Infoscience

Thesis

Membrane elastic heterogeneity studied at nanometrical scale on living cells

The aim of this thesis was to explore the cell mechanical properties using the Atomic Force Microscope (AFM). The cell membrane contains lipids microdomains, called rafts, enriched in cholesterol and sphingolipids. The rafts are believed to play an important role in signal processing by acting as a "signaling platform". Indeed, membrane proteins involved in signal transduction concentrates into these rafts and are coupled with signaling pathways inside the cell. The mechanical properties of these rafts were characterized by targeting one of its component, the glycosylphosphatidylinositol-anchored protein (GPI-AP). During this work, we found these domains to be stiffer than the surrounding membrane. Several control experiments were performed to consolidate this finding. The extraction of cholesterol, one of the major component of raft, was shown to dramatically reduce the stiffness of the raft to reach the surrounding membrane value. The stiffness specificity of rafts may be related to the lower diffusion rate of proteins and can be, therefore, an important property for its role as a signaling platform. During this thesis, we also introduced a new AFM imaging mode, which we called "stiffness tomography". With this imaging mode, we were able to distinguish stiff materials inclusion located into the sample. Different control experiments were done to validate this imaging mode. A virtual experiment was performed with the help of the finite element modeling. This permitted us to validate our methodology, but also pointed us its limitations. The stiffness tomography was also used on living cells and showed significant differences between native and cytoskeleton depolimerized cells. Since no postprocessing tool was available at the beginning of this work, the software development was a very significant part of the project. Its development resulted in a toolbox (a collection of function), that is available for future software development. A non negligible part of the development consisted in the toolbox documentation that is reported in the appendix C. This software permitted to process force volume AFM les and to characterize the elastic properties of the cell membrane with a high precision and reliability.

Related material

Contacts

EPFL authors