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

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.

Time-lapse analysis and mathematical characterization elucidate novel mechanisms underlying muscle morphogenesis

Skeletal muscle morphogenesis transforms short muscle precursor cells into long, multinucleate myotubes that anchor to tendons via the myotendinous junction (MTJ). In vertebrates, a great deal is known about muscle specification as well as how somitic cells, as a cohort, generate the early myotome. However, the cellular mechanisms that generate long muscle fibers from short cells and the molecular factors that limit elongation are unknown. We show that zebrafish fast muscle fiber morphogenesis consists of three discrete phases: short precursor cells, intercalation/elongation, and boundary capture/myotube formation. In the first phase, cells exhibit randomly directed protrusive activity. The second phase, intercalation/elongation, proceeds via a two-step process: protrusion extension and filling. This repetition of protrusion extension and filling continues until both the anterior and posterior ends of the muscle fiber reach the MTJ. Finally, both ends of the muscle fiber anchor to the MTJ (boundary capture) and undergo further morphogenetic changes as they adopt the stereotypical, cylindrical shape of myotubes. We find that the basement membrane protein laminin is required for efficient elongation, proper fiber orientation, and boundary capture. These early muscle defects in the absence of either lamininβ1 or lamininγ1 contrast with later dystrophic phenotypes in lamininα2 mutant embryos, indicating discrete roles for different laminin chains during early muscle development. Surprisingly, genetic mosaic analysis suggests that boundary capture is a cell-autonomous phenomenon. Taken together, our results define three phases of muscle fiber morphogenesis and show that the critical second phase of elongation proceeds by a repetitive process of protrusion extension and protrusion filling. Furthermore, we show that laminin is a novel and critical molecular cue mediating fiber orientation and limiting muscle cell length.

Despite the importance of muscle fiber development and tendon attachment, this process is incompletely understood in vertebrates. One critical step is muscle fiber elongation; muscle precursor cells are short and subsequent elongation/fusion generates long, multinucleate muscle fibers. Using a vertebrate model organism, the zebrafish, we find that single round myoblasts elongate to span the entire width of the myotome prior to fusion. Using rigorous and objective mathematical characterization techniques, we can further divide muscle development into three stages: short precursor cells, intercalation/elongation, and boundary capture/myotube formation. The second phase, elongation, occurs via a two-step mechanism of protrusion extension and filling. Myotube formation involves boundary capture, where the ends of muscle fibers anchor themselves to the myotome boundary and stop elongating. We show that the protein laminin is required for boundary capture, normal fiber length, and proper fiber orientation. Genetic mosaic experiments in laminin-deficient embryos reveal that boundary capture is a cell autonomous phenomenon. Wild-type (normal) cells capture the boundary appropriately and stop elongating in laminin-deficient embryos. Although adhesion to laminin has been implicated in muscular dystrophies where the attachment between muscle cells and tendons fails, no early developmental requirements for laminin in fast muscle morphogenesis have been shown until now.

The atypical Rac activator Dock180 (Dock1) regulates myoblast fusion in vivo

Dock1 (also known as Dock180) is a prototypical member of a new family of atypical Rho GTPase activators. Genetic studies in Drosophila and Caenorhabditis elegans have demonstrated that Dock1 orthologues in these organisms have a crucial role in activating Rac GTPase signaling. We generated mutant alleles of the closely related Dock1 and Dock5 genes to study their function in mammals. We report that while Dock5 is dispensable for normal mouse embryogenesis, Dock1 has an essential role in embryonic development. A dramatic reduction of all skeletal muscle tissues is observed in Dock1-null embryos. Mechanistically, this embryonic defect is attributed to a strong deficiency in myoblast fusion, which is detectable both in vitro and in vivo. Furthermore, we have uncovered a contribution of Dock5 toward myofiber development. These studies identify Dock1 and Dock5 as critical regulators of the fusion step during primary myogenesis in mammals and demonstrate that a specific component of the myoblast fusion machinery identified in Drosophila plays an evolutionarily conserved role in higher vertebrates.

uPAR promotes formation of the p130Cas-Crk complex to activate Rac through DOCK180

The urokinase-type plasminogen activator receptor (uPAR) drives tumor cell membrane protrusion and motility through activation of Rac; however, the pathway leading from uPAR to Rac activation has not been described. In this study we identify DOCK180 as the guanine nucleotide exchange factor acting downstream of uPAR. We show that uPAR cooperates with integrin complexes containing β₃ integrin to drive formation of the p130Cas–CrkII signaling complex and activation of Rac, resulting in a Rac-driven elongated-mesenchymal morphology, cell motility, and invasion. Our findings identify a signaling pathway underlying the morphological changes and increased cell motility associated with uPAR expression.

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.

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

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

Prdm1- and Sox6-mediated transcriptional repression specifies muscle fibre type in the zebrafish embryo

The zebrafish u-boot (ubo) gene encodes the transcription factor Prdm1, which is essential for the specification of the primary slow-twitch muscle fibres that derive from adaxial cells. Here, we show that Prdm1 functions by acting as a transcriptional repressor and that slow-twitch-specific muscle gene expression is activated by Prdm1-mediated repression of the transcriptional repressor Sox6. Genes encoding fast-specific isoforms of sarcomeric proteins are ectopically expressed in the adaxial cells of ubo(tp39) mutant embryos. By using chromatin immunoprecipitation, we show that these are direct targets of Prdm1. Thus, Prdm1 promotes slow-twitch fibre differentiation by acting as a global repressor of fast-fibre-specific genes, as well as by abrogating the repression of slow-fibre-specific genes.

Rho GTPases in osteoclasts: orchestrators of podosome arrangement

Cells from the myeloid lineage, namely macrophages, dendritic cells and osteoclasts, develop podosomes instead of stress fibers and focal adhesions to adhere and migrate. Podosomes share many components with focal adhesions but differ in their molecular organization, with a dense core of polymerized actin surrounded by scaffolding proteins, kinases and integrins. Podosomes are found either isolated both in macrophages and dendritic cells or arranged into superstructures in osteoclasts. When osteoclasts resorb bone, they form an F-actin rich sealing zone, which is a dense array of connected podosomes that firmly anchors osteoclasts to bone. It delineates a compartment in which protons and proteases are secreted to dissolve and degrade the mineralized matrix. Since Rho GTPases have been shown to control F-actin stress fibers and focal adhesions in mesenchymal cells, the question of whether they could also control podosome formation and arrangement in cells from the myeloid lineage, and particularly in osteoclasts, rapidly emerged. This article considers recent advances made in our understanding of podosome arrangements in osteoclasts and how Rho GTPases may control it.

Myoblasts and macrophages share molecular components that contribute to cell-cell fusion

Cell–cell fusion is critical to the normal development of certain tissues, yet the nature and degree of conservation of the underlying molecular components remains largely unknown. Here we show that the two guanine-nucleotide exchange factors Brag2 and Dock180 have evolutionarily conserved functions in the fusion of mammalian myoblasts. Their effects on muscle cell formation are distinct and are a result of the activation of the GTPases ARF6 and Rac, respectively. Inhibition of ARF6 activity results in a lack of physical association between paxillin and β₁-integrin, and disruption of paxillin transport to sites of focal adhesion. We show that fusion machinery is conserved among distinct cell types because Dock180 deficiency prevented fusion of macrophages and the formation of multinucleated giant cells. Our results are the first to demonstrate a role for a single protein in the fusion of two different cell types, and provide novel mechanistic insight into the function of GEFs in the morphological maturation of multinucleated cells.

Genetic control of muscle development: learning from Drosophila

Muscle development involves a complex sequence of time and spatially regulated cellular events leading to the formation of highly specialised syncytial muscle cells displaying a common feature, the capacity of contraction. Analyses of mechanisms controlling muscle development reveals that the main steps of muscle formation including myogenic determination, diversification of muscle precursors, myoblast fusion and terminal differentiation involve the actions of evolutionarily conserved genes. Thus dissecting the genetic control of muscle development in simple model organisms appears to be an attractive way to get insights into core genetic cascade that orchestrate myogenesis. In this respect, particularly insightful have been data generated using Drosophila as a model system. Notably, the interplay between intrinsic and extrinsic cues that determine the early myogenic decisions leading to the specification of muscle progenitors and those controlling myoblasts fusion are much better characterised in Drosophila than in vertebrate species. Also, adult Drosophila myogenesis, which leads to the formation of vertebrate-like multi-fibre muscles, emerges as a particularly well-adapted system to study normal and aberrant muscle development.

Mutation of Dock5, a member of the guanine exchange factor Dock180 superfamily, in the rupture of lens cataract mouse

Rupture of lens cataract (RLC) in the mouse is a spontaneous mutation inherited by a single autosomal recessive gene mapped on chromosome 14. Fine mapping of the mutant locus revealed a nucleotide deletion of 27-bp at the end of 15th exon of Dock5 (Dedicator of cytokinesis-5), a member of the Dock gene superfamily. Since the deletion occurred in-frame, the RLC-DOCK5 protein had a deletion of 9 amino acids (a.a. 506-514) in the DHR1 (DOCK homology region-1) domain that is essential for DOCK5, a GTP-exchanger for Rac1. Although Dock5 mRNA was intensely expressed equally in mutant and wild-type lenses, DOCK5 protein was hardly detectable in the mutant lens. In contrast, expression of Dock180, another member of Dock subfamily A, was not affected in RLC. Immunohistochemically, DOCK5 was stained intensely in the cytoplasm of the anterior epithelial cells and weakly in lens fiber of the wild type lenses, but little in RLC lens. These observations suggest that the mutation may somehow destabilize DOCK5 protein. We propose to designate the mutant allele of rlc as Dock5rlc. Relevance of the signaling pathway involving DOCK5-RAC1 in maintenance of lens integrity of growing lens is discussed.