Identification of fusion-relevant proteins that are involved in the signalling transduction of the first and second temporal fusion phase in founder cells and fusion-competent myoblasts during Drosophila myoblast fusion.
Evolutionary Cell and Developmental Biology (Zoology)
Final Report Abstract
The fusion of mononulceated myoblasts is critical for proper skeletal muscle formation, growth and regeneration. Much of the current molecular mechanism underlying this process has been originated from work on myoblast fusion in Drosophila. Muscle fibers in Drosophila are formed by the subsequent fusion of two myoblast populations: founder cell (FC) and fusion-competent myoblasts (FCMs). A characteristic feature during myoblast fusion is the formation of a dense F-actin focus in fusion-competent myoblasts and a thin actin layer in founder cells. Our preliminary work has shown that branched F-actin formation during myoblast fusion depends on the Arp2/3-based nucleation promoting factors (NPFs) SCAR and WASp of the Wiskott-Aldrich Syndrome family. To allow myoblasts to fuse, founder cells and fusion-competent myoblasts must recognize each other. This process is mediated by myoblast-type specific cell adhesion proteins of the immunoglobulin (Ig) superfamily that transfer the fusion signal from the plasma membrane into the cell and by the cadherin superfamily member N-cadherin. We could show that phosphorylated tyrosines and proline-rich regions in the intracellular domain of the Ig family members Duf, Hbs, Rst and Sns interact with the SH2-SH3 adaptor protein Dock to transfer the signal to SCAR and WASp. In Drosophila, the Arp2/3 complex can control distinct biological events in response to SCAR and WASp regulation. However, during myoblast fusion both NPFs contribute to dense F-actin formation in fusion-competent myoblasts. To understand the role of F-actin formation in more detail, we performed transmission electron microscopy studies and investigated the genetic interaction between the members of the regulatory SCAR complex and WASp. Arp3, SCAR and WASp contribute to the formation of a fusion pore between adhering myoblasts. However, the loss of kette (a member of the SCAR regulatory complex) stops myoblast fusion prior fusion pore formation during the formation of electron-dense plaques. In kette mutants the electron-dense plaques are extended between adhering myoblasts in comparison to the wild-type myoblasts suggesting that Kette is involved in the dissolution of electron-dense plaques. A comparison between Cadherin-containing cellular junctions (e.g. adherens junctions) revealed that electron-dense plaques may serve as myoblast-specific cellular junctions. Moreover, we were able to suppress the myoblast fusion defect of kette mutants by the removal of N-cadherin. Myoblast type-specific rescue experiments of kette mutants showed that Abi, Rac1 and Scar are required for fusion pore formation in founder cells and fusion-competent myoblasts. In contrary, gene dosage experiments showed that fusion pore formation in fusion-competent myoblasts depends on the coordinated activity of SCAR and WASp, which is controlled by Kette. Based on these data we proposed a new model for Kette function in myoblast fusion: First, Kette governs the dissolution of electron-dense plaques. Second, Kette regulates fusion pore formation by coordinating the stoichiometric activity of SCAR and WASp.
Publications
- Actin regulators take the reins in myoblast fusion. Central European Journal of Biology, Vol. 4. 2009, Issue 1, pp. 11–18.
Önel, S. F.
(See online at https://doi.org/10.2478/s11535-008-0059-9) - FuRMAS: triggering myoblast fusion in Drosophila. Dev. Dyn. 238, 1513-1525
Önel, S. F. and Renkawitz-Pohl, R.
(See online at https://doi.org/10.1002/dvdy.21961) - The Arf-GEF Schizo/Loner regulates N-cadherin to induce fusion competence of Drosophila myoblasts. Developmental Biology, Vol. 368. 2012, Issue 1, pp. 18-27.
Dottermusch-Heidel, D., Groth, V., Beck, L. and Önel, S.-F.
(See online at https://doi.org/10.1016/j.ydbio.2012.04.031) - Dock serves as an adaptor in founder cells and fusion-competent myoblasts to induce Arp2/3-based actin polymerization during myoblast fusion. Journal of Cell Science, Vol. 126. 2013, pp. 360-372.
Kaipa, B. R., Shao, H., Schäfer, G., Trinkewitz, T., Groth, V., Liu, J., Beck, L., Abmayr, S. and Önel, S.-F.
(See online at https://doi.org/10.1242/jcs.113860) - Distinct genetic programs guide Drosophila circular and longitudinal visceral myoblast fusion. BMC Cell Biology, Vol. 15. 2014, Article number: 27.
Rudolf, A., Buttgereit, D., Jacobs, M., Wolfstetter, G., Kesper, D., Pütz, M., Berger, S., Renkawitz-Pohl, R., Holz, A., Önel, S.F.
(See online at https://doi.org/10.1186/1471-2121-15-27) - Tethering membrane fusion: common and different players in myoblasts and at the synapse. Journal of Neurogenetics, Vol. 28. 2014, Issue 3-4, pp. 302-315.
Önel, S.F., Rust, M.B., Jacob, R. and Renkawitz-Pohl, R.
(See online at https://doi.org/10.3109/01677063.2014.936014) - Drosophila Kette/Nap1/Hem-2 coordinates Scar- and WASp-dependent Arp2/3 activation in fusion-competent myoblasts. Journal of Cell Science, Vol. 129. 2016, pp. 3426-3436.
Hamp, J., Loewer, A., Dottermusch-Heidel, C., Beck, L., Önel, S.F.
(See online at https://doi.org/10.1242/jcs.175638) - WHAMY is a novel actin polymerase promoting myoblast fusion, macrophage cell motility and sensory organ development in Drosophila. J Cell Sci. 129, 604-620.
Brinkmann, K., Winterhoff, M., Önel, S.F., Schultz, J., Faix, J. and Bogdan, S.
(See online at https://doi.org/10.1242/jcs.179325)
