Myers, Gene, YANG, Xinyi, MAGHELLI, Nicola, ROYER, Loic, Grill, Stephan, FERRARO, Teresa, Labouesse, Michel, PONTABRY, Julien
[
International Worm Meeting,
2017]
The process of morphogenesis in C. elegans embryos is largely driven by epidermal cells. Unlike Drosophila and zebrafish, no cell division or cell rearrangement is involved in C. elegans morphogenesis. Epidermis shape changes, which are characterized by junction lengthening along the anterior/posterior (A/P) direction, play a key role in this process. The nature of junction lengthening and planar polarity establishment, as well as the cellular mechanisms involved in these processes during C. elegans embryonic elongation are the main objectives of this project. Our lab observed that junction elongation along the A/P direction increases after muscle becomes active, and fails in muscle defective embryos. To better understand which role muscles play in polarized junction lengthening, we examined the global and local movement patterns of embryo using Single Plane Illumination Microscopy, focusing on epidermal adherens junctions and muscle nuclei. We found that wild-type embryos rotated strongly soon after muscle became active, and equally frequently to an outward or inward direction. However, muscle defective and Rho-kinase mutant embryos, which stop elongation at the 2-fold stage, scarcely rotated, suggesting that rotations are important for embryo elongation. By comparing the changes of seam cell aspect ratio, we observed that the head, body and tail mechanically behaved as partially independent entities. We next sought to understand how such movements could account for the polarized junction lengthening, keeping in mind that C. elegans embryos are radially symmetric. By measuring the distance between two dorsal or ventral muscle nuclei, respectively, we found that dorsal and ventral muscles mostly contract alternatively, accounting for embryo rotations. Intriguingly, analysis of junction roughness showed that during embryo rotations, junctions along the A/P direction were stretched when seam cells were positioned outwards. Laser ablation targeting DLG-1::GFP results further discovered that these junctions were under higher tension when stretched. These results suggest that asymmetric muscle activity defines the source of polarity in C. elegans embryo and provides the local driving force for epidermis stretching. To study how this tension impacts polarized junction lengthening, we are investigating the insertion of new E-cad molecule into junctions during embryo rotations by single molecule imaging. Altogether, our results suggest that C. elegans embryos extend in a ratchet mode due to the alternating muscle contractions.