. These results suggest that the migratory congregation and persistence of encapsulation of larval mesenchyme cells are intracellularly regulated by ApDOCK protein, and this regulation is associated with organization of cytoskeletal actin.

Do muscle founder cells exist in vertebrates?

Skeletal muscle is formed by the iterative fusion of precursor cells (myocytes) into long multinuclear fibres. Extensive studies of fusion in Drosophila embryos have lead to a paradigm in which myoblasts are divided into two distinct subtypes - founder and fusion-competent myoblasts (FCMs) - that can fuse to each other, but not among themselves. Only founder cells can direct the formation of muscle fibres, while FCMs act as a cellular substrate. Recent studies in zebrafish and mice have demonstrated conservation of the molecules originally identified in Drosophila, but an important question remains: is vertebrate fusion regulated by specifying myocyte subtypes? Stated simply: do vertebrate founder cells exist? In light of recent findings, we argue that a different regulatory mechanism has evolved in vertebrates.

CKIP-1 regulates mammalian and zebrafish myoblast fusion

Multinucleated muscle fibres arise by fusion of precursor cells called myoblasts. We previously showed that CKIP-1 ectopic expression in C2C12 myoblasts increased cell fusion. In this work, we report that CKIP-1 depletion drastically impairs C2C12 myoblast fusion in vitro and in vivo during zebrafish muscle development. Within developing fast-twich myotome, Ckip-1 localises at the periphery of fast precursor cells, closed to the plasma membrane. Unlike wild-type myoblasts that form spatially arrayed multinucleated fast myofibres, Ckip-1-deficient myoblasts show a drastic reduction in fusion capacity. A search for CKIP-1 binding partners identified the ARPC1 subunit of Arp2/3 actin nucleation complex essential for myoblast fusion. We demonstrate that CKIP-1, through binding to plasma membrane phosphoinositides via its PH domain, regulates cell morphology and lamellipodia formation by recruiting the Arp2/3 complex at the plasma membrane. These results establish CKIP-1 as a regulator of cortical actin that recruits the Arp2/3 complex at the plasma membrane essential for muscle precursor elongation and fusion.

Immunocytochemistry to study myogenesis in zebrafish

During myogenesis, cells gradually transition from mesodermal precursors to myoblasts, myocytes, and then to muscle fibers. The molecular characterization of this process requires the ability to identify each of these cell types and the factors that regulate the transitions between them. The most versatile technique for assaying cell identities in situ is immunocytochemistry, because multiple independent molecular markers of differentiation can be assayed simultaneously. The zebrafish has developed into a popular model for the study of myogenesis, and immunocytochemical techniques have been critical. We have adapted existing protocols to optimize immunocytochemistry in zebrafish, and have tested many antibodies developed against mouse, chick, and frog muscle antigens for their cross-reactivity in zebrafish. Here, we present protocols for whole mount immunocytochemistry on both formaldehyde and Carnoy's fixed embryos as well as on sectioned zebrafish tissue. We include a table of antibodies useful for experiments on the molecular biology of myogenesis in zebrafish.

Alkali-like myosin light chain-1 (myl1) is an early marker for differentiating fast muscle cells in zebrafish

During myogenesis, muscle precursors become divided into either fast- or slow-twitch fibres, which in the zebrafish occupy distinct domains in the embryo. Genes encoding sarcomeric proteins specific for fast or slow fibres are frequently used as lineage markers. In an attempt to identify and evaluate early definitive markers for cells in the fast-twitch pathway, we analysed genes encoding proteins contributing to the fast sarcomeric structures. The previously uncharacterized zebrafish alkali-like myosin light chain gene (myl1) was found to be expressed exclusively in cells in the fast-twitch pathway initiated at an early stage of fast fibre differentiation. Myl1 was expressed earlier, and in a more fibre type restricted manner, than any of the previously described and frequently used fast myosin light and heavy chain and troponin muscle markers mylz2, mylz3, tnni2, tnnt3a, fMyHC1.3. In summary, this study introduces a novel marker for early differentiating fast muscle cells.

Jamb and jamc are essential for vertebrate myocyte fusion

Jamb and Jamc are an essential cell surface receptor pair that interact to drive fusion between muscle precursor cells during zebrafish development.

Cellular fusion is required in the development of several tissues, including skeletal muscle. In vertebrates, this process is poorly understood and lacks an in vivo-validated cell surface heterophilic receptor pair that is necessary for fusion. Identification of essential cell surface interactions between fusing cells is an important step in elucidating the molecular mechanism of cellular fusion. We show here that the zebrafish orthologues of JAM-B and JAM-C receptors are essential for fusion of myocyte precursors to form syncytial muscle fibres. Both jamb and jamc are dynamically co-expressed in developing muscles and encode receptors that physically interact. Heritable mutations in either gene prevent myocyte fusion in vivo, resulting in an overabundance of mononuclear, but otherwise overtly normal, functional fast-twitch muscle fibres. Transplantation experiments show that the Jamb and Jamc receptors must interact between neighbouring cells (in trans) for fusion to occur. We also show that jamc is ectopically expressed in prdm1a mutant slow muscle precursors, which inappropriately fuse with other myocytes, suggesting that control of myocyte fusion through regulation of jamc expression has important implications for the growth and patterning of muscles. Our discovery of a receptor-ligand pair critical for fusion in vivo has important implications for understanding the molecular mechanisms responsible for myocyte fusion and its regulation in vertebrate myogenesis.

The fusion of precursor cells is a crucial step in many biological processes, one of which is the development of skeletal muscle. The molecular and cell biology of fusion of muscle precursors has been well described in Drosophila melanogaster larvae, leading to insights into the process in vertebrates. However, the identity and mechanism of action of essential cell surface proteins for fusion between vertebrate muscle precursors has previously been lacking. Here, we describe a vertebrate-specific cell surface receptor pair that is essential for fusion in zebrafish: Jamb and Jamc. Loss of function of either receptor causes a near-complete block in fusion, resulting in an overabundance of mononucleate muscle fibres that are otherwise overtly normal. We demonstrate that Jamb and Jamc physically interact and are co-expressed by muscle precursors. Moreover, we show that the interaction between them is essential for fusion between neighbouring precursors in an embryo. We hypothesise that binding of Jamb to Jamc is a necessary recognition and adhesion step permissive for, but not sufficient to cause, myocyte fusion. Knowledge of these molecular components in vertebrates will lead to better understanding of how fusion is controlled to pattern skeletal muscle tissue.

Connexin 39.9 protein is necessary for coordinated activation of slow-twitch muscle and normal behavior in zebrafish

In many tissues and organs, connexin proteins assemble between neighboring cells to form gap junctions. These gap junctions facilitate direct intercellular communication between adjoining cells, allowing for the transmission of both chemical and electrical signals. In rodents, gap junctions are found in differentiating myoblasts and are important for myogenesis. Although gap junctions were once believed to be absent from differentiated skeletal muscle in mammals, recent studies in teleosts revealed that differentiated muscle does express connexins and is electrically coupled, at least at the larval stage. These findings raised questions regarding the functional significance of gap junctions in differentiated muscle. Our analysis of gap junctions in muscle began with the isolation of a zebrafish motor mutant that displayed weak coiling at day 1 of development, a behavior known to be driven by slow-twitch muscle (slow muscle). We identified a missense mutation in the gene encoding Connexin 39.9. In situ hybridization found connexin 39.9 to be expressed by slow muscle. Paired muscle recordings uncovered that wild-type slow muscles are electrically coupled, whereas mutant slow muscles are not. The further examination of cellular activity revealed aberrant, arrhythmic touch-evoked Ca(2+) transients in mutant slow muscle and a reduction in the number of muscle fibers contracting in response to touch in mutants. These results indicate that Connexin 39.9 facilitates the spreading of neuronal inputs, which is irregular during motor development, beyond the muscle cells and that gap junctions play an essential role in the efficient recruitment of slow muscle fibers.

Growth and the regulation of myotomal muscle mass in teleost fish

Teleost muscle first arises in early embryonic life and its development is driven by molecules present in the egg yolk and modulated by environmental stimuli including temperature and oxygen. Several populations of myogenic precursor cells reside in the embryonic somite and external cell layer and contribute to muscle fibres in embryo, larval, juvenile and adult stages. Many signalling proteins and transcription factors essential for these events are known. In all cases, myogenesis involves myoblast proliferation, migration, fusion and terminal differentiation. Maturation of the embryonic muscle is associated with motor innervation and the development of a scaffold of connective tissue and complex myotomal architecture needed to generate swimming behaviour. Adult muscle is a heterogeneous tissue composed of several cell types that interact to affect growth patterns. The development of capillary and lymphatic circulations and extramuscular organs--notably the gastrointestinal, endocrine, neuroendocrine and immune systems--serves to increase information exchange between tissues and with the external environment, adding to the complexity of growth regulation. Teleosts often exhibit an indeterminate growth pattern, with body size and muscle mass increasing until mortality or senescence occurs. The dramatic increase in myotomal muscle mass between embryo and adult requires the continuous production of muscle fibres until 40-50% of the maximum body length is reached. Sarcomeric proteins can be mobilised as a source of amino acids for energy metabolism by other tissues and for gonad generation, requiring the dynamic regulation of muscle mass throughout the life cycle. The metabolic and contractile phenotypes of muscle fibres also show significant plasticity with respect to environmental conditions, migration and spawning. Many genes regulating muscle growth are found as multiple copies as a result of paralogue retention following whole-genome duplication events in teleost lineages. The extent to which indeterminate growth, ectothermy and paralogue preservation have resulted in modifications of the genetic pathways regulating muscle growth in teleosts compared to mammals largely remains unknown. This review describes the use of compensatory growth models, transgenesis and tissue culture to explore the mechanisms of muscle growth in teleosts and provides some perspectives on future research directions.

Defective cranial skeletal development, larval lethality and haploinsufficiency in Myod mutant zebrafish

Myogenic regulatory factors of the myod family (MRFs) are transcription factors essential for mammalian skeletal myogenesis. Here we show that a mutation in the zebrafish myod gene delays and reduces early somitic and pectoral fin myogenesis, reduces miR-206 expression, and leads to a persistent reduction in somite size until at least the independent feeding stage. A mutation in myog, encoding a second MRF, has little obvious phenotype at early stages, but exacerbates the loss of somitic muscle caused by lack of Myod. Mutation of both myod and myf5 ablates all skeletal muscle. Haploinsufficiency of myod leads to reduced embryonic somite muscle bulk. Lack of Myod causes a severe reduction in cranial musculature, ablating most muscles including the protractor pectoralis, a putative cucullaris homologue. This phenotype is accompanied by a severe dysmorphology of the cartilaginous skeleton and failure of maturation of several cranial bones, including the opercle. As myod expression is restricted to myogenic cells, the data show that myogenesis is essential for proper skeletogenesis in the head.

Generation of a monoclonal antibody reactive to prefusion myocytes

We established a novel monoclonal antibody, Yaksa that is specific to a subpopulation of myogenic cells. The Yaksa antigen is not expressed on the surface of growing myoblasts but only on a subpopulation of myogenin-positive myocytes. When Yaksa antigen-positive mononucleated cells were freshly prepared from a murine myogenic cell by a cell sorter, they fused with each other and formed multinucleated myotubes shortly after replating while Yaksa antigen-negative cells scarcely generated myotubes. These results suggest that Yaksa could segregate fusion-competent, mononucleated cells from fusion–incompetent cells during muscle differentiation. The Yaksa antigen was also expressed in developing muscle and regenerating muscle in vivo and it was localized at sites of cell–cell contact between mono-nucleated muscle cells and between mono-nucleated muscle cells and myotubes. Thus, Yaksa that marks prefusion myocytes before myotube formation can be a useful tool to elucidate the cellular and molecular mechanisms of myogenic cell fusion.

Developmental transition of touch response from slow muscle-mediated coilings to fast muscle-mediated burst swimming in zebrafish

It is well known that slow and fast muscles are used for long-term sustained movement and short bursts of activity, respectively, in adult animal behaviors. However, the contribution of the slow and fast muscles in early animal movement has not been thoroughly explored. In wild-type zebrafish embryos, tactile stimulation induces coilings consisting of 1-3 alternating contractions of the trunk and tail at 24 hours postfertilization (hpf) and burst swimming at 48 hpf. But, embryos defective in flightless I homolog (flii), which encodes for an actin-regulating protein, exhibit normal coilings at 24 hpf that is followed by significantly slower burst swimming at 48 hpf. Interestingly, actin fibers are disorganized in mutant fast muscle but not in mutant slow muscle, suggesting that slower swimming at 48 hpf is attributable to defects of the fast muscle tissue. In fact, perturbation of the fast muscle contractions by eliminating Ca(2+) release only in fast muscle resulted in normal coilings at 24 hpf and slower burst swimming at 48 hpf, just as flii mutants exhibited. In contrast, specific inactivation of slow muscle by knockdown of the slow muscle myosin genes led to complete loss of coilings at 24 hpf, although normal burst swimming was retained by 48 hpf. These findings indicate that coilings at 24 hpf is mediated by slow muscle only, whereas burst swimming at 48 hpf is executed primarily by fast muscle. It is consistent with the fact that differentiation of fast muscle follows that of slow muscle. This is the first direct demonstration that slow and fast muscles have distinct physiologically relevant contribution in early motor development at different stages.

Fine-tuning of Hh signaling by the RNA-binding protein Quaking to control muscle development

The development of the different muscles within the somite is a complex process that involves the Hedgehog (Hh) signaling pathway. To specify the proper number of muscle cells and organize them spatially and temporally, the Hh signaling pathway needs to be precisely regulated at different levels, but only a few factors external to the pathway have been described. Here, we report for the first time the role of the STAR family RNA-binding protein Quaking A (QkA) in somite muscle development. We show in zebrafish that the loss of QkA function affects fast muscle fiber maturation as well as Hh-induced muscle derivative specification and/or morphogenesis. Mosaic analysis reveals that fast fiber maturation depends on the activity of QkA in the environment of fast fiber progenitors. We further show that Hh signaling requires QkA activity for muscle development. By an in silico approach, we screened the 3'UTRs of known Hh signaling component mRNAs for the Quaking response element and found the transcription factor Gli2a, a known regulator of muscle fate development. Using destabilized GFP as a reporter, we show that the gli2a mRNA 3'UTR is a functional QkA target. Consistent with this notion, the loss of QkA function rescued slow muscle fibers in yot mutant embryos, which express a dominant-negative Gli2a isoform. Thus, our results reveal a new mechanism to ensure muscle cell fate diversity by fine-tuning of the Hh signaling pathway via RNA-binding proteins.

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.

An evolutionarily conserved autoinhibitory molecular switch in ELMO proteins regulates Rac signaling

Dedicator of cytokinesis (DOCK) proteins are guanine nucleotide exchange factors (GEFs) controlling the activity of Rac1/Cdc42 during migration, phagocytosis, and myoblast fusion [1-4]. Engulfment and cell motility (ELMO) proteins bind a subset of DOCK members and are emerging as critical regulators of Rac signaling [5-10]. Although formation of a DOCK180/ELMO complex is not essential for Rac1 activation, ELMO mutants deficient in binding to DOCK180 are unable to promote cytoskeleton remodeling [11]. How ELMO regulates signaling through DOCK GEFs is poorly understood. Here, we identify an autoinhibitory switch in ELMO presenting homology to a regulatory unit described for Dia formins. One part of the switch, composed of a Ras-binding domain (RBD) and Armadillo repeats, is positioned N-terminally while the other is housed in the C terminus. We demonstrate interaction between these fragments, suggesting autoinhibition of ELMO. Using a bioluminescence resonance energy transfer biosensor, we establish that ELMO undergoes conformational changes upon disruption of autoinhibition. We found that engagement of ELMO to RhoG, or with DOCK180, promotes the relief of autoinhibition in ELMO. Functionally, we found that ELMO mutants with impaired autoregulatory activity promote cell elongation. These results demonstrate an unsuspected level of regulation for Rac1 signaling via autoinhibition of ELMO.

Evolution of programmed cell fusion: common mechanisms and distinct functions

Eukaryotic cells have evolved diverged mechanisms to merge cells. Here, we discuss three types of cell fusion: (1) Non-self-fusion, cells with different genetic contents fuse to start a new organism and fusion between enveloped viruses and host cells; (2) Self-fusion, genetically identical cells fuse to form a multinucleated cell; and (3) Auto-fusion, a single cell fuses with itself by bringing specialized cell membrane domains into contact and transforming itself into a ring-shaped cell. This is a new type of selfish fusion discovered in C. elegans. We divide cell fusion into three stages: (1) Specification of the cell-fusion fate; (2) Cell attraction, attachment, and recognition; (3) Execution of plasma membrane fusion, cytoplasmic mixing and cytoskeletal rearrangements. We analyze cell fusion in diverse biological systems in development and disease emphasizing the mechanistic contributions of C. elegans to the understanding of programmed cell fusion, a genetically encoded pathway to merge specific cells.

ADP-ribosylation factor 6 regulates mammalian myoblast fusion through phospholipase D1 and phosphatidylinositol 4,5-bisphosphate signaling pathways

Here we show that ARF6 is associated with the multiproteic complex that contains M-cadherin, Trio, and Rac1 and accumulates at sites of myoblast fusion. ARF6 silencing inhibits the association of Trio and Rac1 with M-cadherin. Moreover, we demonstrate that ARF6 regulates myoblast fusion through Phospholipase D activation and PI(4,5)P2 production.

Myoblast fusion is an essential step during myoblast differentiation that remains poorly understood. M-cadherin–dependent pathways that signal through Rac1 GTPase activation via the Rho-guanine nucleotide exchange factor (GEF) Trio are important for myoblast fusion. The ADP-ribosylation factor (ARF)6 GTPase has been shown to bind to Trio and to regulate Rac1 activity. Moreover, Loner/GEP₁₀₀/BRAG2, a GEF of ARF6, has been involved in mammalian and Drosophila myoblast fusion, but the specific role of ARF6 has been not fully analyzed. Here, we show that ARF6 activity is increased at the time of myoblast fusion and is required for its implementation in mouse C2C12 myoblasts. Specifically, at the onset of myoblast fusion, ARF6 is associated with the multiproteic complex that contains M-cadherin, Trio, and Rac1 and accumulates at sites of myoblast fusion. ARF6 silencing inhibits the association of Trio and Rac1 with M-cadherin. Moreover, we demonstrate that ARF6 regulates myoblast fusion through phospholipase D (PLD) activation and phosphatidylinositol 4,5-bis-phosphate production. Together, these data indicate that ARF6 is a critical regulator of C2C12 myoblast fusion and participates in the regulation of PLD activities that trigger both phospholipids production and actin cytoskeleton reorganization at fusion sites.

Regulation of promyogenic signal transduction by cell-cell contact and adhesion

Skeletal myoblast differentiation involves acquisition of the muscle-specific transcriptional program and morphological changes, including fusion into multinucleated myofibers. Differentiation is regulated by extracellular signaling cues, including cell-cell contact and adhesion. Cadherin and Ig adhesion receptors have been implicated in distinct but overlapping stages of myogenesis. N-cadherin signals through the Ig receptor Cdo to activate p38 MAP kinase, while the Ig receptor neogenin signals to activate FAK; both processes promote muscle-specific gene expression and myoblast fusion. M-cadherin activates Rac1 to enhance fusion. Specific Ig receptors (Kirre and Sns) are essential for myoblast fusion in Drosophila, also signaling through Rac, and vertebrate orthologs of Kirre and Sns have partially conserved function. Mice lacking specific cytoplasmic signaling factors activated by multiple receptors (e.g., Rac1) have strong muscle phenotypes in vivo. In contrast, mice lacking individual adhesion receptors that lie upstream of these factors have modest phenotypes. Redundancy among receptors may account for this. Many of the mammalian Ig receptors and cadherins associate with each other, and multivalent interactions within these complexes may require removal of multiple components to reveal dramatic defects in vivo. Nevertheless, it is possible that the murine adhesion receptors rate-limiting in vivo have not yet been identified or fully assessed.

The Rac1 regulator ELMO1 controls vascular morphogenesis in zebrafish

RATIONALE

Angiogenesis is regulated by the small GTPase Rac1. The ELMO1/DOCK180 complex forms a guanine nucleotide exchange factor for Rac1, regulating its activation during cell migration in different biological systems.

OBJECTIVE

To investigate the function of ELMO1/DOCK180 in vascular development.

METHODS AND RESULTS

In situ hybridization studies for elmo1 identified a vascular and neuronal expression in zebrafish. Morpholino-based expression silencing of elmo1 severely impaired the formation of the vasculature, including intersomitic vessels, the dorsal longitudinal anastomotic vessel, the parachordal vessel, and the development of the thoracic duct in tg(fli1:EGFP) embryos. Mechanistically, we identified Netrin-1 and its receptor Unc5B as upstream activators of the ELMO1/DOCK180 complex, regulating its functional interaction and leading to Rac1 activation in endothelial cells and vessel formation in zebrafish.

CONCLUSIONS

Our data have identified a novel signaling cascade regulating vasculature formation in zebrafish.

Structural basis of membrane targeting by the Dock180 family of Rho family guanine exchange factors (Rho-GEFs)

The Dock180 family of atypical Rho family guanine nucleotide exchange factors (Rho-GEFs) regulate a variety of processes involving cellular or subcellular polarization, including cell migration and phagocytosis. Each contains a Dock homology region-1 (DHR-1) domain that is required to localize its GEF activity to a specific membrane compartment where levels of phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P(3)) are up-regulated by the local activity of PtdIns 3-kinase. Here we define the structural and energetic bases of phosphoinositide specificity by the DHR-1 domain of Dock1 (a GEF for Rac1), and show that DHR-1 utilizes a C2 domain scaffold and surface loops to create a basic pocket on its upper surface for recognition of the PtdIns(3,4,5)P(3) head group. The pocket has many of the characteristics of those observed in pleckstrin homology domains. We show that point mutations in the pocket that abolish phospholipid binding in vitro ablate the ability of Dock1 to induce cell polarization, and propose a model that brings together recent mechanistic and structural studies to rationalize the central role of DHR-1 in dynamic membrane targeting of the Rho-GEF activity of Dock180.

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.

Embryonic temperature affects muscle fibre recruitment in adult zebrafish: genome-wide changes in gene and microRNA expression associated with the transition from hyperplastic to hypertrophic growth phenotypes

We investigated the effects of embryonic temperature (ET) treatments (22, 26 and 31 degrees C) on the life-time recruitment of fast myotomal muscle fibres in zebrafish Danio rerio L. reared at 26/27 degrees C from hatching. Fast muscle fibres were produced until 25 mm total length (TL) at 22 degrees C ET, 28 mm TL at 26 degrees C ET and 23 mm TL at 31 degrees C ET. The final fibre number (FFN) showed an optimum at 26 degrees C ET (3600) and was 19% and 14% higher than for the 22 degrees C ET (3000) and 31 degrees C ET (3100) treatments, respectively. Further growth to the maximum TL of approximately 48 mm only involved fibre hypertrophy. Microarray experiments were used to determine global changes in microRNA (miRNA) and mRNA expression associated with the transition from the hyperplasic myotube-producing phenotype (M(+), 10-12 mm TL) to the hypertrophic growth phenotype (M(-), 28-31 mm TL) in fish reared at 26-27 degrees C over the whole life-cycle. The expression of miRNAs and mRNAs obtained from microarray experiments was validated by northern blotting and real-time qPCR in independent samples of fish with the M(+) and M(-) phenotype. Fourteen down-regulated and 15 up-regulated miRNAs were identified in the M(-) phenotype together with 34 down-regulated and 30 up-regulated mRNAs (>2-fold; P<0.05). The two most abundant categories of down-regulated genes in the M(-) phenotype encoded contractile proteins (23.5%) and sarcomeric structural/cytoskeletal proteins (14.7%). In contrast, the most highly represented up-regulated transcripts in the M(-) phenotype were energy metabolism (26.7%) and immune-related (20.0%) genes. The latter were mostly involved in cell-cell interactions and cytokine pathways and included beta-2-microglobulin precursor (b2m), an orthologue of complement component 4, invariant chain-like protein 1 (iclp), CD9 antigen-like (cd9l), and tyrosine kinase, non-receptor (tnk2). Five myosin heavy chain genes that were down-regulated in the M(-) phenotype formed part of a tandem repeat on chromosome 5 and were shown by in situ hybridisation to be specifically expressed in nascent myofibres. Seven up-regulated miRNAs in the M(-) phenotype showed reciprocal expression with seven mRNA targets identified in miRBase Targets version 5 (http://microrna.sanger.ac.uk/targets/v5/), including asporin (aspn) which was the target for four miRNAs. Eleven down-regulated miRNAs in the M(-) phenotype had predicted targets for seven up-regulated genes, including dre-miR-181c which had five predicted mRNA targets. These results provide evidence that miRNAs play a role in regulating the transition from the M(+) to the M(-) phenotype and identify some of the genes and regulatory interactions involved.

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.

Regulation of focal adhesion and cell migration by ANKRD28-DOCK180 interaction

DOCK180 is an atypical guanine nucleotide exchange factor of Rac1 identified originally as one of the two major proteins bound to the SH3 domain of the Crk adaptor protein. DOCK180 induces tyrosine phosphorylation of p130(Cas), and recruits the Crk-p130(Cas) complex to focal adhesions. Recently, we searched for DOCK180-binding proteins with a nano-LC/MS/MS system, and found that ANKRD28, a protein with twenty-six ankyrin domain-repeats, interacts with the SH3 domain of DOCK180. Knockdown of ANKRD28 reduced the migration velocity and altered the distribution of focal adhesion proteins such as Crk, paxillin and p130(Cas). On the other hand, the expression of ANKRD28, p130(Cas), Crk and DOCK180 induced hyper-phosphorylation of p130(Cas), which paralleled the induction of multiple long cellular processes. Depletion of ELMO, another protein bound to the SH3 domain of DOCK180, also retarded cell migration, but its expression together with p130(Cas), Crk and DOCK180 induced extensive lamellipodial protrusion around the entire circumference without 130(Cas) hyperphosphorylation. These data suggest the dual modes of DOCK180-Rac regulation for cell migration.

Modulation of gene expression by human cytosolic tRNase Z(L) through 5'-half-tRNA

A long form (tRNase Z(L)) of tRNA 3' processing endoribonuclease (tRNase Z, or 3' tRNase) can cleave any target RNA at any desired site under the direction of artificial small guide RNA (sgRNA) that mimics a 5'-half portion of tRNA. Based on this enzymatic property, a gene silencing technology has been developed, in which a specific mRNA level can be downregulated by introducing into cells a synthetic 5'-half-tRNA that is designed to form a pre-tRNA-like complex with a part of the mRNA. Recently 5'-half-tRNA fragments have been reported to exist stably in various types of cells, although little is know about their physiological roles. We were curious to know if endogenous 5'-half-tRNA works as sgRNA for tRNase Z(L) in the cells. Here we show that human cytosolic tRNase Z(L) modulates gene expression through 5'-half-tRNA. We found that 5'-half-tRNA(Glu), which co-immunoprecipitates with tRNase Z(L), exists predominantly in the cytoplasm, functions as sgRNA in vitro, and downregulates the level of a luciferase mRNA containing its target sequence in human kidney 293 cells. We also demonstrated that the PPM1F mRNA is one of the genuine targets of tRNase Z(L) guided by 5'-half-tRNA(Glu). Furthermore, the DNA microarray data suggested that tRNase Z(L) is likely to be involved in the p53 signaling pathway and apoptosis.

Focal adhesion kinase signaling regulates the expression of caveolin 3 and beta1 integrin, genes essential for normal myoblast fusion

An essential phase of skeletal myogenesis is the fusion of mononucleated myoblasts to form multinucleated myotubes. Many cell adhesion proteins, including integrins, have been shown to be important for myoblast fusion in vertebrates, but the mechanisms by which these proteins regulate cell fusion remain mostly unknown. Here, we focused on the role of focal adhesion kinase (FAK), an important nonreceptor protein tyrosine kinase involved in integrin signaling, as a potential mediator by which integrins may regulate myoblast fusion. To test this hypothesis in vivo, we generated mice in which the Fak gene was disrupted specifically in muscle stem cells ("satellite cells") and we found that this resulted in impaired myotube formation during muscle regeneration after injury. To examine the role of FAK in the fusion of myogenic cells, we examined the expression of FAK and the effects of FAK deletion on the differentiation of myoblasts in vitro. Differentiation of mouse primary myoblasts was accompanied by a rapid and transient increase of phosphorylated FAK. To investigate the requirement of FAK in myoblast fusion, we used two loss-of-function approaches (a dominant-negative inhibitor of FAK and FAK small interfering RNA [siRNA]). Inhibition of FAK resulted in markedly impaired fusion but did not inhibit other biochemical measures of myogenic differentiation, suggesting a specific role of FAK in the morphological changes of cell fusion as part of the differentiation program. To examine the mechanisms by which FAK may be regulating fusion, we used microarray analysis to identify the genes that failed to be normally regulated in cells that were fusion defective due to FAK inhibition. Several genes that have been implicated in myoblast fusion were aberrantly regulated during differentiation when FAK was inhibited. Intriguingly, the normal increases in the transcript of caveolin 3 as well as an integrin subunit, the beta1D isoform, were suppressed by FAK inhibition. We confirmed this also at the protein level and show that direct inhibition of beta1D subunit expression by siRNA inhibited myotube formation with a prominent effect on secondary fusion. These data suggest that FAK regulation of profusion genes, including caveolin 3 and the beta1D integrin subunit, is essential for morphological muscle differentiation.

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.

Differential requirements for myogenic regulatory factors distinguish medial and lateral somitic, cranial and fin muscle fibre populations

Myogenic regulatory factors of the Myod family (MRFs) are transcription factors essential for mammalian skeletal myogenesis. However, the roles of each gene in myogenesis remain unclear, owing partly to genetic linkage at the Myf5/Mrf4 locus and to rapid morphogenetic movements in the amniote somite. In mice, Myf5 is essential for the earliest epaxial myogenesis, whereas Myod is required for timely differentiation of hypaxially derived muscle. A second major subdivision of the somite is between primaxial muscle of the somite proper and abaxial somite-derived migratory muscle precursors. Here, we use a combination of mutant and morphant analysis to ablate the function of each of the four conserved MRF genes in zebrafish, an organism that has retained a more ancestral bodyplan. We show that a fundamental distinction in somite myogenesis is into medial versus lateral compartments, which correspond to neither epaxial/hypaxial nor primaxial/abaxial subdivisions. In the medial compartment, Myf5 and/or Myod drive adaxial slow fibre and medial fast fibre differentiation. Myod-driven Myogenin activity alone is sufficient for lateral fast somitic and pectoral fin fibre formation from the lateral compartment, as well as for cranial myogenesis. Myogenin activity is a significant contributor to fast fibre differentiation. Mrf4 does not contribute to early myogenesis in zebrafish. We suggest that the differential use of duplicated MRF paralogues in this novel two-component myogenic system facilitated the diversification of vertebrates.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

The extracellular matrix protein TGFBI promotes myofibril bundling and muscle fibre growth in the zebrafish embryo

The extracellular matrix protein Tgfbi has been shown to localise to myotendinous junctions in mouse and postulated to interact with the transmembrane protein Integrin alpha7beta1, which, in parallel with the Dystrophin-associated protein complex, is critical for linkage between the extracellular matrix and the cytoskeleton of muscle fibres. Here we use a GFP-tagged form of Tgfbi to analyse its distribution in the developing skeletal muscle of the zebrafish embryos and antisense morpholino oligonucleotides to investigate the function of the endogenous protein. We find that although tagged Tgfbi accumulates at the myosepta, the attachment of muscle fibres to the myosepta is established and maintained normally in morphant embryos; however, the fibres show a marked reduction in their growth and a disruption of their myofibril bundles.

Lbx2 regulates formation of myofibrils

BACKGROUND

Skeletal muscle differentiation requires assembly of contractile proteins into organized myofibrils. The Drosophila ladybird homeobox gene (lad) functions in founder cells of the segmental border muscle to promote myoblast fusion and muscle shaping. Tetrapods have two homologous genes (Lbx). Lbx1 functions in migration and/or proliferation of hypaxial myoblasts, whereas the function of Lbx2 is poorly understood.

RESULTS

To elucidate the role of Lbx in vertebrate myogenesis, we examined Lbx function in zebrafish. Zebrafish lbx2 transcripts appear in newly formed paraxial mesoderm and become restricted to adaxial cells, precursors of slow muscle. Slow muscles lose lbx2 expression as they differentiate, while a subset of differentiating fast muscle cells transiently expresses lbx2. Fin and hyoid muscle express lbx2 later. In contrast, lbx1b expression first appears lateral to the somites at late segmentation stages and is later restricted to fin muscle. Morpholino knockdown of Lbx1b and Lbx2 suppresses hypaxial muscle development. Moreover, knockdown of Lbx2 results in malformation of muscle fibers and reduced fusion of fast precursors, although no obvious effects on induction or specification are observed. Expression of myofilament genes, including actin and myosin, requires the engrailed repressor domain of Lbx2.

CONCLUSION

Our results elucidate a new function of Lbx2 as a regulator of myofibril formation.

New insights into skeletal muscle development and growth in teleost fishes

Recent research has significantly broadened our understanding of how the teleost somite is patterned to achieve embryonic and postembryonic myogenesis. Medial (adaxial) cells and posterior cells of the early epithelial somite generate embryonic superficial slow and deep fast muscle fibers, respectively, whereas anterior somitic cells move laterally to form an external cell layer of undifferentiated Pax7-positive myogenic precursors surrounding the embryonic myotome. In late embryo and in larvae, some of the cells contained in the external cell layer incorporate into the myotome and differentiate into new muscle fibers, thus contributing to medio-lateral expansion of the myotome. This supports the suggestion that the teleost external cell layer is homologous to the amniote dermomyotome. Some of the signalling molecules that promote lateral movement or regulate the myogenic differentiation of external cell precursors have been identified and include stromal cell-derived factor 1 (Sdf1), hedgehog proteins, and fibroblast growth factor 8 (Fgf8). Recent studies have shed light on gene activations that underlie the differentiation and maturation of slow and fast muscle fibers, pointing out that both adaxially derived embryonic slow fibers and slow fibers formed during the myotome expansion of larvae initially and transiently bear features of the fast fiber phenotype.

Dictyostelium Dock180-related RacGEFs regulate the actin cytoskeleton during cell motility

Cell motility of amoeboid cells is mediated by localized F-actin polymerization that drives the extension of membrane protrusions to promote forward movements. We show that deletion of either of two members of the Dictyostelium Dock180 family of RacGEFs, DockA and DockD, causes decreased speed of chemotaxing cells. The phenotype is enhanced in the double mutant and expression of DockA or DockD complements the reduced speed of randomly moving DockD null cells' phenotype, suggesting that DockA and DockD are likely to act redundantly and to have similar functions in regulating cell movement. In this regard, we find that overexpressing DockD causes increased cell speed by enhancing F-actin polymerization at the sites of pseudopod extension. DockD localizes to the cell cortex upon chemoattractant stimulation and at the leading edge of migrating cells and this localization is dependent on PI3K activity, suggesting that DockD might be part of the pathway that links PtdIns(3,4,5)P(3) production to F-actin polymerization. Using a proteomic approach, we found that DdELMO1 is associated with DockD and that Rac1A and RacC are possible in vivo DockD substrates. In conclusion, our work provides a further understanding of how cell motility is controlled and provides evidence that the molecular mechanism underlying Dock180-related protein function is evolutionarily conserved.

Engineering a multi-nucleated myotube, the role of the actin cytoskeleton

Myoblasts in vitro form characteristic arrays of bipolar-shaped cells prior to fusion. We have shown that the actin cytoskeleton re-organizes in these fusing cells and that the interaction of non-muscle myosin 2A with actin at the plasma membrane helps to generate the bipolar shape of myoblasts, which is key for fusion. Here we discuss how fusion occurs, and in particular how the actin cytoskeleton is involved. Myoblast fusion is essential to form the multi-nucleated muscle fibres that make up the skeletal muscle. Skeletal muscle fibres contain many nuclei, roughly one nucleus to every 15 sarcomeres (35 microm) in adult muscle, although this varies with muscle type (Bruusgaard et al., 2006). Thus a muscle fibre 30 cm long contains about 8000 nuclei and is formed by the fusion of about 8000 cells during development. The formation of multi-nucleated myotubes has been intensively studied for many years using a number of different systems. Many different proteins have been identified using Drosophila as a model system (e.g. see reviews by Taylor, 2000, 2002) that have given an insight into what happens in mammals. However, the process of fusion of mammalian cells is less well understood, and this paper will cover some of the aspects of mammalian myoblast fusion, with a particular focus on the role of the actin cytoskeleton.