Jean-Paul Rieu

Fine tuning of tissues' viscosity and surface tension through contractility suggests a new role for α-catenin.

What governs tissue organization and movement? If molecular and genetic approaches are able to give some answers on these issues, more and more works are now giving a real importance to mechanics as a key component eventually triggering further signaling events. We chose embryonic cell aggregates as model systems for tissue organization and movement in order to investigate the origin of some mechanical constraints arising from cells organization. Steinberg et al. proposed a long time ago an analogy between liquids and tissues and showed that indeed tissues possess a measurable tissue surface tension and viscosity. We question here the molecular origin of these parameters and give a quantitative measurement of adhesion versus contractility in the framework of the differential interfacial tension hypothesis. Accompanying surface tension measurements by angle measurements (at vertexes of cell-cell contacts) at the cell/medium interface, we are able to extract the full parameters of this model: cortical tensions and adhesion energy. We show that a tunable surface tension and viscosity can be achieved easily through the control of cell-cell contractility compared to cell-medium one. Moreover we show that -catenin is crucial for this regulation to occur: these molecules appear as a catalyser for the remodeling of the actin cytoskeleton underneath cell-cell contact, enabling a differential contractility between the cell-medium and cell-cell interface to take place.

Nanomechanical and tribological characterization of the MPC phospholipid polymer photografted onto rough polyethylene implants.

Grafting biomimetic polymers onto biomaterials such as implants is one of the promising approaches to increase their tribological performance and biocompatibility and to reduce wear. In this paper, poly(2-methacryloyloxyethyl phosphorylcholine) (p(MPC)) brushes were obtained by photografting MPC from the rough surface of ultra high molecular weight polyethylene (UHMWPE) joint implants. Such substrates have a high roughness (Ra∼650nm) which often has the same order of magnitude as the brush thickness, so it is very difficult to estimate the vertical density profile of the grafted content. The quality of the p(MPC) grafting was evaluated through a wide range of characterization techniques to reveal the effectiveness of the grafting: atomic force microcopy (AFM) imaging and force spectroscopy, contact angle, SEM/EDX, and confocal microscopy. After testing the methods on smooth glass substrate as reference, AFM nano-indentation proves to be a reliable non destructive method to characterize the thickness and the mechanical properties of the p(MPC) layer in liquid physiological medium. Tribological measurements using a homemade biotribometer confirm that, despite heterogeneity thickness (h=0.5-6μm), the p(MPC) layer covers the roughness of the UHMWPE substrate and acts as an efficient lubricant with low friction coefficient and no wear for 9h of friction.

A Quorum-Sensing Factor in Vegetative Dictyostelium Discoideum Cells Revealed by Quantitative Migration Analysis

Background Many cells communicate through the production of diffusible signaling molecules that accumulate and once a critical concentration has been reached, can activate or repress a number of target genes in a process termed quorum sensing (QS). In the social amoeba Dictyostelium discoideum, QS plays an important role during development. However little is known about its effect on cell migration especially in the growth phase. Methods and Findings To investigate the role of cell density on cell migration in the growth phase, we use multisite timelapse microscopy and automated cell tracking. This analysis reveals a high heterogeneity within a given cell population, and the necessity to use large data sets to draw reliable conclusions on cell motion. In average, motion is persistent for short periods of time (), but normal diffusive behavior is recovered over longer time periods. The persistence times are positively correlated with the migrated distances. Interestingly, the migrated distance decreases as well with cell density. The adaptation of cell migration to cell density highlights the role of a secreted quorum sensing factor (QSF) on cell migration. Using a simple model describing the balance between the rate of QSF generation and the rate of QSF dilution, we were able to gather all experimental results into a single master curve, showing a sharp cell transition between high and low motile behaviors with increasing QSF. Conclusion This study unambiguously demonstrates the central role played by QSF on amoeboid motion in the growth phase.

Changes in the magnitude and distribution of forces at different Dictyostelium developmental stages.

The distribution of forces exerted by migrating Dictyostelium amebae at different developmental stages was measured using traction force microscopy. By using very soft polyacrylamide substrates with a high fluorescent bead density, we could measure stresses as small as 30 Pa. Remarkable differences exist both in term of the magnitude and distribution of forces in the course of development. In the vegetative state, cells present cyclic changes in term of speed and shape between an elongated form and a more rounded one. The forces are larger in this first state, especially when they are symmetrically distributed at the front and rear edge of the cell. Elongated vegetative cells can also present a front-rear asymmetric force distribution with the largest forces in the crescent-shaped rear of the cell (uropod). Pre-aggregating cells, once polarized, only present this last kind of asymmetric distribution with the largest forces in the uropod. Except for speed, no cycle is observed. Neither the force distribution of pre-aggregating cells nor their overall magnitude are modified during chemotaxis, the later being similar to the one of vegetative cells (F(0) approximately 6 nN). On the contrary, both the force distribution and overall magnitude is modified for the fast moving aggregating cells. In particular, these highly elongated cells exert lower forces (F(0) approximately 3 nN). The location of the largest forces in the various stages of the development is consistent with the myosin II localization described in the literature for Dictyostelium (Yumura et al.,1984. J Cell Biol 99:894-899) and is confirmed by preliminary experiments using a GFP-myosin Dictyostelium strain.

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 21u2009°C and of the human breast cancer cell line MDA-MB-231 at 37u2009°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.

Migration of Dictyostelium Slugs: Anterior-like Cells May Provide the Motive Force for the Prespore Zone

The collective motion of cells in a biological tissue originates from their individual responses to chemical and mechanical signals. The Dictyostelium slug moves as a collective of up to 100,000 cells with prestalk cells in the anterior 10-30% and prespore cells, intermingled with anterior-like cells (AL cells), in the posterior. We used traction force microscopy to measure the forces exerted by migrating slugs. Wild-type slugs exert frictional forces on their substratum in the direction of motion in their anterior, balanced by motive forces dispersed down their length. StlB- mutants lack the signal molecule DIF-1 and hence a subpopulation of AL cells. They produce little if any motive force in their rear and immediately break up. This argues that AL cells, but not prespore cells, are the motive cells in the posterior zone. Slugs also exert large outward radial forces, which we have analyzed during looping movement. Each time the anterior touches down after a loop, the outward forces rapidly develop, approximately normal to the almost stationary contact lines. We postulate that these forces result from the immediate binding of the sheath to the substratum and the subsequent application of outward pressure, which might be developed in several different ways.

Multicellular aggregates: a model system for tissue rheology

Morphogenetic processes involve cell flows. The mechanical response of a tissue to active forces is linked to its effective viscosity. In order to decouple this mechanical response from the complex genetic changes occurring in a developing organism, we perform rheometry experiments on multicellular aggregates, which are good models for tissues. We observe a cell softening behavior when submitting to stresses. As our technique is very sensitive, we were able to get access to the measurement of a yield point above which a creep regime is observed obtained for strains above 12%. To explain our rheological curves we propose a model for the cytoskeleton that we represent as a dynamic network of parallel springs, which will break under stress and reattach at null strain. Such a simple model is able to reproduce most of the important behavior of cells under strain. We highlight here the importance of considering cells as complex fluids whose properties will vary with time according to the history of applied stress.Graphical abstract

Charged particles interacting with a mixed supported lipid bilayer as a biomimetic pulmonary surfactant

This study shows the interactions of charged particles with mixed supported lipid bilayers (SLB) as biomimetic pulmonary surfactants. We tested two types of charged particles: positively charged and negatively charged particles. Two parameters were measured: adsorption density of particles on the SLB and the diffusion coefficient of lipids by FRAPP techniques as a measure of interaction strength between particles and lipids. We found that positively charged particles do not adsorb on the bilayer, probably due to the electrostatic repulsion between positively charged parts of the lipid head and the positive groups on the particle surface, therefore no variation in diffusion coefficient of lipid molecules was observed. On the contrary, the negatively charged particles, driven by electrostatic interactions are adsorbed onto the supported bilayer. The adsorption of negatively charged particles increases with the zeta-potential of the particle. Consecutively, the diffusion coefficient of lipids is reduced probably due to binding onto the lipid heads which slows down their Brownian motion. The results are directly relevant for understanding the interactions of particulate matter with pulmonary structures which could lead to pulmonary surfactant inhibition or deficiency causing severe respiratory distress or pathologies.Graphical abstract