Research on the mechanics of epithelial monolayers was supported by an ERC consolidator grant between 09/2015 and 02/2021.
Summary of the achievements of the project
Epithelial monolayers are amongst the simplest tissues in the body, yet they play fundamental roles in adult organisms where they separate the internal environment from the external environment and in development when the intrinsic forces generated by cells within the monolayer drive tissue morphogenesis. The mechanics of these simple tissues is dictated by the cytoskeletal and adhesive proteins that interface the constituent cells into a tissue-scale mechanical syncytium. Mutations in these proteins lead to diseases with fragilised epithelia. However, a quantitative understanding of how subcellular structures govern monolayer mechanics, how cells sense their mechanical environment and what mechanical forces participate in tissue morphogenesis is lacking.
The overall ambition of this project was to characterise the mechanics of cell monolayers at the molecular, cellular, and tissue-scales.
Previous efforts at understanding epithelial tissue mechanics were hampered by the difficulty of deconvolving the mechanics of cell monolayers from those of the substrate they grow on. This project relied on a novel culture system we developed that allows the generation of monolayers devoid of a substrate. These can be subjected to well-controlled deformations while simultaneously monitoring stress in the tissue and imaging at the cellular- or sub-cellular scale, allowing for inherently multi-scale interrogation of epithelial mechanics. In combination with molecular cell biology approaches, these techniques allowed us to characterise the mechanics of epithelia in engineering terms and link mechanical properties with cytoskeletal structures and signalling. This laid the groundwork for the development of multi-scale numerical simulations of the mechanical response of epithelia during physiological function and in embryonic development.
Below we list the highlights of the research:
Stress relaxation in epithelial monolayers is controlled by the actomyosin cortex
As part of their function, tissues must withstand extrinsic mechanical stresses. We show that stress relaxation in monolayers consisting of tens of thousands of cells shows remarkable similarities with relaxation of single cells, suggesting the rheology of epithelial tissues is mediated by molecular turnover of the actomyosin cortex. By contrast, cell-cell junctional complexes and intermediate filaments do not relax tissue stress but form stable connections between cells, allowing monolayers to behave rheologically as single cells.
Epithelial monolayers behave as prestressed elastic sheets
As part of their physiology, epithelia are often subjected to compressive stresses. We showed that epithelial tissues behave as pretensed viscoelastic sheets that can buffer against compression and rapidly recover from buckling. Tissue tension is controlled by the actomyosin cytoskeleton. Epithelial mechanics define a tissue intrinsic buckling threshold that dictates the compressive strain above which tissue folds become permanent.
Curling of epithelial monolayers is controlled by apico-basal gradients in myosin contractility
Tissues change shape and bend during developmental morphogenesis or early in tumor formation. Yet, the amplitude of bending forces and how they integrate with tensile and compressive forces within the plane of the tissue remain largely unknown. By revealing the ability of epithelial monolayers to curl,we demonstrate that the polarization of contractile molecular motors within the tissue thickness generates high spontaneous curvature of the sheet. We quantify the corresponding torques and show that stretch and compression within the tissue plane can substantially impact curling.
Kelkar M, Bohec P, Charras G. “Mechanics of the cellular actin cortex: from signalling to shape change”, Current Opinion in Cell Biology, 66:69-78, (2020).
Recho P, Fouchard J, Wyatt TPJ, Charras G, Kabla A. “Tug-of-war between stretching and bending in living cell sheets”, Physics Review E, 102:012401, (2020).
Bonfanti A, Kaplan L, Charras G, Kabla A. “Fractional viscoelastic models for power-law materials”, Soft Matter, 16:6002-6020, (2020).
Rheinlaender J, Dimitracopoulos A, Wallmeyer B, Kronenberg NM, Chalut KJ, Gather MC, Betz T, Charras G, Franze K. “Cortical stiffness is independent of substrate mechanics”, Nature Materials, 19:1019-1025,(2020).
Lam M, Lisica A, Ramkumar N, Hunter G, Mao Y, Charras G, Baum B. “Isotropic myosin-generated tissue tension is required for the dynamic orientation of the mitotic spindle”, Molecular Biology of the Cell, 31:1370-1379, (2020).
Fouchard J, Wyatt T, Proag A, Lisica A, Khalilgharibi N, Suzanne M, Kabla A, Charras G. “Curling of epithelial monolayers reveals coupling between active bending and tissue tension”, PNAS, 117:9377-9383, (2020).
Cao L, Yonis A, Vaghela M, Barriga EH, Chugh P, Smith MB, Maufront J, Lavoie G, Méant A, Ferber E, Bovellan M, Alberts A, Bertin A, Mayor R, Paluch EK, Roux PP, Jégou A, Romet-Lemonne G, Charras G. “SPIN90 associates with mDia1 and the Arp2/3 complex to control cortical actin organization”. Nature Cell Biology, 22:803-814, (2020).
Bonfanti A, Fouchard J, Khalilgharibi N, Charras G, Kabla A. “A unified rheological model for cells and tissues”. Royal Society Open Science, 7:190920, (2020).
Wyatt T, Fouchard J, Lisica A, Khalilgharibi N, Baum B, Recho P, , Kabla AJ, Charras G. “Actomyosin controls planarity and folding of epithelia in response to compression”. Nature Materials,19:109-117, (2020). Highlighted in commentary in the Journal.
Khalilgharibi N, Fouchard J, Asadipour N, Barrientos R, Duda M, Bonfanti A, Yonis A, Harris A, Mosaffa P, Fujita Y, Kabla A, Mao Y, Baum B, Muñoz JJ, Miodownik M, Charras G. “Stress relaxation in epithelial monolayers is controlled by the actomyosin cortex”. Nature Physics, 15:839-847, (2019). Reviewed in F1000.
Mohammed D, Charras G, Vercruysse E, Versaevel M, Lantoine J, Alaimo L, Bruyère C, Luciano M, Glinel K, Delhaye G, Théodoly o, Gabriele S. “Substrate adhesive area confinement is a key determinant of cell velocity in collective migration”. Nature Physics, 15:858-866, (2019).
Duda M, Kirkland NJ, Khalilgharibi N, Tozluoglu M, Yuen AC, Carpi N, Bove A, Piel M, Charras G, Baum B, Mao Y. “Polarization of Myosin II refines tissue material properties to buffer mechanical stress”, Developmental Cell, 28;48(2):245-260 (2019). Reviewed in F1000.
Barriga E, Franze K, Charras G, Mayor R. “Tissue stiffening coordinates morphogenesis by triggering collective cell migration in vivo.” Nature, 554:523-527, (2018). Reviewed in F1000.
Khalilgharibi N, Fouchard J, Recho P, Charras G*, Kabla A*. “The dynamic mechanical properties of cellularised aggregates”, Current Opinion in Cell Biology, (42):113-120. (2016). * co-corresponding author.
Wyatt TPJ, Baum B,Charras G. “A question of time: tissue adaptation to mechanical forces”, Current Opinion in Cell Biology, 38:68-73, (2016).