Approches physiques de problématiques biologiques

Publications de l’équipe

Année de publication : 2009

J Prost, J-F Joanny, J M R Parrondo (2009 Oct 2)

Generalized fluctuation-dissipation theorem for steady-state systems.

Physical review letters : 090601 En savoir plus
Résumé

The fluctuation-dissipation theorem is a central result of statistical physics, which applies to any system at thermodynamic equilibrium. Its violation is a strong signature of nonequilibrium behavior. We show that for any system with Markovian dynamics, in a nonequilibrium steady state, a proper choice of observables restores a fluctuation-response theorem identical to a suitable version of the equilibrium fluctuation-dissipation theorem. This theorem applies to a broad class of dynamical systems. We illustrate it with linear stochastic dynamics and examples borrowed from the physics of molecular motors and Hopf bifurcations. Finally, we discuss general implications of the theorem.

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Jean-François Joanny, Jacques Prost (2009 Oct 2)

Active gels as a description of the actin-myosin cytoskeleton.

HFSP journal : 94-104 : DOI : 10.2976/1.3054712 En savoir plus
Résumé

This short review presents a qualitative introduction to the hydrodynamic theory of active polar gels and its applications to the mechanics of the cytoskeleton. Active polar gels are viscoelastic materials formed by polar filaments maintained in a nonequilibrium state by constant consumption of energy. In the cytoskeleton of eukaryotic cells, actin filaments are treadmilling and form a viscoelastic gel interacting with myosin molecular motors driven by the hydrolysis of adenosine triphosphate; one can thus consider the actomyosin cytoskeleton as an active polar gel. The hydrodynamic description is generic as it only relies on symmetry arguments. We first use the hydrodynamic approach to discuss the spontaneous generation of flow in an active polar film. Then we give two examples of applications to lamellipodium motility and to instabilities of cortical actin.

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Frank Jülicher, Jacques Prost (2009 Oct 2)

Comment on « Osmotic propulsion: the osmotic motor ».

Physical review letters : 079801; author reply 079802 En savoir plus
Résumé

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Timo Betz, Martin Lenz, Jean-François Joanny, Cécile Sykes (2009 Sep 1)

ATP-dependent mechanics of red blood cells.

Proceedings of the National Academy of Sciences of the United States of America : 15320-5 : DOI : 10.1073/pnas.0904614106 En savoir plus
Résumé

Red blood cells are amazingly deformable structures able to recover their initial shape even after large deformations as when passing through tight blood capillaries. The reason for this exceptional property is found in the composition of the membrane and the membrane-cytoskeleton interaction. We investigate the mechanics and the dynamics of RBCs by a unique noninvasive technique, using weak optical tweezers to measure membrane fluctuation amplitudes with mus temporal and sub nm spatial resolution. This enhanced edge detection method allows to span over >4 orders of magnitude in frequency. Hence, we can simultaneously measure red blood cell membrane mechanical properties such as bending modulus kappa = 2.8 +/- 0.3 x 10(-19)J = 67.6 +/- 7.2 k(B)T, tension sigma = 6.5 +/- 2.1 x 10(-7)N/m, and an effective viscosity eta(eff) = 81 +/- 3.7 x 10(-3) Pa s that suggests unknown dissipative processes. We furthermore show that cell mechanics highly depends on the membrane-spectrin interaction mediated by the phosphorylation of the interconnection protein 4.1R. Inhibition and activation of this phosphorylation significantly affects tension and effective viscosity. Our results show that on short time scales (slower than 100 ms) the membrane fluctuates as in thermodynamic equilibrium. At time scales longer than 100 ms, the equilibrium description breaks down and fluctuation amplitudes are higher by 40% than predicted by the membrane equilibrium theory. Possible explanations for this discrepancy are influences of the spectrin that is not included in the membrane theory or nonequilibrium fluctuations that can be accounted for by defining a nonthermal effective energy of up to E(eff) = 1.4 +/- 0.1 k(B)T, that corresponds to an actively increased effective temperature.

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F Jülicher, K Dierkes, B Lindner, J Prost, P Martin (2009 Aug 25)

Spontaneous movements and linear response of a noisy oscillator.

The European physical journal. E, Soft matter : 449-60 : DOI : 10.1140/epje/i2009-10487-5 En savoir plus
Résumé

A deterministic system that operates in the vicinity of a Hopf bifurcation can be described by a single equation of a complex variable, called the normal form. Proximity to the bifurcation ensures that on the stable side of the bifurcation (i.e. on the side where a stable fixed point exists), the linear-response function of the system is peaked at the frequency that is characteristic of the oscillatory instability. Fluctuations, which are present in many systems, conceal the Hopf bifurcation and lead to noisy oscillations. Spontaneous hair bundle oscillations by sensory hair cells from the vertebrate ear provide an instructive example of such noisy oscillations. By starting from a simplified description of hair bundle motility based on two degrees of freedom, we discuss the interplay of nonlinearity and noise in the supercritical Hopf normal form. Specifically, we show here that the linear-response function obeys the same functional form as for the noiseless system on the stable side of the bifurcation but with effective, renormalized parameters. Moreover, we demonstrate in specific cases how to relate analytically the parameters of the normal form with added noise to effective parameters. The latter parameters can be measured experimentally in the power spectrum of spontaneous activity and linear-response function to external stimuli. In other cases, numerical solutions were used to determine the effects of noise and nonlinearities on these effective parameters. Finally, we relate our results to experimentally observed spontaneous hair bundle oscillations and responses to periodic stimuli.

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Martin Lenz, Daniel J G Crow, Jean-François Joanny (2009 Aug 8)

Membrane buckling induced by curved filaments.

Physical review letters : 038101 En savoir plus
Résumé

We present a novel buckling instability relevant to membrane budding in eukaryotic cells. In this mechanism, curved filaments bind to a lipid bilayer without changing its intrinsic curvature. As more and more filaments adsorb, newly added ones are more and more strained, which destabilizes the flat membrane. We perform a linear stability analysis of filament-dressed membranes and find that the buckling threshold is within reasonable in vivo parameter values. We account for the formation of long tubes previously observed in cells and in purified systems. We study strongly deformed dressed membranes and their bifurcation diagram numerically. Our mechanism could be validated by a simple experiment.

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Jean-Francois Joanny, Ralf Metzler, Felix Ritort, David Weitz (2009 Jul 8)

Polymer physics of the cell. Preface.

Physical biology : 020301 : DOI : 10.1088/1478-3975/6/2/020301 En savoir plus
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P-Y Plaçais, M Balland, T Guérin, J-F Joanny, P Martin (2009 Jun 23)

Spontaneous oscillations of a minimal actomyosin system under elastic loading.

Physical review letters : 158102 En savoir plus
Résumé

Spontaneous mechanical oscillations occur in various types of biological systems where groups of motor molecules are elastically coupled to their environment. By using an optical trap to oppose the gliding motion of a single bead-tailed actin filament over a substrate densely coated with myosin motors, we mimicked this condition in vitro. We show that this minimal actomyosin system can oscillate spontaneously. Our finding accords quantitatively with a general theoretical framework where oscillatory instabilities emerge generically from the collective dynamics of molecular motors under load.

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F Jülicher, J Prost (2009 Apr 9)

Generic theory of colloidal transport.

The European physical journal. E, Soft matter : 27-36 : DOI : 10.1140/epje/i2008-10446-8 En savoir plus
Résumé

We discuss the motion of colloidal particles relative to a two-component fluid consisting of solvent and solute. Particle motion can result from i) net body forces on the particle due to external fields such as gravity; ii) slip velocities on the particle surface due to surface dissipative phenomena. The perturbations of the hydrodynamic flow field exhibit characteristic differences in cases i) and ii) which reflect different patterns of momentum flux corresponding to the existence of net forces, force dipoles or force quadrupoles. In the absence of external fields, gradients of concentration or pressure do not generate net forces on a colloidal particle. Such gradients can nevertheless induce relative motion between particle and fluid. We present a generic description of surface dissipative phenomena based on the linear response of surface fluxes driven by conjugate surface forces. In this framework we discuss different transport scenarios including self-propulsion via surface slip that is induced by active processes on the particle surface. We clarify the nature of force balances in such situations.

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Benoit Sorre, Andrew Callan-Jones, Jean-Baptiste Manneville, Pierre Nassoy, Jean-François Joanny, Jacques Prost, Bruno Goud, Patricia Bassereau (2009 Mar 24)

Curvature-driven lipid sorting needs proximity to a demixing point and is aided by proteins.

Proceedings of the National Academy of Sciences of the United States of America : 5622-6 : DOI : 10.1073/pnas.0811243106 En savoir plus
Résumé

Sorting of lipids and proteins is a key process ALLOWING eukaryotic cells to execute accurate and efficient transportation and intracellular membrane to Maintain homeostasis. It OCCURS DURING THE formation of highly curved transportation intermediates That shuttle entre cell compartments. Protein sorting is reasonably well described, goal lipid sorting is much less Understood. Lipid sorting has-been Proposed to be mediated by a mechanism based on the physical coupling entre membrane composition and high curvature of the transportation intermediates. To test this hypothesis, we-have Performed a combination of strength and fluorescence measurements on membrane tubes of controlled diameters pulled from giant unilamellar vesicles. A model based on membrane elasticity and nonideal solution theory HAS aussi beens Developed to explain our results. We show quantitatively, using 2 independent Approaches, That a difference in lipid composition can build up entre is curved and has noncurved membrane. Importantly, and consistent with our theory, lipid sorting OCCURS only if the system is close to a point of demixing. Remarkably, this process is amplified When low-even a fraction of lipids is clustered upon cholera toxin binding. This Can Be Explained by the reduction of the entropic penalty of lipid sorting When Some lipids are bound together by the toxin. Our results show That curvature-induced lipid sorting results from the collective behavior of lipids and is-even amplified in the presence of lipid-clustering proteins. In addition, They suggest a generic mechanism by qui proteins can Facilitate lipid segregation in vivo.

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Jean-François Joanny, Philip A Pincus (2009 Mar 20)

Biography of Pierre-Gilles de Gennes.

The journal of physical chemistry. B : 3593-4 : DOI : 10.1021/jp901191b En savoir plus
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Padinhateeri Ranjith, David Lacoste, Kirone Mallick, Jean-François Joanny (2009 Mar 18)

Nonequilibrium self-assembly of a filament coupled to ATP/GTP hydrolysis.

Biophysical journal : 2146-59 : DOI : 10.1016/j.bpj.2008.12.3920 En savoir plus
Résumé

We study the stochastic dynamics of growth and shrinkage of single actin filaments or microtubules taking into account insertion, removal, and ATP/GTP hydrolysis of subunits. The resulting phase diagram contains three different phases: two phases of unbounded growth: a rapidly growing phase and an intermediate phase, and one bounded growth phase. We analyze all these phases, with an emphasis on the bounded growth phase. We also discuss how hydrolysis affects force-velocity curves. The bounded growth phase shows features of dynamic instability, which we characterize in terms of the time needed for the ATP/GTP cap to disappear as well as the time needed for the filament to reach a length of zero (i.e., to collapse) for the first time. We obtain exact expressions for all these quantities, which we test using Monte Carlo simulations.

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M D El Alaoui Faris, D Lacoste, J Pécréaux, J-F Joanny, J Prost, P Bassereau (2009 Mar 5)

Membrane tension lowering induced by protein activity.

Physical review letters : 038102 : DOI : 10.1103/PhysRevLett.102.038102 En savoir plus
Résumé

Using videomicroscopy we present measurements of the fluctuation spectrum of giant vesicles containing bacteriorhodopsin pumps. When the pumps are activated, we observe a significant increase of the fluctuations in the low wave vector region, which we interpret as due to a lowering of the effective tension of the membrane.

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D Lacoste, G I Menon, M Z Bazant, J F Joanny (2009 Feb 3)

Electrostatic and electrokinetic contributions to the elastic moduli of a driven membrane.

The European physical journal. E, Soft matter : 243-64 : DOI : 10.1140/epje/i2008-10433-1 En savoir plus
Résumé

We discuss the electrostatic contribution to the elastic moduli of a cell or artificial membrane placed in an electrolyte and driven by a DC electric field. The field drives ion currents across the membrane, through specific channels, pumps or natural pores. In steady state, charges accumulate in the Debye layers close to the membrane, modifying the membrane elastic moduli. We first study a model of a membrane of zero thickness, later generalizing this treatment to allow for a finite thickness and finite dielectric constant. Our results clarify and extend the results presented by D. Lacoste, M. Cosentino Lagomarsino, and J.F. Joanny (EPL 77, 18006 (2007)), by providing a physical explanation for a destabilizing term proportional to [see formula in text] in the fluctuation spectrum, which we relate to a nonlinear (E(2)) electrokinetic effect called induced-charge electro-osmosis (ICEO). Recent studies of ICEO have focused on electrodes and polarizable particles, where an applied bulk field is perturbed by capacitive charging of the double layer and drives the flow along the field axis toward surface protrusions; in contrast, we predict « reverse » ICEO flows around driven membranes, due to curvature-induced tangential fields within a nonequilibrium double layer, which hydrodynamically enhance protrusions. We also consider the effect of incorporating the dynamics of a spatially dependent concentration field for the ion channels.

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Léa-Laetitia Pontani, Jasper van der Gucht, Guillaume Salbreux, Julien Heuvingh, Jean-François Joanny, Cécile Sykes (2009 Jan 13)

Reconstitution of an actin cortex inside a liposome.

Biophysical journal : 192-8 : DOI : 10.1016/j.bpj.2008.09.029 En savoir plus
Résumé

The composite and versatile structure of the cytoskeleton confers complex mechanical properties on cells. Actin filaments sustain the cell membrane and their dynamics insure cell shape changes. For example, the lamellipodium moves by actin polymerization, a mechanism that has been studied using simplified experimental systems. Much less is known about the actin cortex, a shell-like structure underneath the membrane that contracts for cell movement. We have designed an experimental system that mimicks the cell cortex by allowing actin polymerization to nucleate and assemble at the inner membrane of a liposome. Actin shell growth can be triggered inside the liposome, which offers a useful system for a controlled study. The observed actin shell thickness and estimated mesh size of the actin structure are in good agreement with cellular data. Such a system paves the way for a thorough characterization of cortical dynamics and mechanics.

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