From simple to stratifying epithelia: unifying and diverging adhesive and mechanical principles – STRATEPI

Most tissues are lined by an epithelial barrier that are monolayers as in intestine or multilayered as in the skin. To maintain homeostasis, both types of epithelia renew through a balance between cell division and extrusion. Whereas in simple epithelia cell extrusion results in removal of cells from the monolayer, basal cells of stratified epithelia delaminate while differentiating to generate suprabasal layers.
The complex dynamics of epithelia relies on the active nature of their constituent cells giving rise to emergent physical properties, including jamming-unjamming, phase transitions, and active nematics. Whereas our understanding of how physical parameters instruct division and extrusion to make and maintain simple epithelia has advanced greatly in recent years, almost nothing is known on the physical properties that shape the formation and maintenance of stratified epithelia, due to complex 3D nature of the system.
The aim of this project is to understand epithelial stratification by identifying the biomechanical determinants that control cell delamination and suprabasal layer formation and maintenance. We hypothesize that the cellular and supracellular mechanical state of a basal epithelial monolayer determines whether it generates a suprabasal layer and undergo stratification. Using the skin epidermis and primary keratinocytes as paradigms for stratified epithelia we will perform quantitative analysis of the mechanical and molecular parameters of cell delamination, building on our extensive knowledge on these model systems and on cell extrusion from monolayered epithelia, to extract general and tissue specific principles. We will ask which active forces (cytoskeleton contractility, cortical tension) and passive resistive forces (cell-cell and cell-substratum adhesion) are key determinants of stratification versus extrusion and address whether these forces govern the establishment of a boundary between basal and suprabasal layers of the epidermis to make fate compartments. We will integrate experiment and theory to address their global and local impact on traction forces, cell shape and mechanical stress distribution during delamination, 3D cell sorting and boundary formation using in vitro 2D and 3D cellular models. We will consider the role of nematic ordering, cell’s extensile/contractile properties, and of 2D and 3D cell shape disparities to promote local rearrangements, and relate changes in these properties to local and global changes in the adhesive and cytoskeletal repertoire. Finally, we will address the physiological relevance of these in vitro identified principles that cover cell delamination/extrusion for in vivo stratification.
Overall, this interdisciplinary project will provide novel insight into the biomechanical principles that induced epithelia to evolve and switch to stratification to generate a more complex self-renewing barrier.