Live imaging provides new insights on dynamic F-actin filopodia and differential endocytosis during myoblast fusion in Drosophila. 2014

Shruti Haralalka, and Claude Shelton, and Heather N Cartwright, and Fengli Guo, and Rhonda Trimble, and Ram P Kumar, and Susan M Abmayr
Stowers Institute for Medical Research, Kansas City, Missouri, 64110, United States of America.

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.

UI MeSH Term Description Entries
D010609 Pharyngeal Muscles The muscles of the PHARYNX are voluntary muscles arranged in two layers. The external circular layer consists of three constrictors (superior, middle, and inferior). The internal longitudinal layer consists of the palatopharyngeus, the salpingopharyngeus, and the stylopharyngeus. During swallowing, the outer layer constricts the pharyngeal wall and the inner layer elevates pharynx and LARYNX. Palatopharyngeus,Muscles of Pharynx,Palatopharyngeal Muscle,Salpingopharyngeus,Stylopharyngeus,Velopharyngeal Muscle,Muscle, Palatopharyngeal,Muscle, Pharyngeal,Muscle, Velopharyngeal,Muscles, Pharyngeal,Pharyngeal Muscle,Pharynx Muscle,Pharynx Muscles
D011554 Pseudopodia A dynamic actin-rich extension of the surface of an animal cell used for locomotion or prehension of food. Axopodia,Filopodia,Lamellipodia,Lobopodia,Microspikes, Cell Surface,Reticulopodia,Pseudopodium,Cell Surface Microspike,Cell Surface Microspikes,Lamellipodias,Microspike, Cell Surface,Surface Microspike, Cell,Surface Microspikes, Cell
D002352 Carrier Proteins Proteins that bind or transport specific substances in the blood, within the cell, or across cell membranes. Binding Proteins,Carrier Protein,Transport Protein,Transport Proteins,Binding Protein,Protein, Carrier,Proteins, Carrier
D002459 Cell Fusion Fusion of somatic cells in vitro or in vivo, which results in somatic cell hybridization. Cell Fusions,Fusion, Cell,Fusions, Cell
D002462 Cell Membrane The lipid- and protein-containing, selectively permeable membrane that surrounds the cytoplasm in prokaryotic and eukaryotic cells. Plasma Membrane,Cytoplasmic Membrane,Cell Membranes,Cytoplasmic Membranes,Membrane, Cell,Membrane, Cytoplasmic,Membrane, Plasma,Membranes, Cell,Membranes, Cytoplasmic,Membranes, Plasma,Plasma Membranes
D004331 Drosophila melanogaster A species of fruit fly frequently used in genetics because of the large size of its chromosomes. D. melanogaster,Drosophila melanogasters,melanogaster, Drosophila
D004705 Endocytosis Cellular uptake of extracellular materials within membrane-limited vacuoles or microvesicles. ENDOSOMES play a central role in endocytosis. Endocytoses
D005786 Gene Expression Regulation Any of the processes by which nuclear, cytoplasmic, or intercellular factors influence the differential control (induction or repression) of gene action at the level of transcription or translation. Gene Action Regulation,Regulation of Gene Expression,Expression Regulation, Gene,Regulation, Gene Action,Regulation, Gene Expression
D000080865 Formins A family of multidomain microfilament proteins associated with ACTIN FILAMENTS. They generally have multiple Formin-Homology domains (e.g., FH2 domain) and additionally have a RhoGTPase-binding domain and a diaphanous inhibitory domain. Formins and homologs are involved in actin reorganization. DIAPH Protein 1,DIAPH Protein 2,DIAPH Proteins,Diaphanous-Related Formin Homology Proteins,Diaphanous-Related Formins,FMN Protein 1,FMN Protein 2,FMN Proteins,Formin,Formin 1 Protein,Formin 2 Protein,Formin Homology Proteins,Diaphanous Related Formin Homology Proteins,Diaphanous Related Formins,Formins, Diaphanous-Related
D000199 Actins Filamentous proteins that are the main constituent of the thin filaments of muscle fibers. The filaments (known also as filamentous or F-actin) can be dissociated into their globular subunits; each subunit is composed of a single polypeptide 375 amino acids long. This is known as globular or G-actin. In conjunction with MYOSINS, actin is responsible for the contraction and relaxation of muscle. F-Actin,G-Actin,Actin,Isoactin,N-Actin,alpha-Actin,alpha-Isoactin,beta-Actin,gamma-Actin,F Actin,G Actin,N Actin,alpha Actin,alpha Isoactin,beta Actin,gamma Actin

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