Biophysique cellulaire

The Cell Biophysics team at PMMH lab consists of two researchers : Julien Heuvingh (Assistant professor at Paris Diderot University) and Olivia du Roure (CNRS Researcher).

Our current main research focus is on the mechanics of branched actin networks. We use a novel technique with magnetic colloids to deform and characterize the actin networks. Thomas Pujol is a 3 year PhD student working on this subject and Joseph Tavacoli recently joined our group as a postdoc.

We are also interested in the swimming of unicellular organisms by artificial cillia (OdR with Marc Fermigier from PMMH). web site

We work with models of cell membrane : Giant Unilamelar Vesicles (JH with Stéphanie Bonneau from UPMC)

We are part of a collaboration between a medical team working on obesity and physicists to study the influence of the mechanical stress on adipocyte cells and adipose tissue (Emergence project from UPMC).

We are also studying less biology-oriented subjects, such as fluid-structure interaction (OdR with Anke Lindner from PMMH), polymer mechanics, auto-organisation of colloids…

 

Mechanics of actin networks

The actin protein is one of the major component of the cytoskeleton. Actin polymerizes into filaments which forms the cortex beneath the cell membrane. The polymerisation of actin also generates forces, a process used by the cell to move, change its shape, or divide. A large number of associated proteins interacts with actin to control the length of the filaments, accelarate the polymerisation and organise the filaments into bundles or dendritic arrays. More specifically the eukaryotic cells crawl on surface using the directed polymerisation of actin via the protein complex, Arp2/3 at the leading edge of the cell : the membrane is pushed forward by the polymerisation. Some pathogens, like Listeria, are able to recruit the actin of the host cells to assemble a “comet”. This comet is used by the pathogen to move inside the cytoplasm and from a cell to another. In vitro, such comet can be obtained from the growth of a gel at the surface of beads via a mix of purified actin associated protein.

We study the mechanics of actin branched gels via a novel technique using magnetic colloids. Based on dipolar interactions that organise superparamagnetic beads in chain when a magnetic field is applied, this technique allows to apply well-controlled forces in the range of piconewtons to nanonewtons. Such forces are sufficient to deform an actin gel grown on the beads. We can thus study mechanical properties of actin gel. Compared to other existing techniques such as AFM (Chaudhuri et al. Nature 2007), we are here able to obtain easily very large statistics : each link between two beads gives an independent force distance curve, and hundreds of such characterisations can be made in one experiment. We are comparing gels made from different concentrations of branching and capping proteins, to unravel the link between the network architecture and its mechanical properties.

Membres

 

Olivia du Roure

CNRS Researcher

Tel. +33 1 40 79 47 19

olivia.duroure (arobase) espci.fr

 

Julien Heuvingh

Assistant Professor University Paris 7-Denis Diderot

Tel. +33 1 40 79 47 08

julien.heuvingh (arobase) espci.fr

 

Thomas Pujol

PhD Student

Tel. +33 1 40 79 47 22

thomas.pujol (arobase) espci.fr

 

JosephTavacoli

Postdoc

Tel. +33 1 40 79 47 16

joseph.tavacoli (arobase) espci.fr

Publications

**Actin mechanics and force generation**

Brangbour C, du Roure O, Helfer E, Démoulin D, Mazurier A, et al. (2011)

 

Force-Velocity Measurements of a Few Growing Actin Filaments.

PLoS Biol 9(4).
Abstract : The polymerization of actin in filaments generates forces that play a pivotal role in many cellular processes. We introduce a novel technique to determine the force-velocity relation when a few independent anchored filaments grow between magnetic colloidal particles. When a magnetic field is applied, the colloidal particles assemble into chains under controlled loading or spacing. As the filaments elongate, the beads separate, allowing the force-velocity curve to be precisely measured. In the widely accepted Brownian ratchet model, the transduced force is associated with the slowing down of the on-rate polymerization. Unexpectedly, in our experiments, filaments are shown to grow at the same rate as when they are free in solution. However, as they elongate, filaments are more confined in the interspace between beads. Higher repulsive forces result from this higher confinement, which is associated with a lower entropy. In this mechanism, the production of force is not controlled by the polymerization rate, but is a consequence of the restriction of filaments’ orientational fluctuations at their attachment point.
PDF - 397.8 ko

Thomas Pujol, Olivia du Roure, Marc Fermigier & Julien Heuvingh (2012)

 

Impact of branching on the elasticity of actin networks.

PNAS, 2012, Early edition 8 june 2012

>http://www.pnas.org/content/early/2…

Abstract : Actin filaments play a fundamental role in cell mechanics : assembled into networks by a large number of partners, they ensure cell integrity, deformability, and migration. Here we focus on the mechanics of the dense branched network found at the leading edge of a crawling cell. We develop a new technique based on the dipolar attraction between magnetic colloids to measure mechanical properties of branched actin gels assembled around the colloids. This technique allows us to probe a large number of gels and, through the study of different networks, to access fundamental relationships between their microscopic structure and their mechanical properties. We show that the architecture does regulate the elasticity of the network : increasing both capping and branching concentrations strongly stiffens the networks. These effects occur at protein concentrations that can be regulated by the cell. In addition, the dependence of the elastic modulus on the filaments’ flexibility and on increasing internal stress has been studied. Our overall results point toward an elastic regime dominated by enthalpic rather than entropic deformations. This result strongly differs from the elasticity of diluted cross-linked actin networks and can be explained by the dense dendritic structure of lamellipodium-like networks.

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