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Séminaire impromptu - Nagaraju DhanyasiSurface apposition and multiple cell contacts promote myoblast fusion in Drosophila flight muscle

Abstract :

Nagaraju Dhanyasi  (candidat Post-Doc)

 Myoblast fusion is an essential process underlying development, growth and regeneration of muscles in multicellular organisms.
Our work focuses on an ultra-structural description of the fusion process during formation of the indirect flight muscles (IFMs) of Drosophila. These large muscles, which display a variety of structural features analogous to those of vertebrate muscles, are generated through fusion of numerous myoblasts with prominent myotube templates, making them an attractive subject for study of myoblast fusion in an experimental system amenable to genetic investigation.

In order to study fusion between IFM myotubes and myoblasts, we have employed a novel tissue preparation protocol, which allows for high-quality preservation and visualization of cell membranes, and a variety of transmission electron-microscopy (TEM) techniques (FIB/SEM, single section TEM and STEM tomography). Using these methods, we have identified a series of distinct steps in the IFM myoblast fusion process, mediated, as genetic analysis suggests, by different cellular machineries. Cell-adhesion elements are essential for an initial association between the myoblasts and myotubes, bringing them in close (~25nm) proximity to each other. Myoblasts then flatten their surface and move even closer (~10nm) to the myotubes, a transition that is associated with formation of multiple point-like contact sites between the two cell types. Components of the Arp2/3 branched-actin polymerization machinery, established mediators of myoblast fusion in Drosophila, as well as the MARVEL-domain transmembrane protein Singles-bar, are required for these events. The myotube-myoblast contact sites, which form along long stretches of the apposed membranes, and are not associated with membrane protrusions, give rise to multiple nascent fusion pores, which perforate the cell membranes. The pores grow in size, leading to cell membrane break-down and completion of the fusion process.