Cell Dynamics Lab

from molecule to tissue

Pulsed dynamics:
actomyosin in embryonic cells

With Jonathan Michaux, William McFadden and Ed Munro

Movie. Single-molecule movie of a pulse. Each speck corresponds to a single molecule of GFP fused with Actin, and is tracked during a pulse. The pulse itself, in red, is tracked using particle tracking and the extracted velocity flow.

Spatiotemporal patterning of actomyosin contractility plays a key role in cell and tissue morphogenesis during early development. In embryonic cells, actomyosin arrays are highly dynamic structures that remodel on a time scale of 10s of seconds, through a combination of turnover – local filament assembly/disassembly and motor recruitment/inactivation – and motion – rapid spatial redistribution of filaments, motors and other network elements driven by the network's contraction, and caused by myosin activity or actin polymerization. Because of these dynamic and active properties, contractility is complex and intrinsically self-organizing.

We used the C. elegans early embryo to understand how cells pattern force generation through local modulation of self-organized contractility, focusing on pulsed contractility in the C. elegans embryo.

Pulsed contraction represents a widespread mode of actomyosin contractility in which transient, local accumulations of F-actin and Myosin II accompany local contractions of the cell surface. Such pulsed contractions have been described in a wide range of cells and tissues, including the C. elegans zygote1 and later embryonic stages2, in multiple settings in Drosophila3-7, in Xenopus8, or in cell culture9.

We combined two-color fluorescence imaging, live single-molecule imaging, particle tracking, image analysis, and numerical modeling to tease apart the mechanisms of pulse initiation and termination. Strikingly, our results demonstrated that the mechanical component (advection) played little role in pulse initiation or termination, and that the process was mostly governed by Actin and Myosin turnover. In our system, autocatalytic RhoA activation/recruitment is responsible for pulse initiation, while the delayed recruitment of a RhoA inactivator (RGA-3/4) onto Actin filaments drove pulse termination.

This multidisciplinary work uncovered the regulatory mechanism of actin dynamics during pulsed contractions, demonstrating that these pulses are not the consequence of a mechanical feedback (or "cortical instability"), but instead arise through the local activation of the RhoA pathway.

  1. E. Munro, J. Nance, J. R. Priess, Developmental Cell. 7, 413–424 (2004).
  2. M. Roh-Johnson et al., Science. 335, 1232–1235 (2012).
  3. A. C. Martin, Developmental Biology. 341, 114–125 (2010).
  4. D. J. V. David, A. Tishkina, T. J. C. Harris, Development. 137, 1645–1655 (2010).
  5. J. Solon, A. Kaya-Copur, J. Colombelli, D. Brunner, Cell. 137, 1331–1342 (2009).
  6. He, X. Wang, H. L. Tang, D. J. Montell, Nature Cell Biology. 12, 1133–1142 (2010).
  7. M. Rauzi, P. Verant, T. Lecuit, P.-F. Lenne, Nature Cell Biology. 10, 1401–1410 (2008).
  8. H. Y. Kim, L. A. Davidson, Journal of Cell Science. 124, 635–646 (2011).
  9. K. Wu, S. Budnar, A. S. Yap, G. A. Gomez, European Journal of Cell Biology. 93, 396–404 (2014).
  10. F.B. Robin, J.B. Michaux, W. McFadden, E.M. Munro, Excitable RhoA dynamics drive pulsed contractions in the early C. elegans embryo, bioRxiv, 076356 (2016)

IBPS - Laboratoire de Biologie du Développement – ERL 1156
CNRS – Université Pierre et Marie Curie – Inserm
9, quai Saint-Bernard – Bât. C – 6ème Et. – Case 24 – 75252 Paris cedex 05 – France
Tel :+33 (0)1 44 27 42 41 – email : francois.robinATupmc.fr