Peeling model for cell detachment

Trypsin-Free Cultivation of 3D Mini-Tissues in an Adaptive Membrane Bioreactor

The production of large scaffold-free tissues is a key challenge in regenerative medicine. Nowadays, temperature-responsive polymers allow intact tissue harvesting without needing proteolytic enzymes. This method is limited to tissue culture plastic with limited upscaling capacity and plain process control. Here, a thermoresponsive hollow fiber membrane bioreactor is presented to produce large scaffold-free tissues. Intact tissues, rich in cell-to-cell connections and ECM, are harvested from a poly(N-vinylcaprolactam) microgel functionalized poly(ether sulfone)/poly(vinylpyrrolidone) hollow fiber membrane by a temperature shift. The harvested 3D tissues adhere in successive cultivation and exhibit high vitality for several days. The facile adsorptive coating waives the need for extensive surface treatment. The research is anticipated to be a starting point for upscaling the production of interconnected tissues enabling new opportunities in regenerative medicine, large-scale drug screening on physiological relevant tissues, and potentially opening new chances in cell-based therapies.

Effect of the mechanical properties of carbon-based coatings on the mechanics of cell-material interactions

The paper presents an influence of the surface mechanical properties of thin-film materials on blood cell adhesion under shear stress conditions. Physical vapour deposited (PVD) coatings i.e. hydrogenated amorphous carbon (a-C:H) doped with nitrogen or silicon have been investigated. The mechanical properties of materials, namely their microhardness and Young's modulus were measured using indentation test with Rockwell indenter. The adhesion efficiency of blood cells in dynamic conditions were analysed using a radial flow chamber. Red blood cells (RBC) were used as representative cells to analyse cell-material interactions. The biomaterial examinations were performed under physiological flow conditions at the single-cell level. The 3D FVM (finite volume method) model of multi-phase radial flow test was developed to reproduce the physical test and to predict distributions of shear stresses and velocity during blood washout with PBS. Cell-material interactions were found to be strongly associated with the mechanical properties of the thin-film material. The decrease in the hardness of the coatings translated into a weaker cell - material interactions.

Correlation of in vitro cell adhesion, local shear flow and cell density

By combination of particle image velocimetry and live cell imaging in an acoustically driven microfluidic chamber, we study shear and cell density dependent adhesion. We find excellent agreement with simulations considering pure geometrical effects.

Coordination between Intra- and Extracellular Forces Regulates Focal Adhesion Dynamics

Focal adhesions (FAs) are important mediators of cell-substrate interactions. One of their key functions is the transmission of forces between the intracellular acto-myosin network and the substrate. However, the relationships between cell traction forces, FA architecture, and molecular forces within FAs are poorly understood. Here, by combining Förster resonance energy transfer (FRET)-based molecular force biosensors with micropillar-based traction force sensors and high-resolution fluorescence microscopy, we simultaneously map molecular tension across vinculin, a key protein in FAs, and traction forces at FAs. Our results reveal strong spatiotemporal correlations between vinculin tension and cell traction forces at FAs throughout a wide range of substrate stiffnesses. Furthermore, we find that molecular tension within individual FAs follows a biphasic distribution from the proximal (toward the cell nucleus) to distal end (toward the cell edge). Using super-resolution imaging, we show that such a distribution relates to that of FA proteins. On the basis of our experimental data, we propose a model in which FA dynamics results from tension changes along the FAs.

Bleb Nucleation through Membrane Peeling

We study the nucleation of blebs, i.e., protrusions arising from a local detachment of the membrane from the cortex of a cell. Based on a simple model of elastic linkers with force-dependent kinetics, we show that bleb nucleation is governed by membrane peeling. By this mechanism, the growth or shrinkage of a detached membrane patch is completely determined by the linker kinetics, regardless of the energetic cost of the detachment. We predict the critical nucleation radius for membrane peeling and the corresponding effective energy barrier. These may be typically smaller than those predicted by classical nucleation theory, implying a much faster nucleation. We also perform simulations of a continuum stochastic model of membrane-cortex adhesion to obtain the statistics of bleb nucleation times as a function of the stress on the membrane. The determinant role of membrane peeling changes our understanding of bleb nucleation and opens new directions in the study of blebs.

Population ecology of heterotypic tumour cell cultures

OBJECTIVES

Here, we propose a population ecology perspective to describe dynamic interplay between human leukaemia and cervical cancer cells growing together in the same environment.

MATERIALS AND METHODS

MOLT-3 (human T-lymphoblastic leukaemia) and HeLa (human cervical adenocarcinoma) cells were grown together or alone. Living cells were measured using flow cytometry, by counting propidium iodide-negative cells either CD5(+) (MOLT-3) or CD55(+) (HeLa). We developed a mathematical model to take into account possible interactions between cells and among cells and their environmental niches. Model equations were then fitted to growth data.

RESULTS

Ecological interactions that require direct cell contact and indirect mechanisms acting on cell niches, successfully modelled cell population growth. Predicted heterotypic adhesion between the two different cell types was demonstrated experimentally.

CONCLUSIONS

Theoretical ecology can be assayed using human cells and, most importantly, it can provide a conceptual framework to describe and understand evolution of mixed tumour cell populations.

Secondary bjerknes forces deform targeted microbubbles

In this study, we investigated the effect of secondary Bjerknes forces on targeted microbubbles using high-speed optical imaging. We observed that targeted microbubbles attached to an underlying surface and subject to secondary Bjerknes forces deform in the direction of their neighboring bubble, thereby tending toward a prolate shape. The deformation induces an elastic restoring force, causing the bubbles to recoil back to their equilibrium position; typically within 100 μs after low-intensity ultrasound application. The temporal dynamics of the recoil was modeled as a simple mass-spring system, from which a value for the effective spring constant k of the order 10(-3) Nm(-1) was obtained. Moreover, the translational dynamics of interacting targeted microbubbles was predicted by a hydrodynamic point particle model, including a value of the spring stiffness k of the very same order as derived experimentally from the recoiling curves. For higher acoustic pressures, secondary Bjerknes forces rupture the molecular adhesion of the bubbles to the surface. We used this mutual attraction to quantify the binding force between a single biotinylated microbubble and an avidin-coated surface, which was found to be between 0.9 and 2 nanonewtons (nN). The observation of patches of lipids left at the initial binding site suggests that lipid anchors are pulled out of the microbubble shell, rather than biotin molecules unbinding from avidin. Understanding the effect of ultrasound application on targeted microbubbles is crucial for further advances in the realm of molecular imaging.

Shear stress-dependent cell detachment from temperature-responsive cell culture surfaces in a microfluidic device

A new approach to quantitatively estimate the interaction between cells and material has been proposed by using a microfluidic system, which was made of poly(dimethylsiloxane) (PDMS) chip bonding on a temperature-responsive cell culture surface consisted of poly(N-isopropylacrylamide) (PIPAAm) grafted tissue culture polystyrene (TCPS) (PIPAAm-TCPS) having five parallel test channels for cell culture. This construction allows concurrently generating five different shear forces to apply to cells in individual microchannels having various resistance of each channel and simultaneously gives an identical cell incubation condition to all test channels. NIH/3T3 mouse fibroblast cells (MFCs) and bovine aortic endothelial cells (BAECs) were well adhered and spread on all channels of PIPAAm-TCPS at 37 °C. In our previous study, reducing culture temperature below the lower critical solution temperature (LCST) of PIPAAm (32 °C), cells detach themselves from hydrated PIPAAm grafted surfaces spontaneously. In this study, cell detachment process from hydrated PIPAAm-TCPS was promoted by shear forces applied to cells in microchannels. Shear stress-dependent cell detachment process from PIPAAm-TCPS was evaluated at various shear stresses. Either MFCs or BAECs in the microchannel with the strongest shear stress were found to be detached from the substrate more quickly than those in other microchannels. A cell transformation rate constant C(t) and an intrinsic cell detachment rate constant k(0) were obtained through studying the effect of shear stress on cell detachment with a peeling model. The proposed device and quantitative analysis could be used to assess the possible interaction between cells and PIPAAm layer with a potential application to design a cell sheet culture surface for tissue engineering.

A tapered channel microfluidic device for comprehensive cell adhesion analysis, using measurements of detachment kinetics and shear stress-dependent motion

We have developed a method for studying cellular adhesion by using a custom-designed microfluidic device with parallel non-connected tapered channels. The design enables investigation of cellular responses to a large range of shear stress (ratio of 25) with a single input flow-rate. For each shear stress, a large number of cells are analyzed (500-1500 cells), providing statistically relevant data within a single experiment. Besides adhesion strength measurements, the microsystem presented in this paper enables in-depth analysis of cell detachment kinetics by real-time videomicroscopy. It offers the possibility to analyze adhesion-associated processes, such as migration or cell shape change, within the same experiment. To show the versatility of our device, we examined quantitatively cell adhesion by analyzing kinetics, adhesive strength and migration behaviour or cell shape modifications of the unicellular model cell organism Dictyostelium discoideum at 21 °C and of the human breast cancer cell line MDA-MB-231 at 37 °C. For both cell types, we found that the threshold stresses, which are necessary to detach the cells, follow lognormal distributions, and that the detachment process follows first order kinetics. In addition, for particular conditions' cells are found to exhibit similar adhesion threshold stresses, but very different detachment kinetics, revealing the importance of dynamics analysis to fully describe cell adhesion. With its rapid implementation and potential for parallel sample processing, such microsystem offers a highly controllable platform for exploring cell adhesion characteristics in a large set of environmental conditions and cell types, and could have wide applications across cell biology, tissue engineering, and cell screening.

Synchronization of Dictyostelium discoideum adhesion and spreading using electrostatic forces

Synchronization of cell spreading is valuable for the study of molecular events involved in the formation of adhesive contacts with the substrate. At a low ionic concentration (0.17 mM) Dictyostelium discoideum cells levitate over negatively charged surfaces due to electrostatic repulsion. First, a two-chamber device, divided by a porous membrane, allows to quickly increase the ionic concentration around the levitating cells. In this way, a good synchronization was obtained, the onsets of cell spreading being separated by less than 5 s. Secondly applying a high potential pulse (2.5 V/Ref, 0.1s) to an Indium Tin Oxide surface attracts the cells toward the surface where they synchronously spread as monitored by LimE(Deltacoil)-GFP. During spreading, actin polymerizes in series of active spots. On average, the first spot appears 8-11s after the electric pulse and the next ones appear regularly, separated by about 10s. Synchronized actin-polymerization activity continues for 40s. Using an electric pulse to control the exact time point at which cells contact the surface has allowed for the first time to quantify the cellular response time for actin polymerization. Electrochemical synchronization is therefore a valuable tool to study intracellular responses to contact.

Physical model for membrane protrusions during spreading

During cell spreading onto a substrate, the kinetics of the contact area is an observable quantity. This paper is concerned with a physical approach to modeling this process in the case of ameboid motility where the membrane detaches itself from the underlying cytoskeleton at the leading edge. The physical model we propose is based on previous reports which highlight that membrane tension regulates cell spreading. Using a phenomenological feedback loop to mimic stress-dependent biochemistry, we show that the actin polymerization rate can be coupled to the stress which builds up at the margin of the contact area between the cell and the substrate. In the limit of small variation of membrane tension, we show that the actin polymerization rate can be written in a closed form. Our analysis defines characteristic lengths which depend on elastic properties of the membrane-cytoskeleton complex, such as the membrane-cytoskeleton interaction, and on molecular parameters, the rate of actin polymerization. We discuss our model in the case of axi-symmetric and non-axi-symmetric spreading and we compute the characteristic time scales as a function of fundamental elastic constants such as the strength of membrane-cytoskeleton adherence.

Effect of interfacial slippage in peel test: theoretical model

Peel test is an efficient method to assess the performance and characteristics of materials such as adhesives and adhesive tapes. Recent experiments evidenced that the measured adhesive strength is closely related to the shear-induced interfacial slippage near the delamination front due to the concomitant Poisson contraction effect of the adhesive. Based on the experimental observations, a theoretical model is presented in this paper to examine the effect of the shear-induced interfacial slippage in the peel test. The influence of the interfacial slippage, represented by the shear displacement in the cohesive zone, on the fracture energy of decohesive zone is analyzed. An implicit expansion method with a Gauss-Chebyshev quadrature scheme is used to derive the solution. It is found that the length of the slippage zone and the receding contact angle of adhesives are the two most significant contributors to the total fracture energy of the decohesive zone. These results demonstrate that the mechanism of interfacial slippage plays a significant role in the adhesion and peeling behaviors of adhesives.

Signaling through the phosphatidylinositol 3-kinase regulates mechanotaxis induced by local low magnetic forces in Entamoeba histolytica

In micro-organisms, as well as in metazoan cells, cellular polarization and directed migration are finely regulated by external stimuli, including mechanical stresses. The mechanisms sustaining the transduction of such external stresses into intracellular biochemical signals remain mainly unknown. Using an external magnetic tip, we generated a magnetic field gradient that allows migration analysis of cells submitted to local low-intensity magnetic forces (50 pN). We applied our system to the amoeba Entamoeba histolytica. Indeed, motility and chemotaxis are key activities that allow this parasite to invade and destroy the human tissues during amoebiasis. The magnetic force was applied either inside the cytoplasm or externally at the rear pole of the amoeba. We observed that the application of an intracellular force did not affect cell polarization and migration, whereas the application of the force at the rear pole of the cell induced a persistent polarization and strongly directional motion, almost directly opposed to the magnetic force. This phenomenon was completely abolished when phosphatidylinositol 3-kinase activity was inhibited by wortmanin. This result demonstrated that the applied mechanical stimulus was transduced and amplified into an intracellular biochemical signal, a process that allows such low-intensity force to strongly modify the migration behavior of the cell.

Sonoporation from jetting cavitation bubbles

The fluid dynamic interaction of cavitation bubbles with adherent cells on a substrate is experimentally investigated. We find that the nonspherical collapse of bubbles near to the boundary is responsible for cell detachment. High-speed photography reveals that a wall bounded flow leads to the detachment of cells. Cells at the edge of the circular area of detachment are found to be permanently porated, whereas cells at some distance from the detachment area undergo viable cell membrane poration (sonoporation). The wall flow field leading to cell detachment is modeled with a self-similar solution for a wall jet, together with a kinetic ansatz of adhesive bond rupture. The self-similar solution for the delta-type wall jet compares very well with the full solution of the Navier-Stokes equation for a jet of finite thickness. Apart from annular sites of sonoporation we also find more homogenous patterns of molecule delivery with no cell detachment.

The two-pathway model for the catch-slip transition in biological adhesion

Some recently studied biological noncovalent bonds have shown increased lifetime when stretched by mechanical force. In each case these counterintuitive "catch-bonds" have transitioned into ordinary "slip-bonds" that become increasingly shorter lived as the tensile force on the bond is further increased. We describe analytically how these results are supported by a physical model whereby the ligand escapes the receptor binding site via two alternative routes, a catch-pathway that is opposed by the applied force and a slip-pathway that is promoted by force. The model predicts under what conditions and at what critical force the catch-to-slip transition would be observed, as well as the degree to which the bond lifetime is enhanced at the critical force. The model is applied to four experimentally studied systems taken from the literature, involving the binding of P- and L-selectins to sialyl Lewis(X) oligosaccharide-containing ligands. Good quantitative fit to the experimental data is obtained, both for experiments with a constant force and for experiments where the force increases linearly with time.

Enforced detachment of red blood cells adhering to surfaces: statics and dynamics

We investigated the mechanical strength of adhesion and the dynamics of unbinding of red blood cells to solid surfaces. Two different situations were tested: 1), native red blood cells nonspecifically adhered to glass surfaces coated with positively charged polymers and 2), biotinylated red blood cells specifically adhered to glass surfaces decorated with streptavidin, which has a high binding affinity for biotin. We used micropipette manipulation for forming and subsequently breaking the adhesive contact through a stepwise micromechanical procedure. Analysis of cell deformations provided the relation between force and contact radius, which was found to be in good agreement with theoretical predictions. We further demonstrated that the separation energy could be precisely derived from the measure of rupture forces and the cell shape. Finally, the dynamics of detachment was analyzed as a function of the applied force and the initial size of the adhesive patch. Our experiments were supported by original theoretical predictions, which allowed us to correlate the measured separation times with the molecular parameters (e.g., activation barrier, receptor-ligand characteristic length) derived from force measurements at the single bond level.

Shear flow-induced motility of Dictyostelium discoideum cells on solid substrate

Application of a mild hydrodynamic shear stress to Dicytostelium discoideum cells, unable to detach cells passively from the substrate, triggers a cellular response consisting of steady membrane peeling at the rear edge of the cell and periodic cell contact extensions at its front edge. Both processes require an active actin cytoskeleton. The cell movement induced by the hydrodynamic forces is very similar to amoeboid cell motion during chemotaxis, as for its kinematic parameters and for the involvement of phosphatidylinositol(3,4,5)-trisphosphate internal gradient to maintain cell polarity. Inhibition of phosphoinositide 3-kinases by LY294002 randomizes the orientation of cell movement with respect to the flow without modifying cell speed. Two independent signaling pathways are, therefore, induced in D. discoideum in response to external forces. The first increases the frequency of pseudopodium extension, whereas the second redirects the actin cytoskeleton polymerization machinery to the edge opposite to the stressed side of the cell.

Dictyostelium discoideum adhesion and motility under shear flow: experimental and theoretical approaches

Among the different assays to measure cell adhesion, shear-flow detachment chambers offer the advantage to study both passive and active aspects of the phenomena on large cell numbers. Mathematical modeling allows full exploitation of the data by relating molecular parameters to cell mechanics. Using D. discoideum as a model system, we explain how cell detachment kinetics gives access to the rate constants describing the passive association or dissociation of the cell membrane to a given substrate.