Cellular dynamics during myoblast fusion for muscle formation in Drosophila

Fusion between the myogenic cells is a prerequisite for the generation multinucleated muscle fiber in all the animals. However the cellular mechanism of fusion is not well understood. In order to understand the role of cytoskeletal elements during fusion I have used RNAi based gene knockdown to identify components essential for fusion. My study has identified one of the major actin related protein complex Arp2/3and its regulator Wsp/Wip, a major player during myoblast fusion. Mutant analysis of Wsp and Wip show muscle defects due to lack of fusion. I find that Wsp is required at initial steps of fusion as the fusion pore formation was highly compromised in Wsp mutants (Mukherjee et al., 2011). Until now most of the fusion studies have used either ultra structural analysis or fixed preparation of different mutants to identify genes important for fusion. But how a fusing cell behaves in vivo is not addressed carefully. I am using live imaging of thoracic muscles inDrosophila pupa in order to understand the mechanisms of cell-cell interaction and fusion. I am trying to understand the dynamic interactions between the myoblast and myotube that leads to fusion. Also, imaging myoblast fusion in this context gives an opportunity to visualise behaviour of large number of myoblasts which will provide an ample understanding of fusion process. During this study I have identified two morphologically different myoblast cell populations. My observations indicate towards a fusion mechanism different from the one widely believed in the field. 

Figure: Myotube (MT)-Myoblast (MB) interactions during fusion. Snapshots taken from a time-lapse imaging of pupal myogenesis shows MT sends protrusions towards the MB cell.

Collaborators: Ben-Zion Shilo and Eyal Schejter, Weizmann Institute of Sciences, Israel.

Investigating the cellular mechanisms underlying muscle remodeling during the development of Drosophila

Cellular mechanisms responsible for the development, maintenance and repair following injury are central to the proper functioning of muscles. Transformation of larval oblique muscles (LOMs) in to adult indirect flight muscles (IFMs) in fruit flies present a system where removal of unwanted cytoplasmic constituents and fusion of individual muscle cells with existing larval templates are essential to rebuild adult muscles. Our observations using this system, with a belief that the cellular mechanistic underpinnings of these processes are conserved, have shown that a lysosome based catabolic process is involved in cytoplasmic clearance. Then, high-resolution study has shown that the fusion of individual muscle cells with the existing larval templates that results in formation of adult muscles is a multi-step process. Adhesion and branched actin machineries are essential and mediate this process at two different stages of muscle formation.

Figure: High resolution micrograph shows two muscle cells having their cell membranes aligned closely during the adult IFM development in Drosophila

Collaborators: Ben-Zion Shilo and Eyal Schejter, Weizmann Institute of Sciences, Israel.