Morphogenesis of two active tissues in contact, a multi-scale mechanical analysis – EpiMorph

Living tissues constitute materials with extraordinary mechanical properties. Indeed, their cells can deform, regulate their elasticity and even their fluidity by responding to mechanical input. Until very recently, studies on the mechanical properties of tissues had mainly focused on how specific cell types behave in vivo or in vitro, but had rarely integrated the input of different cell types on each other. Likewise, biochemical and biophysical studies have defined how proteins, especially adhesion components, respond to mechanical forces, but such studies have seldom been validated in vivo. Finally, while several theoretical studies have begun to account for the transformation of subcellular entities under physical forces, once again too few of them account for the emergent properties of the active matter in live tissues.

Hence, it becomes essential to bridge scales by encompassing the mechanical input of multiple cell types, by testing whether the unfolding of protein domains predicted from in vitro studies regulates the mechanical response of tissues in vivo, and by proposing models that incorporate the fact that stiffness and stress constantly change.

Epimorph will propose to combine an in vivo model, the C. elegans embryo, a well-defined vertebrate cell culture tissue to mimic C. elegans embryos, and theoretical modeling to achieve the following general goals:
1/ Propose a model for the changes of two mechanically interacting tissues taking their evolving stress and stiffness fields as an input.
2/ Dissect how cortical proteins contribute to mechanosensing and to remodel the cortical actin cytoskeleton.

To achieve these goals, we will combine our expertise with C. elegans embryos, in measuring tension with FRET biosensors, in light-sheet microscopy, and our proven record of physics-biology collaboration. Studies of C. elegans embryo will focus on the stage when its body elongates by 200% through the tight interaction between contracting muscles and the outer epithelium. We will:
(i) Measure the deformation experienced by muscle cells (using calcium imaging and a plasma membrane marker), and the reciprocal effects exerted on the outer epithelium (looking at adherens junctions and the cytoskeleton) – imaging will rely on light-sheet microscopy and spinning disk microscopy;
(ii) Measure the tension experienced by muscle cells using a FRET tension biosensor based on talin (a muscle protein), and by the outer epithelium at the level of the cortex using a FRET tension biosensor based on spectrin (a cortical protein);
(iii) Define how the epithelial actin cytoskeleton gets remodelled in vivo using super-resolution microscopy, dissect the mechanosensing process that leads to actin remodelling;
(iv) Determine if the active mechanical behavior and underlying mechanosensing pathway acting in C. elegans is conserved in vertebrate epithelial cells grown on a PDMS substrate stretched by an automated device;
(v) Elaborate a continuum mechanics model to account for the extension of two connected tissues with different material and active properties based on viscoelasticity and taking apart the active and passive contributions of each cell to global deformation.

This project will make an essential contribution to our understanding of cytoskeletal function, cellular shape memory with important implications for tissue organization and genetic diseases of mechanical origin.