Approches physiques de problématiques biologiques

Publications de l’équipe

Année de publication : 2006

Andrei S Kozlov, Thomas Risler, A J Hudspeth (2006 Dec 19)

Coherent motion of stereocilia assures the concerted gating of hair-cell transduction channels.

Nature neuroscience : 87-92 En savoir plus
Résumé

The hair cell’s mechanoreceptive organelle, the hair bundle, is highly sensitive because its transduction channels open over a very narrow range of displacements. The synchronous gating of transduction channels also underlies the active hair-bundle motility that amplifies and tunes responsiveness. The extent to which the gating of independent transduction channels is coordinated depends on how tightly individual stereocilia are constrained to move as a unit. Using dual-beam interferometry in the bullfrog’s sacculus, we found that thermal movements of stereocilia located as far apart as a hair bundle’s opposite edges showed high coherence and negligible phase lag. Because the mechanical degrees of freedom of stereocilia are strongly constrained, a force applied anywhere in the hair bundle deflects the structure as a unit. This feature assures the concerted gating of transduction channels that maximizes the sensitivity of mechanoelectrical transduction and enhances the hair bundle’s capacity to amplify its inputs.

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Tatsiana Lobovkina, Paul Dommersnes, Jean-François Joanny, Johan Hurtig, Owe Orwar (2006 Dec 13)

Zipper dynamics of surfactant nanotube Y junctions.

Physical review letters : 188105 En savoir plus
Résumé

We investigate the formation of Y junctions in surfactant nanotubes connecting vesicles. Based on experimental observations of the surfactant flow on the nanotubes, we conclude that a Y junction propagates with a zipperlike mechanism. The surfactants from two nanotube branches undergo 1:1 mixing at the junction, and spontaneously form the extension of the third nanotube branch. Taking into account the tension driven surfactant flow, we develop a model for the Y junction dynamics that is in quantitative agreement with the experimental data.

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Rui Pedro Gonçalves, Guillaume Agnus, Pierre Sens, Christine Houssin, Bernard Bartenlian, Simon Scheuring (2006 Oct 25)

Two-chamber AFM: probing membrane proteins separating two aqueous compartments.

Nature methods : 1007-12 En savoir plus
Résumé

Biological membranes compartmentalize and define physical borders of cells. They are crowded with membrane proteins that fulfill diverse crucial functions. About one-third of all genes in organisms code for, and the majority of drugs target, membrane proteins. To combine structure and function analysis of membrane proteins, we designed a two-chamber atomic force microscopy (AFM) setup that allows investigation of membranes spanned over nanowells, therefore separating two aqueous chambers. We imaged nonsupported surface layers (S layers) of Corynebacterium glutamicum at sufficient resolution to delineate a 15 A-wide protein pore. We probed the elastic and yield moduli of nonsupported membranes, giving access to the lateral interaction energy between proteins. We combined AFM and fluorescence microscopy to demonstrate the functionality of proteins in the setup by documenting proton pumping by Halobacterium salinarium purple membranes.

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Otger Campàs, Pierre Sens (2006 Oct 10)

Chromosome oscillations in mitosis.

Physical review letters : 128102 En savoir plus
Résumé

The motion and positioning of chromosomes during eukaryotic cell division is investigated theoretically. We perform a self-contained analysis where the motion of mono-oriented chromosomes results from the competition between the kinetochore and chromokinesin motors on the chromosome arms. We show that the interplay between the asterlike morphology of the mitotic spindle and the collective dynamics of motors accounts for chromosome motion, positioning, and congression. In particular, the characteristic oscillations of chromosomes observed in vivo arise naturally within this description.

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Roger Karlsson, Anders Karlsson, Andrew Ewing, Paul Dommersnes, Jean-Francois Joanny, Aldo Jesorka, Owe Orwar (2006 Sep 26)

Chemical analysis in nanoscale surfactant networks.

Analytical chemistry : 5961-8 En savoir plus
Résumé

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Ewa Paluch, Jasper van der Gucht, Jean-François Joanny, Cécile Sykes (2006 Aug 1)

Deformations in actin comets from rocketing beads.

Biophysical journal : 3113-22 En savoir plus
Résumé

The mechanical and dynamical properties of the actin network are essential for many cellular processes like motility or division, and there is a growing body of evidence that they are also important for adhesion and trafficking. The leading edge of migrating cells is pushed out by the polymerization of actin networks, a process orchestrated by cross-linkers and other actin-binding proteins. In vitro physical characterizations show that these same proteins control the elastic properties of actin gels. Here we use a biomimetic system of Listeria monocytogenes, beads coated with an activator of actin polymerization, to assess the role of various actin-binding proteins in propulsion. We find that the properties of actin-based movement are clearly affected by the presence of cross-linkers. By monitoring the evolution of marked parts of the comet, we provide direct experimental evidence that the actin gel continuously undergoes deformations during the growth of the comet. Depending on the protein composition in the motility medium, deformations arise from either gel elasticity or monomer diffusion through the actin comet. Our findings demonstrate that actin-based movement is governed by the mechanical properties of the actin network, which are fine-tuned by proteins involved in actin dynamics and assembly.

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Pierre Sens, Matthew S Turner (2006 Apr 12)

Budded membrane microdomains as tension regulators.

Physical review. E, Statistical, nonlinear, and soft matter physics : 031918 En savoir plus
Résumé

We propose a mechanism by which changes of the mechanical tension of a composite lipid membrane are buffered by the invagination of membrane domains. We show that domain invagination, driven by differences in chemical composition, is a first-order transition controlled by membrane tension. The invaginated domains play the role of a membrane reservoir, exchanging area with the main membrane, and impose an equilibrium tension entirely controlled by their mechanical properties. The dynamical response of the reservoir reflects the tension-dependent kinetics of the domain shape transition, so that the tension of such a composite membrane is inherently transient and dynamical. The implications of this phenomenon for the mechanical properties of the membranes of living cells, where invaginated membrane domains are known to exist, are discussed.

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Pramod A Pullarkat, Paul Dommersnes, Pablo Fernández, Jean-François Joanny, Albrecht Ott (2006 Feb 21)

Osmotically driven shape transformations in axons.

Physical review letters : 048104 En savoir plus
Résumé

We report a cylindrical-peristaltic shape transformation in axons exposed to a controlled osmotic perturbation. The peristaltic shape relaxes and the axon recovers its original geometry within minutes. We show that the shape instability depends critically on the swelling rate and that volume and membrane area regulation are responsible for the shape relaxation. We propose that volume regulation occurs via leakage of ions driven by elastic pressure, and analyze the peristaltic shape dynamics taking into account the internal structure of the axon. The results obtained provide a framework for understanding peristaltic shape dynamics in nerve fibers occurring in vivo.

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R Voituriez, J F Joanny, J Prost (2006 Feb 21)

Generic phase diagram of active polar films.

Physical review letters : 028102 En savoir plus
Résumé

We study theoretically the phase diagram of compressible active polar gels such as the actin network of eukaryotic cells. Using generalized hydrodynamics equations, we perform a linear stability analysis of the uniform states in the case of an infinite bidimensional active gel to obtain the dynamic phase diagram of active polar films. We predict, in particular, modulated flowing phases and a macroscopic phase separation at high activity. This qualitatively accounts for experimental observations of various active systems, such as actomyosin gels, microtubules and kinesins in vitro solutions, or swimming bacterial colonies.

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Année de publication : 2005

Arnold J Storm, Cornelis Storm, Jianghua Chen, Henny Zandbergen, Jean-François Joanny, Cees Dekker (2005 Sep 24)

Fast DNA translocation through a solid-state nanopore.

Nano letters : 1193-7 En savoir plus
Résumé

We report experiments and modeling of translocation of double-strand DNA through a siliconoxide nanopore. Long DNA molecules with different lengths ranging from 6500 to 97000 base pairs have been electrophoretically driven through a 10 nm pore. We observe a power-law caling of the translocation time with the length, with an exponent of 1.27. This nonlinear scaling is strikingly different from the well-studied linear behavior observed in similar experiments performed on protein pores. We present a theoretical model where hydrodynamic drag on the ection of the polymer outside the pore is the dominant force counteracting the electrical driving force. We show that this applies to our experiments, and we derive a power-law scaling with an exponent of 1.22, in good agreement with the data.

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Thomas Risler, Jacques Prost, Frank Jülicher (2005 Aug 11)

Universal critical behavior of noisy coupled oscillators: a renormalization group study.

Physical review. E, Statistical, nonlinear, and soft matter physics : 016130 En savoir plus
Résumé

We show that the synchronization transition of a large number of noisy coupled oscillators is an example for a dynamic critical point far from thermodynamic equilibrium. The universal behaviors of such critical oscillators, arranged on a lattice in a d -dimensional space and coupled by nearest-neighbors interactions, can be studied using field-theoretical methods. The field theory associated with the critical point of a homogeneous oscillatory instability (or Hopf bifurcation of coupled oscillators) is the complex Ginzburg-Landau equation with additive noise. We perform a perturbative renormalization group (RG) study in a (4-epsilon)-dimensional space. We develop an RG scheme that eliminates the phase and frequency of the oscillations using a scale-dependent oscillating reference frame. Within Callan-Symanzik’s RG scheme to two-loop order in perturbation theory, we find that the RG fixed point is formally related to the one of the model A dynamics of the real Ginzburg-Landau theory with an O2 symmetry of the order parameter. Therefore, the dominant critical exponents for coupled oscillators are the same as for this equilibrium field theory. This formal connection with an equilibrium critical point imposes a relation between the correlation and response functions of coupled oscillators in the critical regime. Since the system operates far from thermodynamic equilibrium, a strong violation of the fluctuation-dissipation relation occurs and is characterized by a universal divergence of an effective temperature. The formal relation between critical oscillators and equilibrium critical points suggests that long-range phase order exists in critical oscillators above two dimensions.

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Michal Tokarz, Björn Akerman, Jessica Olofsson, Jean-Francois Joanny, Paul Dommersnes, Owe Orwar (2005 Jun 18)

Single-file electrophoretic transport and counting of individual DNA molecules in surfactant nanotubes.

Proceedings of the National Academy of Sciences of the United States of America : 9127-32 En savoir plus
Résumé

We demonstrate a complete nanotube electrophoresis system (nanotube radii in the range of 50 to 150 nm) based on lipid membranes, comprising DNA injection, single-molecule transport, and single-molecule detection. Using gel-capped electrodes, electrophoretic single-file transport of fluorescently labeled dsDNA molecules is observed inside nanotubes. The strong confinement to a channel of molecular dimensions ensures a detection efficiency close to unity and identification of DNA size from its linear relation to the integrated peak intensity. In addition to constituting a nanotechnological device for identification and quantification of single macromolecules or biopolymers, this system provides a method to study their conformational dynamics, reaction kinetics, and transport in cell-like environments.

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Laetitia Cohen-Tannoudji, Emanuel Bertrand, Lydie Bressy, Cécile Goubault, Jean Baudry, Jacob Klein, Jean-François Joanny, Jérôme Bibette (2005 Feb 9)

Polymer bridging probed by magnetic colloids.

Physical review letters : 038301 En savoir plus
Résumé

Superparamagnetic particles offer a new way to probe the kinetics of adhesive processes. Two different scenarios of physical adhesion are studied. The thermal activation of van der Waals adhesion is well described by an Arrhenius model. In contrast, it is necessary to go beyond the Arrhenius description to understand the thermal activation of bridging between colloidal particles by a polymer at equilibrium adsorbance. We show that polymer bridging requires some removal of adsorbed polymer and is strongly influenced by the proximity of a glass transition within the adsorbed polymer.

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Max Davidson, Paul Dommersnes, Martin Markström, Jean-Francois Joanny, Mattias Karlsson, Owe Orwar (2005 Jan 27)

Fluid mixing in growing microscale vesicles conjugated by surfactant nanotubes.

Journal of the American Chemical Society : 1251-7 En savoir plus
Résumé

This work addresses novel means for controlled mixing and reaction initiation in biomimetic confined compartments having volume elements in the range of 10(-12) to 10(-15) L. The method is based on mixing fluids using a two-site injection scheme into growing surfactant vesicles. A solid-state injection needle is inserted into a micrometer-sized vesicle (radius 5-25 microm), and by pulling on the needle, we create a nanoscale surfactant channel connecting injection needle and the vesicle. Injection of a solvent A from the needle into the nanotube results in the formation of a growing daughter vesicle at the tip of the needle in which mixing takes place. The growth of the daughter vesicle requires a flow of surfactants in the nanotube that generates a flow of solvent B inside the nanotube which is counterdirectional to the pressure-injected solvent. The volume ratio psi between solvent A and B inside the mixing vesicle was analyzed and found to depend only on geometrical quantities. The majority of fluid injected to the growing daughter vesicle comes from the pressure-based injection, and for a micrometer-sized vesicle it dominates. For the formation of one daughter vesicle (conjugated with a 100-nm radius tube) expanded from 1 to 200 microm in radius, the mixing ratios cover almost 3 orders of magnitude. We show that the system can be expanded to linear strings of nanotube-conjugated vesicles that display exponential dilution. Mixing ratios spanning 6 orders of magnitude were obtained in strings of three nanotube-conjugated micrometer-sized daughter vesicles.

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Année de publication : 2004

Thomas Risler, Jacques Prost, Frank Jülicher (2004 Nov 5)

Universal critical behavior of noisy coupled oscillators.

Physical review letters : 175702 En savoir plus
Résumé

We study the universal thermodynamic properties of systems consisting of many coupled oscillators operating in the vicinity of a homogeneous oscillating instability. In the thermodynamic limit, the Hopf bifurcation is a dynamic critical point far from equilibrium described by a statistical field theory. We perform a perturbative renormalization group study, and show that at the critical point a generic relation between correlation and response functions appears. At the same time, the fluctuation-dissipation relation is strongly violated.

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