LPTMS PhD Proposal: Mechanical response of branched actin networks

Responsable: Martin Lenz 01 69 15 32 62

The architecture of living cells is largely determined by a microscopic networks of semiflexible
filaments : the actin cytoskeleton. In addition to ensuring the cell’s mechanical integrity, its growth
enables cellular motion and force exertion. These crucial roles are played by so-called branched actin
networks, which are random fractal assemblies of filaments and branching points.

Despite its importance within the cell, the rigidity of these networks is not understood from a
theoretical standpoint. Indeed, taking into account the sole rigidity of the filaments and attachment
points, we would predict a vanishing elastic modulus, in contradiction with experiments. We will examine
the origin of these networks’ rigidity, considering in particular the effects of the entanglement of the
network with itself, which generates nonlocal interactions between the points of the elastic network.
Given the difficulty of treating such interactions exactly, we will resort to mean-field approaches whose
validity will be assessed numerically.

From an experimental perspective, our collaborators Olivia du Roure and Julien Heuvingh
(ESPCI) operate a setup allowing the first clean characterization of the branched network. We will work
with them to relate our models to the characteristics of a network grown under force (similar to in vivo
conditions), its nonlinear elasticity etc.

Informal inquiries welcome.