Body axis elongation represents a fundamental morphogenetic process, which involves cell shape changes powered by mechanical forces. Such changes occur through small incremental steps, suggesting the existence of specific mechanisms to stabilize cell shapes and counteract cell elasticity. We are studying C. elegans embryonic elongation to define how the embryo, an elastic material, lengthens progressively through cycles of muscle contractions. To identify the potential morphogenetic lock that would counteract elasticity, we focused on the kinase PAK-1, previously found to mediate a mechanotransduction system downstream of muscle contractions. We performed two screens in a pak-1(-) background, from which ?-spectrin SPC-1 came out as a strong candidate. We found that spc-1(-) pak-1(-) embryos elongate up to 1.5- fold and then retract to 1-fold in a muscle dependent manner. Super-resolution microscopy showed that epidermis circumferential actin filament bundles are discontinuous and not fully oriented perpendicular to adherens junctions in spc-1(-) pak-1(-) embryos. Strikingly, Priess & Hirsh (1986) found that actin depolymerization induces embryo retraction, suggesting that actin rearrangement could account for the lock counteracting elasticity. One additional RNAi screen combined with image analysis suggest the following scenario in normal embryos: (1) muscle activity bends at a sharp angle actin bundles (2) the formin FHOD-1 with actin bundling properties, whose absence combined with spc-1 mutation also induced retraction, blocks further actin depolymerization until the next cycle of muscle activity. In particular, overexpressing a C-terminally truncated FHOD-1(?FH2/DAD) construct, predicted from vertebrate studies to only bundle actin, partially rescued the spc-1(-) pak-1(-) retraction phenotype. To back our results with theory, we modeled the embryo as a Kelvin-Voigt material experiencing acto-myosin force from the epidermis plus muscle tension. We could predict embryo lengthening by introducing a viscoplastic component in the system, which we propose corresponds to actin shortening. Altogether our data identify a cellular network that confers mechanical plasticity to stabilize cell shapes during a ratchet morphogenetic process.

Authors: Lardennois, A., Pasti, G., Ferraro, T., Llense, F., Mahou, P., Pontabry, J., Rodriguez, D., Kim, S., Beaurepaire, E., Gally, C., Labouesse, M.

Affiliations:
- RS2D, France
- Sorbonne Universite, Institut de Biologie Paris-Seine (IBPS), CNRS UMR7622, Paris, France
- Development and Stem Cells Department, IGBMC - CNRS UMR 7104/ INSERM U964/ Universite de Strasbourg, Illkirch, France
- Laboratoire d'Optique et Biosciences, Ecole Polytechnique, INSERM U1182 - CNRS/ UMR7645, Palaiseau, France