Structure et dynamique des forces lors de la migration collective de cellules épithéliales en confinement 1D

Collective cell migration is a fundamental phenomenon in biology involved in many physiological and pathological processes. This phenomenon relies on the ability of cells within a group to interact with each other and to coordinate themselves to move collectively. However, this collective organization during migration varies according to the context, and the mechanisms of cellular self-organization remain poorly understood. This thesis focuses on the propulsion modes employed by cells during their collective migration, asking whether cells tend to self-propel or to form a supracellular structure that allows them to propel themselves collectively. We used an experimental model of MDCK epithelial cells. They are deposited on deformable gels allowing the measurement of the forces exerted on the substrate by Traction Force Microscopy (TFM). These cells are also confined on micro-printed fibronectin line patterns reducing their migration on one dimension. By analysing the trajectories as well as the distribution of forces within cell trains, we have shown several correlation or independence relationships between the speeds, the cell spreading, the collectivity (number of cells in the train) and the forces exerted by these trains. These forces are organised in a continuum between two extreme states, one organised in a long range force transmission and the other in a redistribution of traction forces at the cellular scale; demonstrating a mechanical plasticity of the collective. The destabilisation of cell junctions and the decrease in contractility favour a redistribution of forces at the cellular scale. Our study on the impact of geometrical constraints, particularly the width of the lines, exposes several complex effects on the collective mechanical organisation. This spontaneous redistribution of forces between the substrate and the cell junctions is accompanied by molecular changes, leading to destabilisation or strengthening of the junctions as well as the appearance or disappearance of focal adhesions. The study of the asymmetric organisation of the traction forces shows that a mechanical asymmetry is established between the front and the back of individual cells in migration. In a multicellular context, cell doublets establish a mechanical asymmetry at the multicellular scale favoured by the intensity of the mechanical coupling between cells. This thesis work explains some of the biomechanical couplings necessary for mechanical plasticity during collective migration. It shows that some internal or external factors favour a redistribution of forces towards the cellular or the collective scale and the impact of this plasticity on the mechanical asymmetry during collective migration.La migration cellulaire collective est un phénomène fondamental en biologie impliqué dans de nombreux processus physiologiques, qu'ils soient physiologiques ou pathologiques. Ce phénomène repose sur la capacité des cellules au sein d'un groupe d'interagir entre elles et de se coordonner pour se déplacer collectivement. Cependant, cette organisation collective lors de la migration varie suivant les contextes, et les mécanismes d'auto-organisation cellulaire restent mal connus. Cette thèse s'intéresse plus particulièrement aux modes de propulsion employés par les cellules lors de leur migration collective, en se demandant si les cellules ont plutôt tendance à s'autopropulser ou à former une structure supracellulaire leur permettant de se propulser collectivement. Nous avons utilisé un modèle expérimental de cellules épithéliales MDCK. Elles sont déposées sur des gels déformables permettant de mesurer les forces exercées sur le substrat par Traction Force Microscopy (TFM). Ces cellules sont aussi confinées sur des motifs de ligne de fibronectine micro-imprimée réduisant leur migration à une dimension. En analysant les trajectoires ainsi que la répartition des forces au sein de trains de cellules, nous avons montré plusieurs relations de corrélation ou d'indépendance entre les vitesses, l'étalement cellulaire, la collectivité (nombre de cellules dans le train) et les forces exercées par ces trains. Ces dernières s'organisent en un continuum entre deux états extrêmes l'un organisé en une transmission à longue distance des forces et l'autre en une redistribution des forces de traction sur le substrat à l'échelle cellulaire, démontrant une certaine plasticité mécanique du collectif. La déstabilisation des jonctions cellulaires ainsi que la diminution de la contractilité favorisent une redistribution des forces à l'échelle cellulaire. Notre étude sur l'impact des contraintes géométriques, notamment la largeur des lignes, expose plusieurs effets complexes sur l'organisation mécanique collective. Cette redistribution spontanée des forces entre le substrat et les jonctions cellulaires s'accompagne de changements moléculaires, entraînant une déstabilisation ou un renforcement des jonctions ainsi que l'apparition ou la disparition des adhésions focales. L'étude de l'organisation asymétrique des forces de traction montre qu'une asymétrie mécanique s'établit entre l'avant et l'arrière de des cellules individuelles en migration. Dans un contexte multicellulaire, les doublets de cellule établissent une asymétrie mécanique à l'échelle multicellulaire qui est favorisée par l'intensité du couplage mécanique entre cellules. Ce travail de thèse explicite certains couplages biomécaniques nécessaires à la plasticité mécanique lors de la migration collective. Il montre ainsi que certains facteurs internes ou externes favorisent une redistribution des forces vers l'échelle cellulaire ou bien collective, et l'impact de cette plasticité sur l'asymétrie mécanique lors de la migration collective

Cell migration guided by long-lived spatial memory

International audienceLiving cells actively migrate in their environment to perform key biological functions-from unicellular organisms looking for food to single cells such as fibroblasts, leukocytes or cancer cells that can shape, patrol or invade tissues. Cell migration results from complex intracellular processes that enable cell self-propulsion, and has been shown to also integrate various chemical or physical extracellular signals. While it is established that cells can modify their environment by depositing biochemical signals or mechanically remodelling the extracellular matrix, the impact of such self-induced environmental perturbations on cell trajectories at various scales remains unexplored. Here, we show that cells can retrieve their path: by confining motile cells on 1D and 2D micropatterned surfaces, we demonstrate that they leave long-lived physicochemical footprints along their way, which determine their future path. On this basis, we argue that cell trajectories belong to the general class of self-interacting random walks, and show that self-interactions can rule large scale exploration by inducing long-lived ageing, subdiffusion and anomalous first-passage statistics. Altogether, our joint experimental and theoretical approach points to a generic coupling between motile cells and their environment, which endows cells with a spatial memory of their path and can dramatically change their space exploration

Myosin II isoforms play distinct roles in adherens junction biogenesis

\u3cp\u3eAdherens junction (AJ) assembly under force is essential for many biological processes like epithelial monolayer bending, collective cell migration, cell extrusion and wound healing. The acto-myosin cytoskeleton acts as a major force-generator during the de novo formation and remodeling of AJ. Here, we investigated the role of non-muscle myosin II isoforms (NMIIA and NMIIB) in epithelial junction assembly. NMIIA and NMIIB differentially regulate biogenesis of AJ through association with distinct actin networks. Analysis of junction dynamics, actin organization, and mechanical forces of control and knockdown cells for myosins revealed that NMIIA provides the mechanical tugging force necessary for cell-cell junction reinforcement and maintenance. NMIIB is involved in E-cadherin clustering, maintenance of a branched actin layer connecting E-cadherin complexes and perijunctional actin fibres leading to the building-up of anisotropic stress. These data reveal unanticipated complementary functions of NMIIA and NMIIB in the biogenesis and integrity of AJ.\u3c/p\u3

The role of single-cell mechanical behaviour and polarity in driving collective cell migration

International audienceThe directed migration of cell collectives is essential in various physiological processes, such as epiboly, intestinal epithelial turnover, and convergent extension during morphogenesis as well as during pathological events like wound healing and cancer metastasis 1,2 . Collective cell migration leads to the emergence of coordinated movements over multiple cells. Our current understanding emphasizes that these movements are mainly driven by large-scale transmission of signals through adherens junctions 3,4 . In this study, we show that collective movements of epithelial cells can be triggered by polarity signals at the single cell level through the establishment of coordinated lamellipodial protrusions. We designed a minimalistic model system to generate one-dimensional epithelial trains confined in ring shaped patterns that recapitulate rotational movements observed in vitro in cellular monolayers and in vivo in genitalia or follicular cell rotation 5–7 . Using our system, we demonstrated that cells follow coordinated rotational movements after the establishment of directed Rac1-dependent polarity over the entire monolayer. Our experimental and numerical approaches show that the maintenance of coordinated migration requires the acquisition of a front-back polarity within each single cell but does not require the maintenance of cell-cell junctions. Taken together, these unexpected findings demonstrate that collective cell dynamics in closed environments as observed in multiple in vitro and in vivo situations 5,6,8,9 can arise from single cell behavior through a sustained memory of cell polarity