title: Live imaging provides new insights on dynamic F-actin filopodia and differential endocytosis during myoblast fusion in Drosophila.
abstract: 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.
authors.affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, 64110, United States of America.
authors.forename: Shruti
authors.initials: S
authors.lastname: Haralalka
authors.affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, 64110, United States of America.
authors.forename: Claude
authors.initials: C
authors.lastname: Shelton
authors.affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, 64110, United States of America.
authors.forename: Heather N
authors.initials: HN
authors.lastname: Cartwright
authors.affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, 64110, United States of America.
authors.forename: Fengli
authors.initials: F
authors.lastname: Guo
authors.affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, 64110, United States of America.
authors.forename: Rhonda
authors.initials: R
authors.lastname: Trimble
authors.affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, 64110, United States of America.
authors.forename: Ram P
authors.initials: RP
authors.lastname: Kumar
authors.affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, 64110, United States of America; Department of Anatomy and Cell Biology, University of Kansas School of Medicine, Kansas City, Kansas, 66160, United States of America.
authors.forename: Susan M
authors.initials: SM
authors.lastname: Abmayr
mesh_terms: D051376:Actin-Related Protein 2-3 Complex / Q000378:metabolism; D000199:Actins / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D002459:Cell Fusion; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D029721:Drosophila Proteins / Q000378:metabolism*; D004331:Drosophila melanogaster / Q000166:cytology* / Q000196:embryology / Q000648:ultrastructure; D004705:Endocytosis; D000080865:Formins; D005786:Gene Expression Regulation; D057054:Molecular Imaging; D018485:Muscle Fibers, Skeletal / Q000166:cytology; D032446:Myoblasts / Q000166:cytology* / Q000648:ultrastructure; D010609:Pharyngeal Muscles / Q000166:cytology / Q000196:embryology; D011554:Pseudopodia / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D051376:Actin-Related Protein 2-3 Complex; D000199:Actins; D002352:Carrier Proteins; D029721:Drosophila Proteins; D000080865:Formins; C106890:diaphanous protein, Drosophila