Elsa Bazellières

Control of cell–cell forces and collective cell dynamics by the intercellular adhesome

Dynamics of epithelial tissues determine key processes in development, tissue healing and cancer invasion. These processes are critically influenced by cell–cell adhesion forces. However, the identity of the proteins that resist and transmit forces at cell–cell junctions remains unclear, and how these proteins control tissue dynamics is largely unknown. Here we provide a systematic study of the interplay between cell–cell adhesion proteins, intercellular forces and epithelial tissue dynamics. We show that collective cellular responses to selective perturbations of the intercellular adhesome conform to three mechanical phenotypes. These phenotypes are controlled by different molecular modules and characterized by distinct relationships between cellular kinematics and intercellular forces. We show that these forces and their rates can be predicted by the concentrations of cadherins and catenins. Unexpectedly, we identified different mechanical roles for P-cadherin and E-cadherin; whereas P-cadherin predicts levels of intercellular force, E-cadherin predicts the rate at which intercellular force builds up.

Rigidity sensing and adaptation through regulation of integrin types.

Tissue rigidity regulates processes in development, cancer and wound healing. However, how cells detect rigidity, and thereby modulate their behaviour, remains unknown. Here, we show that sensing and adaptation to matrix rigidity in breast myoepithelial cells is determined by the bond dynamics of different integrin types. Cell binding to fibronectin through either α5β1 integrins (constitutively expressed) or αvβ6 integrins (selectively expressed in cancer and development) adapts force generation, actin flow, and integrin recruitment to rigidities associated with healthy or malignant tissue, respectively. In vitro experiments and theoretical modelling further demonstrate that this behaviour is explained by the different binding and unbinding rates of both integrin types to fibronectin. Moreover, rigidity sensing through differences in integrin bond dynamics applies both when integrins bind separately and when they compete for binding to fibronectin.

Crumbs proteins in epithelial morphogenesis.

Cell polarity is an essential feature of most eukaryotic cells, especially epithelial cells in multicellular animals. Polarity protein complexes that regulate epithelial organization have been identified. In this review, it is proposed to describe how the Crumbs complex acts in the process of cell polarity and epithelial organization. During the last decade, several partners of Crumbs, an apical transmembrane protein, have been identified and their direct or indirect associations with the cytoplasmic domain of Crumbs have been dissected. In addition, mutants of several of the genes encoding proteins belonging to the Crumbs network have been obtained in animals ranging from flies to mouse, which have led to a better understanding of their functions in vivo. These functions include polarity axis formation, stabilization of epithelial apico-lateral junctions, photoreceptor organization and ciliogenesis. Since human CRUMBS1 mutations are associated with retina degeneration, it has become essential to define Crumbs network and to understand exactly how this network acts in polarized cells, with a view to developing suitable therapeutic approaches for treating this severe degenerative disease.

P-cadherin promotes collective cell migration via a Cdc42-mediated increase in mechanical forces

P-cadherin induces polarization and collective cell migration through an increase in the strength and anisotropy of mechanical forces, which is mediated by the P-cadherin/β-PIX/Cdc42 axis.

Apico-basal elongation requires a drebrin-E–EB3 complex in columnar human epithelial cells

Although columnar epithelial cells are known to acquire an elongated shape, the mechanisms involved in this morphological feature have not yet been completely elucidated. Using columnar human intestinal Caco2 cells, it was established here that the levels of drebrin E, an actin-binding protein, increase in the terminal web both in vitro and in vivo during the formation of the apical domain. Drebrin E depletion was found to impair cell compaction and elongation processes in the monolayer without affecting cell polarity or the formation of tight junctions. Decreasing the drebrin E levels disrupted the normal subapical F-actin–myosin-IIB–βII-spectrin network and the apical accumulation of EB3, a microtubule-plus-end-binding protein. Decreasing the EB3 levels resulted in a similar elongation phenotype to that resulting from depletion of drebrin E, without affecting cell compaction processes or the pattern of distribution of F-actin–myosin-IIB. In addition, EB3, myosin IIB and βII spectrin were found to form a drebrin-E-dependent complex. Taken together, these data suggest that this complex connects the F-actin and microtubule networks apically during epithelial cell morphogenesis, while drebrin E also contributes to stabilizing the actin-based terminal web.

Active Tensile Modulus of an Epithelial Monolayer

A general trait of cell monolayers is their ability to exert contractile stresses on their surroundings. The scaling laws that link such contractile stresses with the size and geometry of constituent cells remain largely unknown. In this Letter, we show that the active tension of an epithelial monolayer scales linearly with the size of the constituent cells, a surprisingly simple relationship. The slope of this relationship defines an active tensile modulus, which depends on the concentration of myosin and spans more than 2 orders of magnitude across cell types and molecular perturbations.

TRPV4 participates in the establishment of trailing adhesions and directional persistence of migrating cells.

Calcium signaling participates in different cellular processes leading to cell migration. TRPV4, a non-selective cation channel that responds to mechano-osmotic stimulation and heat, is also involved in cell migration. However, the mechanistic involvement of TRPV4 in cell migration is currently unknown. We now report that expression of the mutant channel TRPV4-121AAWAA (lacking the phosphoinositide-binding site 121KRWRK125 and the response to physiological stimuli) altered HEK293 cell migration. Altered migration patterns included periods of fast and persistent motion followed by periods of stalling and turning, and the extension of multiple long cellular protrusions. TRPV4-WT overexpressing cells showed almost complete loss of directionality with frequent turns, no progression, and absence of long protrusions. Traction microscopy revealed higher tractions forces in the tail of TRPV4-121AAWAA than in TRPV4-WT expressing cells. These results are consistent with a defective and augmented tail retraction in TRPV4-121AAWAA- and TRPV4-WT-expressing cells, respectively. The activity of calpain, a protease implicated in focal adhesion (FA) disassembly, was decreased in TRPV4-121AAWAA compared with TRPV4-WT-expressing cells. Consistently, larger focal adhesions were seen in TRPV4-121AAWAA compared with TRPV4-WT-expressing HEK293 cells, a result that was also reproduced in T47D and U87 cells. Similarly, overexpression of the pore-dead mutant TRPV4-M680D resumed the TRPV4-121AAWAA phenotype presenting larger FA. The migratory phenotype obtained in HEK293 cells overexpressing TRPV4-121AAWAA was mimicked by knocking-down TRPC1, a cationic channel that participates in cell migration. Together, our results point to the participation of TRPV4 in the dynamics of trailing adhesions, a function that may require the interplay of TRPV4 with other cation channels or proteins present at the FA sites.

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.

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

Intestinal 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.

Binding of ZO-1 to α5β1 integrins regulates the mechanical properties of α5β1–fibronectin links

Interaction between tight junction protein ZO-1 and integrin α5β1 reduces the resistance to force of α5β1–fibronectin bonds while simultaneously increasing their affinity. This effect is specific to subconfluent cells in which ZO-1 is displaced from its normal localization in cell–cell junctions.