Biomimetism of cellular movement

Keywords: actin, polymerisation, blebbing, VASP, motility, biomimicking systems

Read the scientific activity report. (pdf 165Ko, last update 26th, march 2010)

Cell motility is important for maintaining the health of an organism, for example, in wound healing where cells move rapidly to repair the damaged epithelium. Cell motility is also important in disease, however, particularly in cancer metastasis where malignant cells leave the primary tumour, migrate through the extracellular matrix, and go through the vasculature to colonise a new site. The goal of our work is to understand the biophysical mechanisms of cell motility. We use interdisciplinary methods to treat this subject: an experimental physics approach to acto-myosin based cell motility led by Cécile Sykes and the biochemical study of actin and Major Sperm Protein-powered motility led by Julie Plastino.

We design and study experimental systems that mimic cell motility and allow us to perform controlled studies. In one approach, we reconstitute movement by assembling the molecular building blocks one by one. For example, we can reproduce cellular actin polymerization and movement in a test tube: treated polystyrene beads activate the polymerisation of actin once placed in cellular extracts or in a mixture of purified proteins. In another approach, by simplifying cellular systems we can unveil the necessary components of a dynamic process. For example, changes in the shape of cells or cell fragments in which the microtubules are destroyed, we have observed, are based on dynamic rearrangements of the actomyosin cortex.

We can use the bead system to measure the force generated by actin polymerisation that propels the bead in a mixture of purified proteins. A microfibre attached to a bead acts as a spring to measure the force. The ‘comet tail' of polymerised actin that grows from the bead is held by a micropipette that can pull on the comet at a controlled velocity (Fig. 1). These experiments allowed us to measure the force exerted by actin polymerization on the bead (on the order of a nanoNewton) and understand the mechanism of force production.

By replacing the beads with oil droplets, we obtain behaviour that may be closer to that of a fluid cell membrane. We found that these oil droplets are deformed when the growing actin network squeezes the droplet (Fig. 2). By varying the amount of the actin binding protein VASP (Vasodilator stimulated phosphoprotein) on the surface, we observed saltatory motion due to detachment and diffusion of the polymerization activating molecules on the surface (Fig. 3).

Fig. 2

Fig. 3

Also, we have studied how comet tails are formed from the initially spherical growth of actin around a polystyrene bead. We found that the actin shell breaks due to the spherical geometry and the tension that builds up - as seen by relaxation of the broken gel (Video 1). Breakage occurs at a weak point in the gel, and the comet tail then forms at the opposite side of the bead. The point at which breakage occurs depends upon the actin network composition.

Spontaneous symmetry breaking of the spherical actin gel that forms around polystyrene beads. Time-lapse fluorescence microscopy of a bead of radius 7 µm coated with VCA and placed in a medium containing fluorescent F-actin and actin-related proteins.

We have observed an analogous phenomenon of broken symmetry in cells growing in suspension that display enhanced contractility after treatment with nocodazole to depolymerize their microtubules. The actomyosin cortex (a thin shell of actin network underlying the plasma membrane) of the cells breaks causing an oscillation of cell shape (Video 2). In these cells, as in the bead experiments, the actin gel breaks under tension due to local heterogeneity in the stresses placed upon it. In the case of the oscillating cell, tension is produced by the action of myosin motors in the cortex, whereas in the case of the bead, tension develops due to actin polymerisation in a sphere. The whole oscillation is due to a breakage-reformation cycle of the actin cortex.

Comparison of the oscillations of a KE37 lymphoblast (left panel) and a L929 cell fragment (right panel). Note the presence of a lamellipodium opposite the ring in the KE37 cell but not in the L929 cell fragment. Scale bars : 10 µm, time in min.

Our future work will be oriented towards reconstituting an artificial cell from functional modules in order to understand how they work. Moreover, we study other examples of movement by the principle of polymer assembly, for example, the assembly and disassembly of fibres of the major sperm protein (MSP), which is responsible for the amoeboid movement of Caenorhabditis elegans sperm.

Last update: March 2010

Key publications

2009

• T. Betz, M. Lenz, J.-F. Joanny, C. Sykes
  ATP-dependent mechanics of red blood cell
  P.N.A.S., 106(36), 15312-15317
• L.-L. Pontani, J. van der Gucht, G. Salbreux, J. Heuvingh, J.-F. Joanny, C. Sykes
  Reconstitution of an actin cortex inside a liposome
  Biophysical Journal, 96, 192-198

2008

• L. Trichet, C. Sykes, J. Plastino
  Relaxing the actin cytoskeleton for adhesion and movement with Ena/VASP
  Journal of Cell Biology, 181(1):19-25

2007

• Y. Marcy, J. Prost, J.-F. Joanny, C. Sykes
  Probing friction in actin-based motility
  New Journal of Physics, 431(9)
• C. Revenu, M. Courtois, C. Sykes, D. Louvard, S. Robine
  Villin severing activity enhances actin-based motility in vivo
  Molecular biology of the cell, 18(3):827-38
• J. Heuvingh, M. Franco, P. Chavrier, C. Sykes
  ARF1-mediated actin polymerization produces movement of artificial vesicles
  P.N.A.S., 104(43)16928-16933(2007)
• L. Trichet, O. Campas, C. Sykes, J. Plastino
  VASP governs actin dynamics by modulating filament anchoring
  Biophysical Journal, 92(3):1081-9

2006

• E. Paluch, C. Sykes, J. Prost, M. Bornens
  Cortical actomyosin as an active gel for cell deformations during locomotion and division
  Trends in Cell Biology, 16(1), 5-10
• E. Paluch, J. Van der Gucht, J.-F. Joanny, C. Sykes
  Deformations in actin comets from rocketing beads
  Biophysical Journal, 91: 3113-3122
• E. Paluch, J. van der Gucht, C. Sykes
  Cracking up : symmetry breaking in cellular systems
  Journal of Cell Biology (review), 175(5), 687-92