Control of Myoblast Fusion by a Guanine Nucleotide Exchange Factor, Loner, and Its Effector ARF6

Department of Molecular Biology, University of Texas Southwestern Medical Center at Dallas, 6000 Harry Hines Boulevard, Dallas, TX 75390, USA.
Cell (Impact Factor: 33.12). 10/2003; 114(6):751-62. DOI: 10.1016/S0092-8674(03)00720-7
Source: PubMed

ABSTRACT Myoblast fusion is essential for the formation and regeneration of skeletal muscle. In a genetic screen for regulators of muscle development in Drosophila, we discovered a gene encoding a guanine nucleotide exchange factor, called loner, which is required for myoblast fusion. Loner localizes to subcellular sites of fusion and acts downstream of cell surface fusion receptors by recruiting the small GTPase ARF6 and stimulating guanine nucleotide exchange. Accordingly, a dominant-negative ARF6 disrupts myoblast fusion in Drosophila embryos and in mammalian myoblasts in culture, mimicking the fusion defects caused by loss of Loner. Loner and ARF6, which also control the proper membrane localization of another small GTPase, Rac, are key components of a cellular apparatus required for myoblast fusion and muscle development. In muscle cells, this fusigenic mechanism is coupled to fusion receptors; in other fusion-competent cell types it may be triggered by different upstream signals.

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    • "In heterologous cells, BRAG1 catalytic activity appears to be constitutive and is not affected by mutations in the IQ motif that abrogate CaM binding. Similarly, disruption of the catalytic domain, but not the IQ motif, of the single Drosophila BRAG gene Loner was found to cause defects in myoblast fusion (Chen et al., 2003). "
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    ABSTRACT: Activity-dependent modifications of excitatory synapses contribute to synaptic maturation and plasticity, and are critical for learning and memory. Consequently, impairments in synapse formation or synaptic transmission are thought to be responsible for several types of mental disabilities. BRAG1 is a guanine nucleotide exchange factor for the small GTP-binding protein Arf6 that localizes to the postsynaptic density of excitatory synapses. Mutations in BRAG1 have been identified in families with X-linked intellectual disability (XLID). These mutations mapped to either the catalytic domain or an IQ-like motif; however, the pathophysiological basis of these mutations remains unknown. Here, we show that the BRAG1 IQ motif binds apo-calmodulin (CaM), and that calcium-induced CaM release triggers a reversible conformational change in human BRAG1. We demonstrate that BRAG1 activity, stimulated by activation of NMDA-sensitive glutamate receptors, depresses AMPA receptor (AMPA-R)-mediated transmission via JNK-mediated synaptic removal of GluA1-containing AMPA-Rs in rat hippocampal neurons. Importantly, a BRAG1 mutant that fails to activate Arf6 also fails to depress AMPA-R signaling, indicating that Arf6 activity is necessary for this process. Conversely, a mutation in the BRAG1 IQ-like motif that impairs CaM binding results in hyperactivation of Arf6 signaling and constitutive depression of AMPA transmission. Our findings reveal a role for BRAG1 in response to neuronal activity with possible clinical relevance to nonsyndromic XLID.
    The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 08/2012; 32(34):11716-26. DOI:10.1523/JNEUROSCI.1942-12.2012 · 6.75 Impact Factor
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    • "structure , we stained wild - type ( Fig . 1D ) and homozygous siz C1 – 28 mutant embryos with anti - Duf ( Fig . 1E ) . These data indicated that cell recognition and adhesion as well as ring formation occur in schizo / loner mutants . Based on conflicting reports on Schizo / Loner protein localization placing Schizo / Loner specifically in FCs ( Chen et al . , 2003 ) or in FCs and FCMs ( Richardson et al . , 2007 ) , we next determined in which myoblast population Schizo / Loner functions . Therefore , we per - formed myoblast type - specific rescue experiments by employing the UAS / GAL4 system ( Brand and Perrimon , 1993 ) . The expression of Schizo / Loner with the FC - specific driver line rP2"
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    ABSTRACT: Myoblast fusion is a key process in multinucleated muscle formation. Prior to fusion, myoblasts recognize and adhere to each other with the aid of cell-adhesion proteins integrated into the membrane. Their intracellular domains participate in signal transduction by binding to cytoplasmic proteins. Here we identified the calcium-dependent cell-adhesion protein N-cadherin as the binding partner of the guanine-nucleotide exchange factor Schizo/Loner in Drosophila melanogaster. N-cadherin was expressed in founder cells and fusion-competent myoblasts of Drosophila during the first fusion phase. Our genetic analyses demonstrated that the myoblast fusion defect of schizo/loner mutants is rescued in part by the loss-of-function mutation of N-cadherin, which suggests that Schizo/Loner is a negative regulator of N-cadherin. Based on our findings, we propose a model where N-cadherin must be removed from the myoblast membrane to induce a protein-free zone at the cell-cell contact point to permit fusion.
    Developmental Biology 05/2012; 368(1):18-27. DOI:10.1016/j.ydbio.2012.04.031 · 3.64 Impact Factor
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    • "Though Rols can interact with Mbc biochemically, prompting the model that Rols functions to recruit Mbc to Kirre at sites of fusion (Chen and Olson, 2001), the relevance of this interaction is not clear, as Mbc does not appear to be required in the founder cells for fusion (Haralalka et al., 2011). Schizo/Loner might facilitate Arf-mediated recruitment of Rac1 to the membrane (Chen et al., 2003), probably resulting in activation of the Scar pathway by Rac1 (see Box 3). A Schizoactivated Arf GTPase might also, like Rac1, directly activate the Scar complex, as recently shown in mammalian cells (Koronakis et al., 2011). "
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    ABSTRACT: 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.
    Development 02/2012; 139(4):641-56. DOI:10.1242/dev.068353 · 6.27 Impact Factor
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