Biology subjects

Fouchard, J.

Publications and source records attributed to Fouchard, J..

10 recordsLinked to original sources

bioRxiv2025

Osmotic pressure induces unexpected relaxation of contractile 3D microtissue

Cell contraction and proliferation, matrix secretion and external mechanical forces induce compression during embryogenesis and tumor growth, which in turn regulate cell proliferation, metabolism or differentiation. How compression affects tissue contractility, a hallmark of tissue function, is however unknown. Here we apply osmotic compression to microtissues of either mouse colon adenocarcinoma CT26 cells, mouse NIH 3T3 fibroblasts, or human primary colon cancer-associated fibroblasts. Microtissues are anchored to flexible pillars that serve as force transducers. We observe that low-amplitude osmotic compression induces a rapid relaxation of tissue contractility, primed by the deformation of the extracellular matrix. Furthermore, we show that this compression-induced relaxation is independent of the cell type, proportional to the initial tissue contractility, and depends on RhoA-mediated myosin activity. Together, our results demonstrate that compressive stress can relax active tissue force, and points to a potential role of this feedback mechanism during morphogenetic events such as onco- or embryogenesis.

biophysics↗

bioRxiv2025

Allometric spreading and focal adhesion collective organization are coordinated by cell-scale geometrical constraints

Focal adhesions are protein complexes that transmit actin cytoskeleton forces to the extracellular matrix and serve as signaling hubs that regulate cell physiology. While their growth is achieved through a local force-dependent process, the requirement of sustaining stress at the cell scale suggests a global regulation of the collective organization of focal adhesions. To investigate evidence of such large-scale regulation, we compared changes in cell shape and the organization of focal adhesion-like structures during the early spreading of fibroblasts either on a two-dimensional substrate or confined between two parallel plates, and for cells of different volumes. In this way, we reveal that the areal density of focal adhesions is conserved regardless of cell size or third-dimensional confinement, despite different absolute values of the surface covered by adhesion clusters. In particular, the width of the focal adhesions ring, which fills the flat lamella at the cell front, adapts to cell size and third-dimensional confinement and scales with cell-substrate contact radius. We find that this contact radius also adapts in the parallel-plate geometry so that the cumulated area of cellsubstrate contact is conserved at the cell scale. We suggest that this behavior is the result of 3D cell shape changes which govern spreading transitions. Indeed, because of volume conservation constraints, the evolution of cell-body contact angle adjusts according to cell size and confinement, whereas the rate of early spreading at the cell-substrate contact is not affected by thirddimensional geometry. Overall, our data suggest that a coordination between global and local scales mediates the adaptation of cell-substrate contacts and focal adhesions distribution to large scale geometrical constraints, which allows an invariant cell-substrate adhesive energy.

biophysics↗

bioRxiv2024

Disentangling the contributions of stress fibres and the unbundled actin meshwork to the anisotropy of cortical tension in response to cell shape

Many fundamental biological processes, in particular development and morphogenetic movements, involve tissue and cell deformation, as well as the generation of anisotropic mechanical stresses. They are often accompanied by the appearance of oriented contractile actomyosin structures resembling the stress fibres (SF) observed in vitro. Here, we investigate, at the single cell level, how cell shape -- by itself -- could control the structure and tension of the actomyosin cortex. Using a unique combination of 3D micropatterning, single peripheral SF (PSF) tension measurement, laser ablation and image analysis, we show that cell shape anisotropy, e.g. its 2D aspect ratio, is indeed sufficient to induce anisotropy of the cortical structure and tension. In particular, taking into account the experimentally measured anisotropy of the cortical meshwork, we could quantify cortical tension and decouple the contribution originating from bundled actin (oriented cortical stress fibres, CSF) and the contribution of the unbundled actin meshwork (UAM). We show that the increase of cortical tension anisotropy with the cells aspect ratio depends on the CSF alignment and orientation, the contribution of the isotropic mesh being independent of cell shape. Remarkably, while experimental data from single stress fibre measurements and laser ablation were analysed through different theoretical frameworks, namely that of negative pressure in nematics and hole drilling in prestressed materials, we found quantitatively the same composite material behaviour. In sum, we decipher here the very material properties of the actomyosin cortex, and its sensitivity to cell shape which is at the root of many mechanobiological processes, in particular morphogenesis.

biophysics↗

bioRxiv2024

Human adipose stromal cells differentiate towards a tendon phenotype with extracellular matrix production and adapted visco-elastic properties in a 3D-culture system

Tendon cell differentiation relies on molecular and mechanical parameters that control the expression of tendon-associated transcription factors and extracellular matrix proteins. However, the minimal cues able to initiate tendon differentiation from progenitor cells remains unknown. We analysed the tendon differentiation program in human adipose stromal cells (hASCs) cultured in a minimal 3D-system. We generated 3D-hASC constructs by embedding hASCs in a type-I collagen gel under a static uniaxial geometrical constraint with no additional molecular and mechanical cues, and assessed tendon-associated gene expression and mechanical properties for up to three weeks in culture. Analysis of tendon-associated genes revealed a molecular progression consistent with the acquisition of a tendon phenotype. The analysis of viscoelastic properties of 3D-hASC constructs by nano-indentation indicated a progressive increase in tissue stiffness up to 10 kPa, concomitant with a reduced stress relaxation indicative of solid-like mechanical properties. These changes in mechanical properties parallel the molecular change of matrix genes during the time of cultures. In summary, we have established that hASCs cultured in a minimal 3D-system progress into the tendon differentiation program associated with variations of mechanical properties.

developmental biology↗

bioRxiv2023

Fracture in Living Cell Monolayers

The ability of tissues to sustain mechanical stress and avoid rupture is a fundamental pillar of their function. Rupture in response to physiological levels of stress can be undesired, for example resulting from disease or genetic mutations, or be an integral part of developmental processes, such as during blastocoel formation in mouse or leg eversion in flies. Despite its importance, we know very little about rupture in cellularised tissues because it is a multi-scale phenomenon that necessitates comprehension of the interplay between mechanical forces and processes at the molecular and cellular scales. Using a combination of mechanical measurements, live imaging and computational modelling, we characterise rupture in epithelial monolayers. We show that, despite consisting of only a single layer of cells, monolayers can withstand surprisingly large deformations, often accommodating several-fold increases in their length before rupture. At large deformation, epithelia increase their stiffness multiple-fold in a process controlled by a supracellular network of keratin filaments. Perturbing keratin organisation fragilised monolayers and prevented strain stiffening. Using computational approaches, we show that, although the kinetics of adhesive bond rupture ultimately control tissue strength, tissue rheology and the history of deformation prior to failure set the strain and stress that the tissue reaches at the onset of fracture. Our data paint a picture of epithelia as versatile materials that combine resistance to shocks with deformability when subjected to low strain rates.

biophysics↗

bioRxiv2022

Tension at intercellular junctions is necessary for accurate orientation of cell division in the epithelium plane

The direction in which a cell divides is set by the orientation of its mitotic spindle and is important for determining cell fate, controlling tissue shape and maintaining tissue architecture. Division perpendicular to the plane of the substrate can promote tissue stratification during development or wound healing, but also metastasis when orientation is aberrant. Much is known about the molecular mechanisms involved in setting the spindle orientation. However, less is known about the contribution of mechanical factors, such as tissue tension, in ensuring spindle orientation in the plane of the epithelium, despite epithelia being continuously subjected to mechanical stresses. Here, we used suspended epithelial monolayers devoid of extracellular matrix and subjected to varying levels of tissue tension to study the orientation of division relative to the tissue plane. We found that decreasing tissue tension by compressing the monolayers or by inhibiting myosin contractility leads to a higher frequency of out-of-plane divisions. Reciprocally, accurate in-plane division can be restored by increasing tissue tension by increasing cell contractility or by tissue stretching. By considering the full three-dimensional geometry of the epithelium, we show that spindles are sensitive to tissue tension, independently of cell shape, through its impact on the tension at subcellular surfaces. Overall, our data suggest that accurate spindle orientation in the plane of the epithelium necessitates the presence of a sufficiently large tension at intercellular junctions. Significance statementIn growing epithelia, divisions are typically oriented in the plane of the tissue to drive expansion. In some organs, divisions are then re-oriented so that they occur perpendicular to the epithelium plane to drive tissue stratification and cell differentiation. When uncontrolled, this switch in orientation can lead to defects in tissue organisation and, in cancer, likely contribute to metastasis. While much is known about the molecular mechanisms controlling mitotic spindle orientation, less is known about the role of mechanical factors. Here we use mechanical and chemical perturbations to show that mechanics plays a role in controlling the plane of division. Overall, our data suggest that the orientation of spindles in the epithelium plane requires sufficient tension across intercellular junctions.

biophysics↗

bioRxiv2019

Curling of epithelial monolayers reveals coupling between active bending and tissue tension

Epithelial monolayers are two-dimensional cell sheets which compartmentalise the body and organs of multi-cellular organisms. Their morphogenesis during development or pathology results from patterned endogenous and exogenous forces and their interplay with tissue mechanical properties. In particular, bending of epithelia is thought to results from active torques generated by the polarization of myosin motors along their apico-basal axis. However, the contribution of these out-of-plane forces to morphogenesis remains challenging to evaluate because of the lack of direct mechanical measurement. Here, we use epithelial curling to characterize the out-of-plane mechan ics of epithelial monolayers. We find that curls of high curvature form spontaneously at the free edge of epithelial monolayers devoid of substrate in vivo and in vitro. Curling originates from an enrichment of myosin in the basal domain that generates an active spontaneous curvature. By measuring the force necessary to flatten curls, we can then estimate the active torques and the bending modulus of the tissue. Finally, we show that the extent of curling is controlled by the interplay between in-plane and out-of-plane stresses in the monolayer. Such mechanical coupling implies an unexpected role for in-plane stresses in shaping epithelia during morphogenesis.

biophysics↗

bioRxiv2019

A unified rheological model for cells and cellularised materials

The mechanical response of single cells and tissues exhibits a broad distribution of time scales that gives often rise to a distinctive power-law regime. Such complex behaviour cannot be easily captured by traditional rheological approaches, making material characterisation and predictive modelling very challenging. Here, we present a novel model combining conventional viscoelastic elements with fractional calculus that successfully captures the macroscopic relaxation response of epithelial monolayers. The parameters extracted from the fitting of the relaxation modulus allow prediction of the response of the same material to slow stretch and creep, indicating that the model captured intrinsic material properties. Two characteristic times can be derived from the model parameters, and together these explain different qualitative behaviours observed in creep after genetic and chemical treatments. We compared the response of tissues with the behaviour of single cells as well as intra and extra-cellular components, and linked the power-law behaviour of the epithelium to the dynamics of the cell cortex. Such a unified model for the mechanical response of biological materials provides a novel and robust mathematical approach for diagnostic methods based on mechanical traits as well as more accurate computational models of tissues mechanics.

biophysics↗

bioRxiv2018

Actomyosin controls planarity and folding of epithelia in response to compression

Throughout embryonic development and adult life, epithelia are subjected to external forces. The resulting deformations can have a profound impact on tissue development and function. These include compressive deformations which, although hard to study in model systems due to the confounding effects of the substrate, are thought to play an important role in tissue morphogenesis by inducing tissue folding and by triggering mechanosensitive responses including cell extrusion and cell differentiation. Here, using suspended epithelia, we are able to uncover the immediate response of epithelial tissues to the application of large (5-80%) in-plane compressive strains. We show that fast compression induces tissue buckling followed by active tissue flattening which erases the buckle within tens of seconds. Strikingly, there is a well-defined limit to this second response, so that stable folds form in the tissue for compressive strains larger than [~]35%. Finally, a combination of experiment and modelling shows that the response to compression is orchestrated by the automatic adaptation of the actomyosin cytoskeleton as it re-establishes tension in compressed tissues. Thus, tissue pre-tension allows epithelia to both buffer against fast compression and regulate folding.

biophysics↗

bioRxiv2018

Stress relaxation in epithelial monolayers is controlled by actomyosin

Epithelial monolayers are one-cell thick tissue sheets that separate internal and external environments. As part of their function, they withstand extrinsic mechanical stresses applied at high strain rate. However, little is known about how monolayers respond to mechanical deformations. In stress relaxation tests, monolayers respond in a biphasic manner and stress dissipation is accompanied by an increase in monolayer resting length, pointing to active remodelling of cell architecture during relaxation. Consistent with this, actomyosin remodels at a rate commensurate with mechanical relaxation and governs the rate of monolayer stress relaxation - as in single cells. By contrast, junctional complexes and intermediate filaments form stable connections between cells, enabling monolayers to behave rheologically as single cells. Together, these data show actomyosin cytoskeletal dynamics govern the rheological properties of monolayers by enabling active, ATP-dependent changes in the resting length. These findings have far-reaching consequences for our understanding of developmental morphogenesis and tissue response to mechanical stress.

biophysics↗