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The immune synapse is the tight contact zone between a lymphocyte and a cell presenting its cognate antigen. This structure serves as a signaling platform and entails a polarization of intracellular components necessary to the immunological function of the cell. While the surface properties of the presenting cell are known to control the formation of the synapse, their impact on polarization has not yet been studied. Using functional lipid droplets as tunable artificial presenting cells combined with a microfluidic pairing device, we simultaneously observe synchronized synapses and dynamically quantify polarization patterns of individual B cells. By assessing how ligand concentration, surface fluidity, and substrate rigidity impact lysosome polarization, we show that its onset and kinetics depend on the local antigen concentration at the synapse and on substrate rigidity. Our experimental system enables a fine phenotyping of monoclonal cell populations based on their synaptic readout.

Cell mechano-sensation and adaptation are supported by the actin network. The microtubule network is not considered to be directly sensitive to mechanical forces acting on a cell. However, recent studies on isolated microtubulesin vitrohave shown that bending forces have an impact on their structure, composition and lifespan, suggesting that, in a cellular context, microtubules may react to mechanical forces. We tested this hypothesis in living cells by subjecting them to cycles of compressive forces and found that microtubules became distorted, less dynamic and more stable. This mechano-stabilisation depends on CLASP2, which relocates from the end to the deformed shaft of microtubules. These results demonstrate that microtubules in living cells have mechano-responsive properties that allow them to resist and even counteract the forces to which they are subjected.

The immune synapse is the tight contact zone between a lymphocyte and a cell presenting its cognate antigen. This structure serves as a signaling platform and entails a polarization of intra-cellular components, necessary to the immunological function of the cell. While the surface properties of the presenting cell are known to control the formation of the synapse, their impact on polarization has not yet been studied.Using functional lipid droplets as tunable artificial presenting cells combined with a microfluidic pairing device, we simultaneously observe synchronized synapses and dynamically quantify polarization patterns of individual B cells. By assessing how ligand concentration, surface fluidity and substrate rigidity impact lysosome polarization, we show that its onset and kinetics depend on the local antigen concentration at the synapse and on substrate rigidity. Our experimental system enables a fine phenotyping of monoclonal cell populations based on their synaptic readout.

Mechanics has been a central focus of physical biology in the past decade. In comparison, the osmotic and electric properties of cells are less understood. Here we show that a parameter central to both the physics and the physiology of the cell, its volume, depends on a mechano-osmotic coupling. We found that cells change their volume depending on the rate at which they change shape, when they spread, migrate or are externally deformed. Cells undergo slow deformation at constant volume, while fast deformation leads to volume loss. We propose a mechano-sensitive pump and leak model to explain this phenomenon. Our model and experiments suggest that volume modulation depends on the state of the actin cortex and the coupling of ion fluxes to membrane tension. This mechano-osmotic coupling defines a membrane tension homeostasis module constantly at work in cells, causing volume fluctuations associated with fast cell shape changes, with potential consequences on cellular physiology.

This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms/). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 International license, as described at https://creativecommons.org/licenses/by-nc-sa/4.0/).; International audience; The fate of hematopoietic stem and progenitor cells (HSPCs) is regulated by their interaction with stromal cells in the bone marrow. However, the cellular mechanisms regulating HSPC interaction with these cells and their potential impact on HSPC polarity are still poorly understood. Here we evaluated the impact of cell–cell contacts with osteoblasts or endothelial cells on the polarity of HSPC. We found that an HSPC can form a discrete contact site that leads to the extensive polarization of its cytoskeleton architecture. Notably, the centrosome was located in proximity to the contact site. The capacity of HSPCs to polarize in contact with stromal cells of the bone marrow appeared to be specific, as it was not observed in primary lymphoid or myeloid cells or in HSPCs in contact with skin fibroblasts. The receptors ICAM, VCAM, and SDF1 were identified in the polarizing contact. Only SDF1 was independently capable of inducing the polarization of the centrosome–microtubule network.

Although textbook pictures depict the cell nucleus as a simple ovoid object, it is now clear that it adopts a large variety of shapes in tissues. When cells deform, because of cell crowding or migration through dense matrices, the nucleus is subjected to large constraints that alter its shape. In this review, we discuss recent studies related to nuclear fragility, focusing on the surprising finding that the nuclear envelope can form blebs. Contrary to the better-known plasma membrane blebs, nuclear blebs are unstable and almost systematically lead to nuclear envelope opening and uncontrolled nucleocytoplasmic mixing. They expand, burst, and repair repeatedly when the nucleus is strongly deformed. Although blebs are a major source of nuclear instability, they are poorly understood so far, which calls for more in-depth studies of these structures.

The microscopic environment inside a metazoan organism is highly crowded. Whether individual cells can tailor their behavior to the limited space remains unclear. In this study, we found that cells measure the degree of spatial confinement by using their largest and stiffest organelle, the nucleus. Cell confinement below a resting nucleus size deforms the nucleus, which expands and stretches its envelope. This activates signaling to the actomyosin cortex via nuclear envelope stretch-sensitive proteins, up-regulating cell contractility. We established that the tailored contractile response constitutes a nuclear ruler–based signaling pathway involved in migratory cell behaviors. Cells rely on the nuclear ruler to modulate the motive force that enables their passage through restrictive pores in complex three-dimensional environments, a process relevant to cancer cell invasion, immune responses, and embryonic development.

© 2020 Déjardin et al. This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms/). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 International license, as described at https://creativecommons.org/licenses/by-nc-sa/4.0/)., LINC complexes are transmembrane protein assemblies that physically connect the nucleoskeleton and cytoskeleton through the nuclear envelope. Dysfunctions of LINC complexes are associated with pathologies such as cancer and muscular disorders. The mechanical roles of LINC complexes are poorly understood. To address this, we used genetically encoded FRET biosensors of molecular tension in a nesprin protein of the LINC complex of fibroblastic and epithelial cells in culture. We exposed cells to mechanical, genetic, and pharmacological perturbations, mimicking a range of physiological and pathological situations. We show that nesprin experiences tension generated by the cytoskeleton and acts as a mechanical sensor of cell packing. Moreover, nesprin discriminates between inductions of partial and complete epithelial-mesenchymal transitions. We identify the implicated mechanisms, which involve α-catenin capture at the nuclear envelope by nesprin upon its relaxation, thereby regulating β-catenin transcription. Our data thus implicate LINC complex proteins as mechanotransducers that fine-tune β-catenin signaling in a manner dependent on the epithelial-mesenchymal transition program., This material is based on work supported by the Centre national de la recherche scientifique (CNRS), Agence nationale de la recherche (ANR; grants ANR-13-JSV5-0007 and ANR-14-CE09-0006), France BioImaging (ANR-10-INBS-04), la Ligue contre le Cancer (REMX17751 to P.M. Davidson), and the Fondation ARC pour la Recherche sur le Cancer (PDF20161205227 to P.M. Davidson). P.S. Carollo has received funding from the European Union’s Horizon 2020 Framework Programme for Research and Innovation (Marie Skłodowska-Curie grant agreement 665850-INSPIRE) and acknowledges the Ecole Doctorale Frontières de l'Innovation en Recherche et Éducation (FIRE) Programme Bettencourt. E.R. Gomes was supported by a European Research Council consolidator grant (617676).

The microscopic environment inside a metazoan organism is highly crowded. Whether individual cells can tailor their behavior to the limited space remains unclear. Here, we found that cells measure the degree of spatial confinement using their largest and stiffest organelle, the nucleus. Cell confinement below a resting nucleus size deforms the nucleus, which expands and stretches its envelope. This activates signaling to the actomyosin cortexvianuclear envelope stretch-sensitive proteins, upregulating cell contractility. We established that the tailored contractile response constitutes a nuclear ruler-based signaling pathway involved in migratory cell behaviors. Cells rely on the nuclear ruler to modulate the motive force enabling their passage through restrictive pores in complex three-dimensional (3D) environments, a process relevant to cancer cell invasion, immune responses and embryonic development.One Sentence SummaryNuclear envelope expansion above a threshold triggers a contractile cell response and thus acts as a ruler for the degree of cell deformation.

LINC complexes are transmembrane protein assemblies that physically connect the nucleo- and cytoskeletons through the nuclear envelope. Dysfunctions of LINC complexes are associated with pathologies such as cancer and muscular disorders. The mechanical roles of LINC complexes in these contexts are poorly understood. To address this, we used genetically encoded FRET biosensors of molecular tension in LINC complex proteins of fibroblastic and epithelial cells in culture. We exposed cells to mechanical, genetic and pharmacological perturbations, mimicking a range of physiological and pathological situations. We show that LINC complex proteins experience tension generated by the cytoskeleton and act as mechanical sensors of cell packing. Moreover, the LINC complex discriminates between inductions of partial and complete epithelial-mesenchymal transitions (EMT). We identify the implicated mechanisms, which associate nesprin tension sensing with α-catenin capture at the nuclear envelope, thereby regulating β-catenin transcription. Our data thus implicate that LINC complexes are mechanotransducers that fine-tune β-catenin signaling in a manner dependent on the Epithelial-Mesenchymal Transition program.

International audience; Despite extensive knowledge on the mechanisms that drive singlecell migration, those governing the migration of cell clusters, as occurring during embryonic development and cancer metastasis,remain poorly understood. Here, we investigate the collective migration of cell on adhesive gels with variable rigidity, using 3D cellular aggregates as a model system. After initial adhesion to the substrate, aggregates spread by expanding outward a cell monolayer, whose dynamics is optimal in a narrowrange of rigidities. Fast expansion gives rise to the accumulation of mechanical tension that leads to the rupture of cell–cell contacts and the nucleation of holes within the monolayer, which becomes unstable and undergoes dewetting like a liquid film. This leads to a symmetry breaking and causes the entire aggregate to move as a single entity. Varying the substrate rigidity modulates the extent of dewetting and induces different modes of aggregate motion: “giant keratocytes,” where the lamellipodium is a cell monolayer that expands at the front and retracts at the back; “penguins,” characterized by bipedal locomotion; and “running spheroids,” for nonspreading aggregates. We characterize these diversemodes of collectivemigration by quantifying the flows and forces that drive them, andwe unveil the fundamental physical principles that govern these behaviors, which underscore the biological predisposition of living material to migrate, independent of length scale.

Maintaining cell cohesiveness within tissues requires that intercellular adhesions develop sufficient strength to support traction forces applied by myosin motors and by neighboring cells. Cadherins are transmembrane receptors that mediate intercellular adhesion. The cadherin cytoplasmic domain recruits several partners, including catenins and vinculin, at sites of cell-cell adhesion. Our study used force measurements to address the role of αE-catenin and vinculin in the regulation of the strength of E-cadherin-based adhesion. αE-catenin-deficient cells display only weak aggregation and fail to strengthen intercellular adhesion over time, a process rescued by the expression of αE-catenin or chimeric E-cadherin·αE-catenins, including a chimera lacking the αE-catenin dimerization domain. Interestingly, an αE-catenin mutant lacking the modulation and actin-binding domains restores cadherin-dependent cell-cell contacts but cannot strengthen intercellular adhesion. The expression of αE-catenin mutated in its vinculin-binding site is defective in its ability to rescue cadherin-based adhesion strength in cells lacking αE-catenin. Vinculin depletion or the overexpression of the αE-catenin modulation domain strongly decreases E-cadherin-mediated adhesion strength. This supports the notion that both molecules are required for intercellular contact maturation. Furthermore, stretching of cell doublets increases vinculin recruitment and α18 anti-αE-catenin conformational epitope immunostaining at cell-cell contacts. Taken together, our results indicate that αE-catenin and vinculin cooperatively support intercellular adhesion strengthening, probably via a mechanoresponsive link between the E-cadherin·β-catenin complexes and the underlying actin cytoskeleton.

The response of cells to forces is essential for tissue morphogenesis and homeostasis. This response has been extensively investigated in interphase cells, but it remains unclear how forces affect dividing cells. We used a combination of micro-manipulation tools on human dividing cells to address the role of physical parameters of the micro-environment in controlling the cell division axis, a key element of tissue morphogenesis. We found that forces applied on the cell body direct spindle orientation during mitosis. We further show that external constraints induce a polarization of dynamic subcortical actin structures that correlate with spindle movements. We propose that cells divide according to cues provided by their mechanical micro-environment, aligning daughter cells with the external force field.

Micropatterned poly(dimethylsiloxane) substrates fabricated by soft lithography led to large-scale orientation of myoblasts in culture, thereby controlling the orientation of the myotubes they formed. Fusion occurred on many chemically identical surfaces in which varying structures were arranged in square or hexagonal lattices, but only a subset of patterned surfaces yielded aligned myotubes. Remarkably, on some substrates, large populations of myotubes oriented at a reproducible acute angle to the lattice of patterned features. A simple geometrical model predicts the angle and extent of orientation based on maximizing the contact area between the myoblasts and patterned topographic surfaces. Micropatterned substrates also provided short-range cues that influenced higher-order functions such as the localization of focal adhesions and accumulation of postsynaptic acetylcholine receptors. Our results represent what we believe is a new approach for musculoskeletal tissue engineering, and our model sheds light on mechanisms of myotube alignment in vivo.

Cells actively produce contractile forces for a variety of processes including cytokinesis and motility. Contractility is known to rely on myosin II motors which convert chemical energy from ATP hydrolysis into forces on actin filaments. However, the basic physical principles of cell contractility remain poorly understood. We reconstitute contractility in a simplified model system of purified F-actin, muscle myosin II motors, and alpha-actinin cross-linkers. We show that contractility occurs above a threshold motor concentration and within a window of cross-linker concentrations. We also quantify the pore size of the bundled networks and find contractility to occur at a critical distance between the bundles. We propose a simple mechanism of contraction based on myosin filaments pulling neighboring bundles together into an aggregated structure. Observations of this reconstituted system in both bulk and low-dimensional geometries show that the contracting gels pull on and deform their surface with a contractile force of approximately 1 microN, or approximately 100 pN per F-actin bundle. Cytoplasmic extracts contracting in identical environments show a similar behavior and dependence on myosin as the reconstituted system. Our results suggest that cellular contractility can be sensitively regulated by tuning the (local) activity of molecular motors and the cross-linker density and binding affinity.

We discuss the pulling force f required to extrude a lipid tube from a living cell as a function of the extrusion velocity L̇ . The main feature is membrane friction on the cytoskeleton. As recently observed for neutrophils, the tether force exhibits a “shear thinning” response over a large range of pulling velocities, which was previously interpreted by assuming viscoelastic flows of the sliding membrane. Here, we propose an alternative explanation based on purely Newtonian flow: The diameter of the tether decreases concomitantly with the increase of the membrane tension in the lipid tube. The pulling force is found to vary as L̇ 1 3 , which is consistent with reported experimental data for various types of cells.

We used optical tweezers to measure the force-extension curve for the elongation of nanotubes from adhered giant vesicles. We show that the force increases significantly with the length of the tube, which is drastically different from what is observed when the membrane tension is kept constant, e.g. by pipette aspiration. The absence of any force plateau is quantitatively analysed in the framework of the material model of membranes. In particular, we rationalize a counter-intuitive weaker force rise for long tubes and demonstrate that the measured force-length trace allows us to probe both the entropic regime (characterised by the bending rigidity) and the elastic regime (characterised by the area expansion modulus) of the lipid membrane.

Tethers are nanocylinders of lipid bilayer membrane, arising in situations ranging from micromanipulation experiments on synthetic vesicles to the formation of dynamic tubular networks in the Golgi apparatus. Relying on the extensive theoretical and experimental works aimed to understand the physics of individual tethers formation, we addressed the problem of the interaction between two nanotubes. By using a combination of micropipette manipulation and optical tweezers, we quantitatively studied the process of coalescence that occurred when the separation distance between both vesicle-tether junctions became smaller than a threshold length. Our experiments, which were supported by an original theoretical analysis, demonstrated that the measurements of the tether force and angle between tethers at coalescence directly yield the bending rigidity, kappa, and the membrane tension, sigma, of the vesicles. Contrary to other methods used to probe the bending rigidity of vesicles, the proposed approach permits a direct measurement of kappa without requiring any control of the membrane tension. Finally, after validation of the method and proposal of possible applications, we experimentally investigated the dynamics of the coalescence process.

We have recently developed a minimal system for generating long tubular nanostructures that resemble tubes observed in vivo with biological membranes. Here, we studied membrane tube pulling in ternary mixtures of sphingomyelin, phosphatidylcholine and cholesterol. Two salient results emerged: the lipid composition is significantly different in the tubes and in the vesicles; tube fission is observed when phase separation is generated in the tubes. This shows that lipid sorting may depend critically on both membrane curvature and phase separation. Phase separation also appears to be important for membrane fission in tubes pulled out of giant liposomes or purified Golgi membranes.

The interest in physical approaches to the study of cell adhesion has generated numerous recent works on the development of substrates mimicking the extracellular matrix and the use of giant synthetic liposomes, commonly considered as basic models of living cells. The use of well-characterized bioactive substrates and artificial cells should allow us to gain new insight into the cell–extracellular matrix interactions, provided that their biomimetic relevance has been really proved. The aim of this paper is to define some minimal requirements for effective biomimetic features and to propose simple adhesion assays. We show, for instance, that immobilization of specific ligands is sometimes not sufficient t oe nsure specific adhesion of cells expressing the corresponding receptors. B yi nvestigating comparatively the adhesive behaviour of decorated erythrocytes and vesicles, we also discuss the potentialities and limitations of synthetic vesicles as test cells.

LINC complexes are transmembrane protein assemblies that physically connect the nucleo- and cytoskeletons through the nuclear envelope. Dysfunctions of LINC complexes are associated with pathologies such as cancer and muscular dystrophies. The mechanical roles of LINC complex proteins in these contexts are poorly understood. To address this issue, we have developed genetically encoded FRET biosensors that measure molecular tension in LINC complex proteins.We have evidenced that LINC complex proteins are under tension generated by the cytoskeleton and sensitive to various external and internal mechanical constraints in fibroblastic and epithelial cells. In addition, we have shown that some of the same constraints result in the localization at the nuclear envelope of alpha-catenin, an actin- binding protein required for intercellular adhesion that can also act as a transcription regulator. These results are consistent with a mechanotransduction activity of LINC complex proteins, the targets of which we currently investigate.

Like liquid droplets, cellular aggregates, also called “living droplets,” spread onto adhesive surfaces. When deposited onto fibronectin-coated glass or polyacrylamide gels, they adhere and spread by protruding a cellular monolayer (precursor film) that expands around the droplet. The dynamics of spreading results from a balance between the pulling forces exerted by the highly motile cells at the periphery of the film, and friction forces associated with two types of cellular flows: (i) permeation, corresponding to the entry of the cells from the aggregates into the film; and (ii) slippage as the film expands. We characterize these flow fields within a spreading aggregate by using fluorescent tracking of individual cells and particle imaging velocimetry of cell populations. We find that permeation is limited to a narrow ring of width ξ (approximately a few cells) at the edge of the aggregate and regulates the dynamics of spreading. Furthermore, we find that the subsequent spreading of the monolayer depends heavily on the substrate rigidity. On rigid substrates, the migration of the cells in the monolayer is similar to the flow of a viscous liquid. By contrast, as the substrate gets softer, the film under tension becomes unstable with nucleation and growth of holes, flows are irregular, and cohesion decreases. Our results demonstrate that the mechanical properties of the environment influence the balance of forces that modulate collective cell migration, and therefore have important implications for the spreading behavior of tissues in both early development and cancer.

Pathogenic bacteria can cross from blood vessels to host tissues by opening transendothelial cell macroapertures (TEMs). To induce TEM opening, bacteria intoxicate endothelial cells with proteins that disrupt the contractile cytoskeletal network. Cell membrane tension is no longer resisted by contractile fibers, leading to the opening of TEMs. Here we model the opening of TEMs as a new form of dewetting. While liquid dewetting is irreversible, we show that cellular dewetting is transient. Our model predicts the minimum radius for hole nucleation, the maximum TEM size, and the dynamics of TEM opening, in good agreement with experimental data. The physical model is then coupled with biological experimental data to reveal that the protein missing in metastasis (MIM) controls the line tension at the rim of the TEM and opposes its opening.

International audience; Induced spontaneous curvature is a new mechanism of microcapsules bursting by nucleation and growth of a hole surrounded by a curling rim. Here we study the dynamics of curling on a macroscopic scale induced by the elastic curvature of a bilayer of tracing paper and tape after soaking by water. The ribbon which is fully stretched at time t = 0 is immersed in a viscous oil. We observe the winding from the free end and find two regimes: rolling at constant velocity at short times, and a slowdown at long times. We interpret these two regimes by a balance of the elastic driving force and the friction force on the roll.

We describe a simple and robust method for high-throughput surface patterning of deformable substrates such as silicone rubber films covered with a thin layer of protein and cell antifouling hydrogel (PLL-g-PEG). The irradiation with deep UV (

We study the spreading of spheroidal aggregates of cells, expressing a tunable level of E-cadherin molecules, on glass substrates decorated with mixed fibronectin and polyethylene glycol. We observe the contact area by optical interferometry and the profile by side-view microscopy. We find a universal law of aggregate spreading at short times, which we interpret through an analogy with the spreading of viscoelastic droplets. At long times, we observe either partial wetting or complete wetting, with a precursor film of cells spreading around the aggregate with two possible states. In strongly cohesive aggregates this film is a cellular monolayer in the liquid state, whereas in weakly cohesive aggregates, cells escape from the aggregate, forming a 2D gas. The escape of isolated cells is a physical mechanism that appears also to be present in the progression of a noninvasive tumor into a metastatic malignant carcinoma, known as the epithelial-mesenchymal transition.

International audience; Reconstituted systems mimicking cells are interesting tools for understanding the details of cell behavior. Here, we use an experimental system that mimics cellular actin cortices, namely liposomes developing an actin shell close to their inner membrane, and we study their dynamics of spreading. We show that depending on the morphology of the actin shell inside the liposome, spreading dynamics is either reminiscent of a bare liposome (in the case of a sparse actin shell) or of a cell (in the case of a continuous actin shell). We use a mechanical model that qualitatively accounts for the shape of the experimental curves. From the data on spreading dynamics, we extract characteristic times that are consistent with mechanical estimates. The mechanical characterization of such stripped-down experimental systems paves the way for a more complex design closer to a cell. We report here the first step in building an artificial cell and studying its mechanics.

International audience; Upon light exposure, photosensitizers generate reactive chemical species that lead to cellular membrane alteration. On the one hand, this property is exploited in photodynamic therapy to irreversibly destroy diseased tissues. On the other hand, the reactivity of photosensitizers with lipid membranes prevents encapsulation or loading in liposomes for delivery applications. Polymersomes, which are vesicles made of amphiphilic polymers, have been used as drug carriers due to their superior robustness over liposomes. We have investigated the photo-response of prototypical polymersomes loaded with a classical chlorine photosensitizer. We have observed a complex sequence of light-induced morphological changes. Using micromechanical assays based on micropipette manipulation, we have quantitatively monitored the different phases of the photo-response, which include membrane area variation, osmotic swelling, membrane cross-linking and vesicle deflation. We have thus gained insight into the complex cascade of chemical reactions involved in photosensitization. Finally, our findings suggest that composite chlorine-copolymer vesicles may be used as a new class of light-sensitive drug carriers.

International audience; Polymersomes, which are stable and robust vesicles made of block copolymer amphiphiles, are good candidates for drug carriers or micro/nanoreactors. Polymer chemistry enables almost unlimited molecular design of responsive polymersomes whose degradation upon environmental changes has been used for the slow release of active species. Here, we propose a strategy to remotely trigger instantaneous polymersome bursting. We have designed asymmetric polymer vesicles, in which only one leaflet is composed of responsive polymers. In particular, this approach has been successfully achieved by using a UV-sensitive liquid-crystalline copolymer. We study experimentally and theoretically this bursting mechanism and show that it results from a spontaneous curvature of the membrane induced by the remote stimulus. The versatility of this mechanism should broaden the range of applications of polymersomes in fields such as drug delivery, cosmetics and material chemistry.

Considerable effort over the past three decades has allowed the identification of the protein families that control the cellular machinery responsible for intracellular transport within eukaryotic cells. These proteins are estimated to represent about 10-20% of the human "proteome." The complexity of intracellular transport makes useful the development of model membranes. We describe here experimental systems based on lipid giant unilamellar vesicles (GUVs), which are attached to kinesin molecules. These systems give rise to thin membrane tubes and to complex tubular networks when incubated in vitro with microtubules and ATP. This type of assay, which mimics key events occurring during intracellular transport, allows physicists and biologists to understand how the unique mechanical properties of lipid membranes could be involved in the budding process, the sorting of cargo proteins and lipids, and the separation of the buds from a donor membrane.

SummaryCell adhesion and motility depend strongly on the interactions between cells and extracellular matrix (ECM) substrates. When plated onto artificial adhesive surfaces, cells first flatten and deform extensively as they spread. At the molecular level, the interaction of membrane-based integrins with the ECM has been shown to initiate a complex cascade of signaling events [1], which subsequently triggers cellular morphological changes and results in the generation of contractile forces [2]. Here, we focus on the early stages of cell spreading and probe their dynamics by quantitative visualization and biochemical manipulation with a variety of cell types and adhesive surfaces, adhesion receptors, and cytoskeleton-altering drugs. We find that the dynamics of adhesion follows a universal power-law behavior. This is in sharp contrast with the common belief that spreading is regulated by either the diffusion of adhesion receptors toward the growing adhesive patch [3–5] or by actin polymerization [6–8]. To explain this, we propose a simple quantitative and predictive theory that models cells as viscous adhesive cortical shells enclosing a less viscous interior. Thus, although cell spreading is driven by well-identified biomolecular interactions, it is dynamically limited by its mesoscopic structure and material properties.

We experimentally studied the adhesion dynamics of vesicles decorated with specific stickers onto bioactive surfaces. The growth laws were found to strongly depend upon the chemical preparation of the substrate and were rationalized with theoretical descriptions based on scaling law arguments. By using a micropipette-free approach, we demonstrate that two-dimensional binding rates between receptor and ligand can be lower than three dimensional on rates by orders of magnitude due to reduced accessibility of the immobilized protein.

The formation of membrane tubes (or tethers), which is a crucial event in many biological processes, is intrinsically a dynamic process. In this paper, we discuss both theoretically and experimentally the dynamical laws that govern extrusion and retraction of tubes extracted from lipid vesicles at high speed and under strong flows. A detailed description of the tether shape provides the first evidence that the tension along the tube increases from the vesicle body to the tip of the tube, while the tube radius decreases. As the pulling force is suppressed suddenly, the tube can relax only from the free end, and the velocity of retraction is a direct measurement of the frozen tension along the tube. We also report experiments on tethers pulled out either by mechanical point-forces or by hydrodynamic (electroosmosis-induced) flow, and we show that the observed dynamical laws for retraction are in good quantitative agreement with our theoretical predictions.

International audience; The dynamics of cellular adhesion and deadhesion, which play key roles in many cellular processes, have most often been studied at the scale of single bonds or single cells. However, multicellular adhesion and deadhesion are also central processes in tissue mechanics, morphogenesis, and pathophysiology, where collective tissue phenomena may introduce additional effects that are absent at the single-cell level. In this paper we present experiments on the adhesion of cellular aggregates and a laboratory model system to study tissue mechanics. We introduce a technique to measure the forces and energies involved in the detachment of an aggregate from a substrate (which can be viewed as a cellular tack assay) and in the fracture between two partially fused aggregates, as a function of the adhesion time, the pulling speed, and the cadherin density at the cell surface. We develop a model based on polymer physics to interpret the observations. We identify a significant contribution to the adhesion energy of viscous dissipation mechanisms present at the tissue scale that are absent at the single-cell level, as well as a significant effect of the speed at which the separation force is applied.

We study the spontaneous curving observed when a strip of tracing paper is deposited on a water surface. The differential swelling across the ribbon thickness leads to a curling of the sheet, with the curvature increasing to a maximum and then relaxing as the sheet becomes completely wet. We develop a theoretical model, in good agreement with our experimental observations and relevant to the understanding of curling instabilities induced by asymmetric swelling of natural or synthetic shells. © 2011 The Royal Society of Chemistry.

International audience; Polymersomes are vesicles made of amphiphilic diblock copolymers. Giant polymersomes of several tens of microns in diameter can be prepared from low T(g) (glass transition temperature) flexible (coil-coil) copolymers by processes such as rehydration swelling or electroformation. These techniques are, however, inefficient in producing giant polymersomes composed of high T(g) and/or rigid-flexible (rod-coil) copolymers. We have used an alternative method based on the formation of an inverted emulsion to produce giant unilamellar rod-coil polymersomes. We have selected copolymers whose hydrophobic moieties are glassy liquid crystalline polymers. The viscoelasticity of individual polymersomes has been measured by micropipette aspiration. Whereas the elastic modulus was found to be of the same order of magnitude as the one of prototypical vesicles made of coil-coil copolymers, the membrane viscosity of this new class of polymersomes was about three orders of magnitude more viscous than their coil-coil counterparts. The versatile method used here to form giant polymersomes could be useful for designing and studying novel functional polymer capsules. The results highlight the possibility of widely tuning the mechanical properties of polymersomes by selecting or synthesizing the proper copolymer.