We work to understand how embryos change shape during development, and in particular, we are interested in the biological processes that control the mechanical properties of cells.
The cortical actomyosin cytoskeleton is a major determinant of the mechanical properties of embryonic cells and tissues. Spatial and temporal modulations of these properties define the mechanical landscape that drives morphogenesis. Yet, an integrated view linking the biochemical properties of the cortex, the structures it then assembles and their mechanical properties is still missing. We want to characterize how from the local biochemical dynamics of the cortex in vivo emerges a structured, active and dynamic polymer material that controls the cell mechanics and morphogenesis.
For this, we use a small worm, the nematode C. elegans, an extremely powerful model system, combining a variety of approaches from different fields: quantitative live cell imaging, genetics, optogenetics, biophysics and biochemistry.
During the development of an embryo, each cell plays its part in well-orchestrated ballet that shapes the organism. But to shape the embryo, cells must tune their mechanical properties in a concerted and tightly regulated manner.
A thin layer of a polymer at the cell surface – the “cortex” – is responsible in a large part for defining cell mechanics – not unlike how the mechanics of a balloon is defined by the nature of the plastic that makes it. And the mechanical properties of this polymer come from the fibers it’s made of, like how the fabric of a shirt or a sweater makes it stiff or stretchy.
But in addition to these traditional properties, the cortex has two additional “skills”. First, it builds up and breaks down very rapidly: it is “dynamic”. If you leave a water balloon on a shelf and then decide a year later to pick it up again, the same material would be there, the same polymer fibers. Same’s true for the fabric of your shirt or your sweater. In the cortex, on the other hand, a filament has a shelf life of seconds to minutes. Take 5, go take a coffee for two minutes, come back: all the filaments that make it have been replaced by new ones. This has a very important consequence: the cortex is visco-elastic. At short time scale, that is if you do something quick, it will behave like a spring – we talk of an elastic material. But at long time scales it behaves like a fluid. If you pull on it slowly, it will change its shape and adjust to a new one, just like honey. The second skill of the cortex is its ability to contract: it is “active”. The cortex is made of filaments, like fibers in a plastic or a shirt, but also of miniature motor that can contract. This give rise to a very interesting property of the cortex: it can actively shrink. Imagine your sweater shrinking in the back, while new fabric is being weaved in the front, or your kid’s waterballoon shrinking on one end while new rubber forms on the other side. Well, that’s basically what happens to some cells as they migrate.
Our aim is to understand how the assembly of the cortex, this dynamic and active polymer, controls cell mechanics.
We study these things because we find them marvelous and fascinating. We do it because we find it amazing to imagine that complex organisms emerge from a single cell, that changes into a ball of cells, that then forms limbs. But also because we believe that, although they probably have no direct or immediate practical applications to our work, understanding how something works is the first step to manipulating, engineering or exploiting it!
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