UMR168 – Laboratoire Physico-Chimie Curie

Publications de l’UMR 168

Année de publication : 2000

P Martin, A D Mehta, A J Hudspeth (2000 Oct 12)

Negative hair-bundle stiffness betrays a mechanism for mechanical amplification by the hair cell.

Proceedings of the National Academy of Sciences of the United States of America : 12026-31 En savoir plus

Hearing and balance rely on the ability of hair cells in the inner ear to sense miniscule mechanical stimuli. In each cell, sound or acceleration deflects the mechanosensitive hair bundle, a tuft of rigid stereocilia protruding from the cell’s apical surface. By altering the tension in gating springs linked to mechanically sensitive transduction channels, this deflection changes the channels’ open probability and elicits an electrical response. To detect weak stimuli despite energy losses caused by viscous dissipation, a hair cell can use active hair-bundle movement to amplify its mechanical inputs. This amplificatory process also yields spontaneous bundle oscillations. Using a displacement-clamp system to measure the mechanical properties of individual hair bundles from the bullfrog’s ear, we found that an oscillatory bundle displays negative slope stiffness at the heart of its region of mechanosensitivity. Offsetting the hair bundle’s position activates an adaptation process that shifts the region of negative stiffness along the displacement axis. Modeling indicates that the interplay between negative bundle stiffness and the motor responsible for mechanical adaptation produces bundle oscillation similar to that observed. Just as the negative resistance of electrically excitable cells and of tunnel diodes can be embedded in a biasing circuit to amplify electrical signals, negative stiffness can be harnessed to amplify mechanical stimuli in the ear.

C Rauch, E Farge (2000 May 29)

Endocytosis switch controlled by transmembrane osmotic pressure and phospholipid number asymmetry.

Biophysical journal : 3036-47 En savoir plus

The dynamics of endocytosis in living K562 cells was investigated after the osmotic pressure of the external medium was decreased and the transmembrane phospholipid number asymmetry was increased. When the external pressure was decreased by a factor of 0.54, a sudden inhibition of endocytosis was observed. Under these conditions, the endocytosis suddenly recovered after the phospholipid number asymmetry was increased. The phospholipid asymmetry was generated by the addition of exogenous phosphatidylserine, which is translocated by the endogenous flippase activity to the inner layer of the membrane. The recovery of endocytosis is thus consistent with the view that the phospholipid number asymmetry can act as a budding force for endocytosis. Moreover, we quantitatively predict both the inhibition and recovery of endocytosis as first-order phase transitions, using a general model that assumes the existence of a transmembrane surface tension asymmetry as the budding driving force. In this model, the tension asymmetry is considered to be elastically generated by the activity of phospholipid pumping. We finally propose that cells may trigger genetic transcription responses after the internalization of cytokine-receptor complexes, which could be controlled by variations in the cytosolic or external pressure.

V Noireaux, R M Golsteyn, E Friederich, J Prost, C Antony, D Louvard, C Sykes (2000 Feb 29)

Growing an actin gel on spherical surfaces.

Biophysical journal : 1643-54 En savoir plus

Inspired by the motility of the bacteria Listeria monocytogenes, we have experimentally studied the growth of an actin gel around spherical beads grafted with ActA, a protein known to be the promoter of bacteria movement. On ActA-grafted beads F-actin is formed in a spherical manner, whereas on the bacteria a « comet-like » tail of F-actin is produced. We show experimentally that the stationary thickness of the gel depends on the radius of the beads. Moreover, the actin gel is not formed if the ActA surface density is too low. To interpret our results, we propose a theoretical model to explain how the mechanical stress (due to spherical geometry) limits the growth of the actin gel. Our model also takes into account treadmilling of actin. We deduce from our work that the force exerted by the actin gel on the bacteria is of the order of 10 pN. Finally, we estimate from our theoretical model possible conditions for developing actin comet tails.


Année de publication : 1999

P Martin, A J Hudspeth (1999 Dec 10)

Active hair-bundle movements can amplify a hair cell’s response to oscillatory mechanical stimuli.

Proceedings of the National Academy of Sciences of the United States of America : 14306-11 En savoir plus

To enhance their mechanical sensitivity and frequency selectivity, hair cells amplify the mechanical stimuli to which they respond. Although cell-body contractions of outer hair cells are thought to mediate the active process in the mammalian cochlea, vertebrates without outer hair cells display highly sensitive, sharply tuned hearing and spontaneous otoacoustic emissions. In these animals the amplifier must reside elsewhere. We report physiological evidence that amplification can stem from active movement of the hair bundle, the hair cell’s mechanosensitive organelle. We performed experiments on hair cells from the sacculus of the bullfrog. Using a two-compartment recording chamber that permits exposure of the hair cell’s apical and basolateral surfaces to different solutions, we examined active hair-bundle motion in circumstances similar to those in vivo. When the apical surface was bathed in artificial endolymph, many hair bundles exhibited spontaneous oscillations of amplitudes as great as 50 nm and frequencies in the range 5 to 40 Hz. We stimulated hair bundles with a flexible glass probe and recorded their mechanical responses with a photometric system. When the stimulus frequency lay within a band enclosing a hair cell’s frequency of spontaneous oscillation, mechanical stimuli as small as +/-5 nm entrained the hair-bundle oscillations. For small stimuli, the bundle movement was larger than the stimulus. Because the energy dissipated by viscous drag exceeded the work provided by the stimulus probe, the hair bundles powered their motion and therefore amplified it.

M. C. Fauré, P. Bassereau, L. T. Lee, A. Menelle, C. Lheveder (1999 Nov 24)

Phase Transitions in Monolayers of PS−PEO Copolymer at the Air−Water Interface.

Macromolecules : 32 : 8538-8550 : DOI : 10.1021/ma9900840 En savoir plus

The structures and phase transitions occurring during the compression of polystyrene−poly(ethylene oxide) (PS−PEO) diblock copolymer monolayers at the air−water interface have been studied by surface pressure isotherms, neutron reflectivity, and Brewster angle microscopy. At low coverage, the EO monomers adsorb at the air−water interface, but the PEO layer is not purely bidimensional. At high coverage, the EO−interface interaction becomes repulsive. A brush structure has been observed with differences between the long and the short PEO chains attributed to the difference in the accessible range of surface concentration. For the longest chain, the PEO concentration profiles exhibit a superposition of a depletion layer near the interface and discontinuous profile characteristics of a two-phase brush described by the “n-cluster” theory. For the shortest chains, the profiles are “pseudoparabolic”. Finally, Brewster angle microscopy shows that, for the long chains, the transition between the adsorbed and the brush structures is first order.

J.-B. Manneville, P. Bassereau, D. Lévy, and J. Prost (1999 May 24)

Activity of transmembrane proteins induces magnification of shape fluctuations of lipid membranes.

Phys. Rev. Lett. : 82 : 4356 : DOI : 10.1103/PhysRevLett.82.4356 En savoir plus

We report the first experimental evidence of the effect of the activity of transmembrane proteins on shape fluctuations of a lipid membrane. We incorporate a light-driven proton pump, the bacteriorhodopsin (BR), inside the phospholipid bilayer of fluctuating giant vesicles. Using the micropipet technique, we measure the excess surface area due to the fluctuations of the vesicles. The excess surface area is larger when the BR pumps protons than when it is not activated.


Année de publication : 1998

M.C. Fauré, P. Bassereau, M. A. Carignano, I. Szleifer, Y. Gallot, D. Andelman (1998 Jun 1)

Monolayers of diblock copolymer at the air-water interface: the attractive monomer-surface case.

The European Physical Journal B - Condensed Matter and Complex Systems : 3 : 365-375 : DOI : 10.1007/s100510050324 En savoir plus

We have studied both experimentally and theoretically the surface pressure isotherms of copolymers of polystyrene-polyethyleneoxide (PS-PEO) at the air-water interface. The SCMF (single chain mean-field) theory provides a very good agreement with the experiments for the entire range of surface densities and is consistent with the experiments if an adsorption energy per PEO monomer at the air-water interface of about one kB T is taken. In addition, the chain density profile has been calculated for a variety of surface densities, from the dilute to the very dense ones. The SCMF approach has been complemented by a mean-field approach in the low density regime, where the PEO chains act as a two-dimensional layer. Both theoretical calculations agree with the experiments in this region.


Année de publication : 1997

Patricia Bassereau, Frédéric Pincet (1997 Dec 24)

Quantitative Analysis of Holes in Supported Bilayers Providing the Adsorption Energy of Surfactants on Solid Substrate.

Langmuir : 13 : 7003-7007 : DOI : 10.1021/la970515c En savoir plus

We investigated the topography of mixed bilayers consisting of a first monolayer of DMPE (dimyristoylphosphatidylethanolamine) and of a second monolayer of DOPC (dioleoylphosphatidylcholine) that were Langmuir−Blodgett deposited on mica. Using transfer ratio measurements and tapping mode atomic force microscopy experiments, we show that the subnanometric holes in the bilayers result from the desorption of lipids of the first monolayer during the transfer of the second monolayer. We present a new simple technique based on the quantitative analysis of these holes that allows determination of the adsorption energy of amphiphilic molecules on solid surfaces. This technique is valid for relatively low adsorption energies in the range 1 to 10 kBT.


Année de publication : 1995

P. Bassereau, T. P. Russell (1995 Feb 19)

Thermal expansion of thin diblock copolymer films.

Isreal Journal of Chemistry : 35 : 13-19 : DOI : 10.1002/ijch.199500004 En savoir plus

The thermal expansion of thin films of symmetric diblock copolymers of polystyrene (PS) and poly(methyl methácrylate) (PMMA) was investigated by X‐ray reflectivity. The confinement of the copolymer to the substrate, coupled with the multilayering of the copolymer where PS and PMMA layers are oriented parallel to the substrate, gives rise to unusual thermal expansion characteristics. The total thickness of the film increases as 3αL, where αL is the linear thermal expansion coefficient of the copolymer. Unlike homopolymer films, the thermal expansion of an ordered block copolymer film results in an excessive stretching of the copolymer chains at the interface between the PS and PMMA layers. This excess stretching is a result of the confinement of the junction points of the copolymer chains to the interfaces and the suppression of the lateral expansion of the copolymer. When the stretching of the chains becomes too high, relaxation occurs by transporting copolymer chains to the surface. This is evidenced by a reduction in the period of the multilayer. After the copolymer chains have relaxed, the change in the multilayer period with temperature closely follows αL.


Année de publication : 2013

E Hannezo, J Prost, J-F Joanny (1970 Jan 1)

Growth, homeostatic regulation and stem cell dynamics in tissues.

Journal of the Royal Society, Interface / the Royal Society : 20130895 : DOI : 10.1098/rsif.2013.0895 En savoir plus

The regulation of cell growth in animal tissues is a question of critical importance: most tissues contain different types of cells in interconversion and the fraction of each type has to be controlled in a precise way, by mechanisms that remain unclear. Here, we provide a theoretical framework for the homeostasis of stem-cell-containing epithelial tissues using mechanical equations, which describe the size of the tissue and kinetic equations, which describe the interconversions of the cell populations. We show that several features, such as the evolution of stem cell fractions during intestinal development, the shape of a developing intestinal wall, as well as the increase in the proliferative compartment in cancer initiation, can be studied and understood from generic modelling which does not rely on a particular regulatory mechanism. Finally, inspired by recent experiments, we propose a model where cell division rates are regulated by the mechanical stresses in the epithelial sheet. We show that pressure-controlled growth can, in addition to the previous features, also explain with few parameters the formation of stem cell compartments as well as the morphologies observed when a colonic crypt becomes cancerous. We also discuss optimal strategies of wound healing, in connection with experiments on the cornea.