Local Mechanical Forces Promote Polarized Junctional Assembly and Axis Elongation in Drosophila

Yu, Jessica C. 1, 2 ;  Fernandez-Gonzalez, Rodrigo 1, 2, 3, 4

1. Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON, Canada; 2. Translational Biology and Engineering Program, Ted Rogers Centre for Heart Research, University of Toronto, ON, Canada; 3. Cell and Systems Biology, University of Toronto, Toronto, ON, Canada; 4. Developmental and Stem Cell Biology Program, Hospital for Sick Kids, Toronto, ON, Canada

Axis elongation is a conserved process in which the head-to-tail or anterior-posterior (AP) axis of an embryo extends. Axis elongation defects in humans cause developmental malformations, including spina bifida and anencephaly. In Drosophila, cellular rearrangements drive axis elongation. Cells exchange neighbours by converging into transient multicellular vertices which resolve through the assembly of new cell interfaces parallel to the AP axis. We previously showed that contraction of the cells anterior and posterior to multicellular vertices generates mechanical signals that promote vertex resolution. Using a laser-based approach to disrupt ectopic tension specifically in the cells around multicellular vertices decreased the rate of new interface assembly. Similar results were obtained when we used pharmacological treatments to globally disrupt myosin activity or actin polymerization. Notably, when actin polymerization was impaired, 68% of vertices persisted for at least 10 minutes and upwards of 40 minutes, suggesting a role for actin polymerization in the resolution of vertices. To investigate whether mechanical forces could promote actin assembly during vertex resolution, we quantified actin fluorescence in new interfaces under ectopic AP tension and found that total actin fluorescence was 2.3-fold greater in new interfaces under ectopic tension than in control interfaces. Our results suggest that local mechanical forces promote directional actin polymerization during vertex resolution. We are currently conducting an RNAi-based screen for regulators of actin dynamics during axis elongation. To date, we have found six actin regulators that affect axis elongation, including formins and actin-severing proteins. Identifying how actin is regulated during axis elongation will be crucial in understanding the mechanisms of tissue extension during embryonic development, and help lead to the development of therapeutic strategies to treat congenital disease.