We initially focused on apoptotic cell dynamics. Using Drosophila developing leg as a model system to study apoptosis-dependent epithelium folding, we reveal that apoptotic cells, far from been eliminate without consequence on their surroundings, generate a traction force within tissues that drive tissue remodelling and folding (Monier et al, Nature, 2015). While apoptosis was known to contribute to tissue shape, it was generally assumed that it was a consequence of local elimination of cells that were no longer required. Our work identifies apoptotic cells as inductor of mechanical signals and brought apoptotic cells into the biomechanics field.
We next dissected the cellular mechanisms governing the generation of the apoptotic force. We found that it relies on the formation of a dynamic actomyosin contractile tether connecting the apical surface to a basally anchored nucleus, which constitutes an essential component of the force-producing machinery. This work reveals that the nucleus, in addition to its role in genome protection, actively participates in mechanical force production and connects the contractile actomyosin cytoskeleton to basal junctions (Ambrosini, Rayer et al, Developmental Cell, 2019).
Having discovered that apoptotic cells were generating forces, pulling on their neighbours and influencing tissue remodeling, we asked if this active role was conserved in higher organisms. Using chicken and quail embryos, we found that apoptosis mainly occurs shortly before or at the time of dorsal bending of the neural tube and is required for this bending. We further showed that apoptotic cells generate an apico-basal force before being extruded from the neuro-epithelium through the formation of a tensile acto-myosin cable that connect the nucleus to the apical surface of the cell. At the subcellular level, this force deforms the nucleus and precipitates its fragmentation. All together these data reveal that apoptotic cells influence their surroundings mechanically and strongly suggest their active contribution to neural tube bending. Furthermore, it suggests that the same force plays a role in tissue morphogenesis in fly and chicken embryos (Roellig et al, under review in Developmental Cell).
We further identified a network of genes orchestrating apoptosis dependent morphogenesis through a genetic screen. From around 1600 genes screened, we selected around 120 candidates required for leg morphogenesis. The rational was that these genes were potentially involved either in apoptosis regulation, apoptotic force generation and/or transmission, or apoptotic cell elimination (Barbaste, Schott et al, manuscript in preparation).
A new project in the team, regarding the amazing reproducibility of tissue shape from one individual to another originated from this screen. We could highlight a new role for mechanical forces in shape robustness. Indeed, in order to acquire a stereotyped 3D structure, a mechanical fence is built between different portions of the remodeling tissue ensuring the insensitivity to perturbations in the nearby environment. This mechanical fence relies on Arp2/3 complex, which drives Myosin II planar polarity and favors directional force transmission (Martin et al, Developmental Cell, 2021).
In parallel, we asked if the force exerted during apoptotic cell delamination was specific to apoptotic cells or share with other type of delaminating cells. To tackle this question we analyzed the dynamic of cell delamination during epithelial-mesenchymal transition (EMT) and combined state-of-the-art live imaging techniques, together with robust biophysical modelling.
EMT is an essential process in physiological and pathological contexts. While EMT is often associated with tissue invagination during development, the impact of EMT on tissue remodelling remains unexplored. We show that at the initiation of the EMT process, cells produce a force orthogonal to the surface of the epithelium that constitutes an essential driving force for tissue invagination. Using both laser microdissection and in silico physical modelling, we show that mesoderm invagination does not proceed if orthogonal forces are impaired, indicating that they constitute essential driving forces in the folding process. These data will help understand the initiation of metastasis (Gracia et al, Nature Communication, 2019).
