The reproducible shape and spatial organization of organs imply the existence of physical rules directing the assembly of complex biological structures. Organ shape and function depend on cell architecture and polarity, which are both supported by cell cytoskeleton networks. The formation of controlled and reproducible geometrical structures relies on the self-organization properties of these networks. Our aim is to unravel the physical processes underlying cytoskeleton self-organization processes and to formulate the rules directing their spatial organization.
Actin filaments and microtubules form such complex intricate networks in cells that it is difficult to identify the principles of their self-organization. Our rationale is that these principles can manifest themselves in a reproducible, and therefore understandable, way only in response to defined geometrical cues. Thus, to study the geometrical and mechanical rules underlying cytoskeleton self-organization we used microfabrication tools in order to control and manipulate the spatial boundary conditions the cytoskeleton networks are sensitive to.
These tools allow us to analyze and quantify actin and microtubule networks in cells of controled and regular shapes. Considering that the complexity of the intra-cellular biochemical conditions may partially hinder the physical rules we want to investigate, we are also developing alternative methods to analyze cytoskeleton self-organization in controlled biochemical conditions in vitro by mixing, in defined proportions, the individual cytoskeleton components. Cells extracts and cytoplastes allow us to bridge the gap between these two approaches. The simplification of the cellular approach, the top-down way, and the complexification of the biochemical approach, the bottom-up way, should eventually encounter and provide us a continuous experimental platform to analyze the physics of cytoskeleton networks and morphogenesis from molecules to cells.
Actin is an asymmetric protein that can self-assemble to form polarized actin filaments. Actin filaments can interact to form actin networks. Actin networks can self-organize into two main types of structures in cells: bundles (or fibers) made of aligned long filaments and meshworks made of branched and intermingled short filaments. Our aim is to reconstitute actin networks in vitro and investigate : 1- how the spatial distribution of actin nucleators directs the growth and assembly of filaments to form defined network geometries in 2D and 3D, 2- how the contraction and disassembly processes regulate networks turn-over and permanent renewal. In both cases we try to relate network dynamics to its physical properties and the eventual production of mechanical forces. A particular attention is devoted to the study of the role of actin network geometry and size.
We are interested in the regulation of mechanical forces production by the actin cytoskeleton. Using traction force microscopy with micropatterned cells we can precisely quantify the forces developed on cell-matrix and on cell-cell adhesions. We investigate the role of proteins associated to cell adhesions and actin cables as well as the effect of actin cable’s size, number and spatial organization in cells. We are interested in the mechanical changes associated to key morphogenetic event such as the epithelial to mesenchymal transition.
Similar to the formation of actin filaments from the self-assembly of actin monomers, tubulin forms asymmetric dimers that can self-assemble into microtubules. Compared with actin filaments, microtubules are much more rigid, and almost straight in the dimensions of a single cell. Microtubules can sustain much higher compression forces than actin filament but are not as numerous as actin filaments. In most animal cells, the MT network forms as an aster in which microtubules radiate from the centrosome, the main microtubule organizing center. As cells divide, the centrosome is duplicated and the network forms a bipolar spindle. Our aim is to investigate the role of microtubule mechanics and microtubule-associated proteins in the establishment of the various microtubule network architectures during interphase and mitosis.
The centrosome got its name from its geometrical position “at the center of the soma”. It is indeed generally observed at the geometrical cell centre in culture conditions or during specific developmental stages. This centering mechanism is remarkably robust and can adapt to asymmetric external adhesive cues. However, in some specific conditions, the centrosome can be off-centred and close to the cell periphery. It is notably the case in quiescent cells where it supports the growth of the primary cilium. Our aim is to understand the mechanisms regulating centrosome centering and off-centering. In particular we would like to characterize the force balance acting on centrosome that stem from mechanical forces developed in both actin and microtubule networks. To that end we investigate the mechanical and geometrical changes in actin and microtubule networks during two interesting events: the ciliogenesis and the formation of cell-cell junctions, since they are both coupled to centrosome position changes.
