Cytoskeletal dynamics during collective cell migration in Drosophila embryos

 Kobb, Anna 1, 2 ; Fernandez-Gonzalez, Rodrigo 1, 2, 3, 4

 1.  University of Toronto, Toronto, ON, Canada; 2.  Translational Biology and Engineering Program, Ted Rogers Centre for Heart Research, University of Toronto, Toronto, ON, M5G 1M1, Canada; 3.  Department of Cell and Systems Biology, University of Toronto, Toronto, ON, M5S 3G5, Canada; 4.  Developmental and Stem Cell Biology Program, The Hospital for Sick Children, Toronto, ON, M5G 1X8, Canada

Coordinated cell movements drive embryonic development and tissue repair, and are implicated in cancer metastasis. Cell movements are often coordinated by supracellular cytoskeletal networks formed by actin and the molecular motor non-muscle myosin II. Embryonic wound closure is a conserved process driven by the collective migration of cells to rapidly seal a lesion. During embryonic wound repair, actin and myosin are polarized in the cells adjacent to the wound, accumulating at the interface with the wounded cells and forming a supracellular cable around the wound. The actomyosin cable contracts and coordinates the migratory behaviours of the cells adjacent to the wound. However, the mechanisms that regulate actomyosin dynamics in supracellular networks are not well understood. Using fluorescence recovery after photobleaching (FRAP) in Drosophila embryos, we found that myosin turns over as the cable at the wound margin contracts. Myosin turnover at the wound margin was slower than in actomyosin networks with reduced contractility. Mathematical modelling suggested that myosin assembly and disassembly rates were both reduced by tension at the wound edge. We used laser ablation to show that tension at the wound margin increased as wound closure progressed, and the increase in tension was associated with reduced myosin turnover. Reducing tension by lasersevering resulted in increased turnover and loss of myosin from the wound edge. Finally, myosin motor activity was necessary for its stabilization around the wound and for rapid wound closure. Our results indicate that mechanical forces regulate myosin dynamics during embryonic wound repair, however we cannot not discount that myosin may be regulated through its binding partner at the wound margin, actin. In our recent work, we used FRAP to show that actin was stabilized around embryonic wounds. Loss of tension through laser ablation led to loss of actin fluorescence, suggesting that tension may be necessary for actin localization. To explore this we have begun experiments to increase or decrease tension at the wound margin and to measure affects on actin turnover. To begin to discern the relationship between actin and myosin at the wound margin we inhibited myosin activity and measured actin turnover. Actin stabilization was unaffected by myosin inhibition, suggesting that actin may be regulated independently. We are now further exploring the interplay between actin and myosin in wound repair by inhibiting actin dynamics and measuring changes in myosin turnover. This work will provide insights into the mechanisms used by cells to coordinate their behavior and could have implications for development and disease.