Thin film microrheology of the living actomyosin cortex in the C. elegans embryo
Cell Biology
Final Report Abstract
The cell cortex is a thin layer of cross-linked actin filaments and myosin motor proteins beneath the cell membrane. In addition to providing mechanical stability to the cell, this dynamic network also drives large-scale cortical flows. Cortical flows are crucial to key cellular processes such as cell polarization, cell division and morphogenesis. Despite our detailed understanding of individual cortical components, their interplay within the cortex remains obscure because of the difficulty to infer molecular contributions to cortical properties from the observable large-scale behavior of the cortex alone. An intermediate level of description is required, detailing the emergent physical properties of the cortex. With this grant we aimed to understand the biophysical mechanisms governing the large-scale behavior of the actomyosin cortex by characterizing cortical mechanics in vivo. We wanted to measure the large-scale physical properties of the cortex using magnetics beads embedded in the cortex of the one-cell stage C. elegans embryo before the establishment of the anterior-posterior cortical flow. It was possible to generate single cell C. elegans embryos where single beads were injected. We were able to track the motion of beads within the surrounding cortical network but could not perform measurements to investigate mesoscale mechanical properties of the cortex due to the challenging nature of these experiments. An attempt to use larger cells and Zebrafish embryos also failed. However, we achieved to track single myosin minifilaments over time and in cortical regions with different levels of contractility. We found that both distribution and size of myosin minifilaments are regulated by the cell in order to spatiotemporally control the contractile forces at the cortex. The optimised tracking method enables us to follow other properties of minifilaments, such as their local motion, actin binding state and exchange kinetics and examine their correlation with the force levels at the cortex. Finally, we completed an RNAi screen for characterising the impact of actin-binding-proteins (ABPs) on large-scale actomyosin cortex dynamics. By performing a candidate RNAi screening of 33 ABPs and actomyosin regulators, we demonstrated that perturbing distinct ABPs can lead to similar flow phenotypes. This is indicative for a ’morphogenetic degeneracy’ where multiple molecular processes contribute to the same large-scale physical property. We speculate that morphogenetic degeneracies contribute to the robustness of bulk biological matter in development. Moreover, through the work pursued in this grant we made an additional important discovery, that the actomyosin cortex is undergoing chiral rotatory flow. By a combination of imaging and theory we have found that this chiral flow is driven by a novel emergent active mechanical property of the actomyosin cytoskeleton, active torque generation, and guides organismal L/R symmetry breaking. In summary, this proposal provided a link from molecules to the mechanics of cortical behavior, which will be crucial to the understanding of cortical flows in living systems.
Publications
- (2018) Morphogenetic degeneracies in the actomyosin cortex. eLife 2018 7 e37677
Naganathan, Sundar Ram; Fürthauer, Sebastian; Rodriguez, Josana; Fievet, Bruno Thomas; Jülicher, Frank; Ahringer, Julie; Cannistraci, Carlo Vittorio; Grill, Stephan W.
(See online at https://doi.org/10.7554/elife.37677) - (2014). Active torque generation by the actomyosin cell cortex drives left-right symmetry breaking. eLife 3: e04165
Naganathan, S.R., Furthauer, S., Nishikawa, M., Jülicher, F., and Grill, S.W.
(See online at https://doi.org/10.7554/eLife.04165) - (2016). Actomyosin-driven leftright asymmetry: from molecular torques to chiral self organization. Curr Opin Cell Biol 38, 24-30
Naganathan, S.R., Middelkoop, T.C., Furthauer, S., and Grill, S.W.
(See online at https://doi.org/10.1016/j.ceb.2016.01.004) - (2016). Cortical flow aligns actin filaments to form a furrow. eLife 5:e17807
Reymann, A.C., Staniscia, F., Erzberger, A., Salbreux, G., and Grill, S.W.
(See online at https://doi.org/10.7554/eLife.17807) - (2016). Determining Physical Properties of the Cell Cortex. Biophys. J. 110, 1421-1429
Saha, A., Nishikawa, M., Behrndt, M., Heisenberg, C.P., Jülicher, F., and Grill, S.W.
(See online at https://doi.org/10.1016/j.bpj.2016.02.013)
