Doctoral thesis viva voce: Damien Démoulin

Contact : damien.demoulin (arobase)

10 May 2012 10:00 » 14:00 — ESPCI ParisTech

Magnetic beads as force sensors : application to actin filaments growth pressure

Damien Démoulin, doctorant - PhD student Crédits : ESPCI ParisTech
Damien Démoulin, doctorant - PhD student Crédits : ESPCI ParisTech

Superparamagnetic colloids are used in many studies to apply a force or a torque on biological systems such as cells or DNA molecules. The physical process by which magnetization appears in those beads, allowing them to exert forces, is still poorly understood.

In a first part, with an original experimental setup based on optical anisotropy measurements, we show that the apparently magnetically isotropic beads can orient themselves in a magnetic field. Equilibrium and out of equilibrium measurements allowed to develop a model explaining how a few big grains in the beads are responsible for the whole coupling. The robustness of this model lies in its generalization to several sizes of beads and its consistency with the properties of the ferrofluid used to make them.

In a second part, the reaction of a few actin filaments to the pressure applied by superparamagnetic colloids is studied. This setup represents a direct way to probe the physical mechanism underlying the production of force by actin filaments polymerization in cellular motility. Controlling the filaments’ organization allows to study an ideal case where the filaments fluctuate a lot and an opposite ideal case where they are very stiff. We experimentally show that the former situation can be described by the entropic model because the length of the filaments is independent of the applied force. This is no longer true in the latter situation.

Thus, we experimentally show for the first time the effect of force on actin filaments polymerization. A Brownian ratchet based numerical model with no load sharing can describe the production of force in that situation.