Laboratoire Matière et Systèmes Complexes

MSC : Miglena Angelova, Nicolas Puff, Jean-Baptiste Fournier, Anne-Florence Bitbol.

Studies of the physiology of living cells have revealed a large number of events based on the interaction of biological membranes (plamic and intra-cellular) with endogenic active drugs or exogenic drugs delivered locally on a small part of the membrane. These active molecules, or chemical agents, are probably able to modulate in an inhomogeneous way the lateral and transversal distributions of the various membrane constituents (lipids, proteins, polysaccharides), inducing specific phenomena that cannot be expected from an environment where only homogeneous bulk interactions would be present. Thus, by locally applying a drug capable to adsorb onto the membrane or to get inserted into it, or by locally applying an enzyme transforming the molecules forming the membrane, it is possible to locally modify the mechanical equilibrium of the membrane. The study of the resulting shape instabilities open new perspectives for the understanding of the relationship between structure and function in biological membranes. For example, we have discovered a new type of curvature instability, which is able to trigger the ejection of a tubule growing exponentially toward a chemical source (1). This dynamical instability is caused by a local dilation of one of the two monolayers, that relaxes slowly because of the inter-monolayer friction. Precisely, the mechanism is the following. A local chemical modification of the lipids is induced in one of the two monolayers by the local injection of a NaOH solution at pH 13. It increases the preferred area per lipid (and/or modifies the spontaneous curvature of the lipids), inducing a local and transient curvature of the membrane. Because only one monolayer is affected, the lateral redistribution of the lipids of the exposed monolayer is strongly slowed down by the intermonolayer friction. Hence, after a rapid expansion of both monolayers, the instability creates a large deformation, which finally relaxes once the exposed monolayer attains its equilibrium density. When applied onto a sufficiently small region, this instability develops a non-linearity that produces the ejection of a tubule. Amazingly, the tubule aims at the chemical source. A dynamical model of the deformation in the linear regime allows to fit the height of the instability versus time and to deduce a measurement of the intermonolayer friction coefficient.

(1) J.-B. Fournier, N. Khalifat, N. Puff end M. I. Angelova. Phys. Rev. Lett. 101, 018102 (2008)