Lipid Nanotubes as a Model for Highly Curved Cellular Membrane Structures

Doctoral thesis, 2013

Cells and their organelles show a variety of membrane morphologies with multiple submicrometer features, for example, tubules, vesicles, folds and pores. The shape of the cellular membranes can dynamically change to support a variety of functions, such as cargo transport, transmission of signals between the cells, cell movement and division. A convenient route to understanding the complexity of cellular membranes is to study artificially created lipid bilayer membrane systems. The work presented in this thesis is focused on highly curved membrane structures in the form of lipid bilayer nanotubes. Firstly, the shape transformation mechanism for free floating lipid nanotubes was investigated. Driven by their high curvature energy, nanotubes contract in length and eventually transform into tubular stomatocyte-like structures. Secondly, diffusion, electric field and Marangoni-flow-driven modes of transport through lipid nanotubes are described. Then, an important improvement in the characterization of lipid nanotubes was achieved by developing a new technique for the measurement of lipid nanotube radii. This technique is based on monitoring the translocation of a photobleached tube region between two nanotube-connected vesicles during the growth of a receiving/daughter vesicle. The validity of this measurement technique was confirmed using super resolution microscopy. In addition, our technique has proven useful for tracking membrane bending rigidity changes in response to environmental and compositional alterations, both in cell plasma membranes and in model vesicle systems. Finally, a microfluidic pipette with a self-confining volume at the tip was presented. It allows for selectively affecting a chosen cell and accessing membranes on the single cell level.