Traction Forces of Endothelial Cells under Slow Shear Flow

Endothelial cells are constantly exposed to fluid shear stresses that regulate vascular morphogenesis, homeostasis, and disease. The mechanical responses of endothelial cells to relatively high shear flow such as that characteristic of arterial circulation has been extensively studied. Much less is known about the responses of endothelial cells to slow shear flow such as that characteristic of venous circulation, early angiogenesis, atherosclerosis, intracranial aneurysm, or interstitial flow. Here we used a novel, to our knowledge, microfluidic technique to measure traction forces exerted by confluent vascular endothelial cell monolayers under slow shear flow. We found that cells respond to flow with rapid and pronounced increases in traction forces and cell-cell stresses. These responses are reversible in time and do not involve reorientation of the cell body. Traction maps reveal that local cell responses to slow shear flow are highly heterogeneous in magnitude and sign. Our findings unveil a low-flow regime in which endothelial cell mechanics is acutely responsive to shear stress

Active wetting of epithelial tissues

Development, regeneration and cancer involve drastic transitions in tissue morphology. In analogy with the behavior of inert fluids, some of these transitions have been interpreted as wetting transitions. The validity and scope of this analogy are unclear, however, because the active cellular forces that drive tissue wetting have been neither measured nor theoretically accounted for. Here we show that the transition between 2D epithelial monolayers and 3D spheroidal aggregates can be understood as an active wetting transition whose physics differs fundamentally from that of passive wetting phenomena. By combining an active polar fluid model with measurements of physical forces as a function of tissue size, contractility, cell-cell and cell-substrate adhesion, and substrate stiffness, we show that the wetting transition results from the competition between traction forces and contractile intercellular stresses. This competition defines a new intrinsic lengthscale that gives rise to a critical size for the wetting transition in tissues, a striking feature that has no counterpart in classical wetting. Finally, we show that active shape fluctuations are dynamically amplified during tissue dewetting. Overall, we conclude that tissue spreading constitutes a prominent example of active wetting --- a novel physical scenario that may explain morphological transitions during tissue morphogenesis and tumor progression

Crumbs proteins in epithelial morphogenesis

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Rôle de la Drebine dans la morphogenèse épithéliale

La morphogenèse épithéliale est un processus complexe qui résulte en une organisation particulière des épithélia, leur permettant ainsi d assurer leurs fonctions physiologiques. Les cellules épithéliales sont polarisées et présentent un domaine apical en contact avec le milieu extérieur et un domaine basal en contact avec la lame basale. Ces deux domaines sont séparés par un complexe jonctionnel qui permet de maintenir une cohésion du tissu, et qui joue un rôle de barrière physique entre l extérieur et l intérieur du corps. Du côté apical, ces cellules présentent une bordure en brosse composée de microvillosités hautement organisées, qui leur permet ainsi d augmenter leur surface d échange avec le milieu extérieur. Dans la cellule, ces microvillosités vont s ancrer au niveau d un réseau dense d actine, le terminal web. Au cours de ma thèse, je me suis intéressée à la morphogenèse des cellules épithéliales intestinales en utilisant comme modèle la lignée cellulaire Caco2 qui en culture est capable de se différencier en entérocytes. L acquisition de leur polarité ainsi que de leur forme colonnaire nécessite des remodelages du cytosquelette d actine et la régulation du trafic intracellulaire des composants de la membrane plasmique. Dans cette étude, j ai identifié la Drebrine, qui est une protéine de liaison à l actine, comme une protéine nécessaire à la formation du terminal web et de la bordure en brosse. Parallèlement, j ai montré que la Drebrine est impliquée dans la redistribution de différentes vésicules d endocytose, et/ou de recyclage, lors de la morphogenèse épithélialeEpithelial morphogenesis is a complex process that provides a unique organization to epithelial cells in order to perform their physiological functions. Epithelial cells are highly polarized cells with an apical domain facing the outside environment and a basolateral domain contacting the underlying basal lamina. These two cell surfaces are delimited by a set of junctions providing tissue integrity and a controlled physical barrier between the outside and the inside of the body. In particular, in single layered columnar epithelia, cells are highly organized along the apico-basal axis with an apical surface that is often covered by microvilli developed to enlarge the apical surface and the exchanges with the outside medium. These microvilli in the cells are anchored in a dense actin-based network called terminal web. This cell polarity relies on the local organization of the cell cytoskeleton and the regulation of intracellular trafficking of plasma membrane components. Here we showed that Drebrin, which is an actin-binding protein, is required for the formation of the terminal web and thus for the brush border organization. In parallel, we also demonstrate a role for Drebrin in the correct distribution of endocytic and recycling vesicle during the process of cell morphogenesisAIX-MARSEILLE2-BU Sci.Luminy (130552106) / SudocSudocFranceF

Polarity complex proteins

AbstractThe formation of functional epithelial tissues involves the coordinated action of several protein complexes, which together produce a cell polarity axis and develop cell–cell junctions. During the last decade, the notion of polarity complexes emerged as the result of genetic studies in which a set of genes was discovered first in Caenorhabditis elegans and then in Drosophila melanogaster. In epithelial cells, these complexes are responsible for the development of the apico-basal axis and for the construction and maintenance of apical junctions. In this review, we focus on apical polarity complexes, namely the PAR3/PAR6/aPKC complex and the CRUMBS/PALS1/PATJ complex, which are conserved between species and along with a lateral complex, the SCRIBBLE/DLG/LGL complex, are crucial to the formation of apical junctions such as tight junctions in mammalian epithelial cells. The exact mechanisms underlying their tight junction construction and maintenance activities are poorly understood, and it is proposed to focus in this review on establishing how these apical polarity complexes might regulate epithelial cell morphogenesis and functions. In particular, we will present the latest findings on how these complexes regulate epithelial homeostasis

Developmental Upregulation of Ephrin-B1 Silences Sema3C/Neuropilin-1 Signaling during Post-crossing Navigation of Corpus Callosum Axons.

International audienceThe corpus callosum is the largest commissure in the brain, whose main function is to ensure communication between homotopic regions of the cerebral cortex. During fetal development, corpus callosum axons (CCAs) grow toward and across the brain midline and then away on the contralateral hemisphere to their targets. A particular feature of this circuit, which raises a key developmental question, is that the outgoing trajectory of post-crossing CCAs is mirror-symmetric with the incoming trajectory of pre-crossing axons. Here, we show that post-crossing CCAs switch off their response to axon guidance cues, among which the secreted Semaphorin-3C (Sema3C), that act as attractants for pre-crossing axons on their way to the midline. This change is concomitant with an upregulation of the surface protein Ephrin-B1, which acts in CCAs to inhibit Sema3C signaling via interaction with the Neuropilin-1 (Nrp1) receptor. This silencing activity is independent of Eph receptors and involves a N-glycosylation site (N-139) in the extracellular domain of Ephrin-B1. Together, our results reveal a molecular mechanism, involving interaction between the two unrelated guidance receptors Ephrin-B1 and Nrp1, that is used to control the navigation of post-crossing axons in the corpus callosum

Drebrin E depletion in human intestinal epithelial cells mimics Rab8a loss of function

International audienceIntestinal epithelial cells are highly polarized and exhibit a complex architecture with a columnar shape and a specialized apical surface supporting microvilli organized in a brush border. These microvilli are rooted in a dense meshwork of acto-myosin called the terminal web. We have shown recently that Drebrin E, an F-actin-binding protein, is a key protein for the organization of the terminal web and the brush border. Drebrin E is also required for the columnar cell shape of Caco2 cells (human colonic cells). Here, we found that the subcellular localization of several apical markers including dipeptidyl peptidase IV (DPPIV) was strikingly modified in Drebrin E-depleted Caco2 cells. Instead of being mostly present at the apical surface, these proteins are accumulated in an enlarged subapical compartment. Using known intracellular markers, we show by both confocal and electron microscopy that this compartment is related to lysosomes. We also demonstrate that the enrichment of DPPIV in this compartment originates from apical endocytosis and that depletion of Rab8a induces an accumulation of apical proteins in a similar compartment. Consistent with this, the phenotype observed in Drebrin E knock-down Caco2 cells shares some features with a pathology called microvillar inclusion disease (MVID) involving both Myosin Vb and Rab8a. Taken together, these results suggest that Drebrin E redirects the apical recycling pathway in intestinal epithelial cells to the lysosomes, demonstrating that Drebrin E is a key regulator in apical trafficking in Caco2 cells