Interaction of Calcium Ions with Lipid Membranes
Abstract: Bilayer membranes enclose and shield the biological cell and its inner compartments, as well as the tubular networks that exist within and between the cells. Due to their fluidic nature, the membranes are incredibly dynamic and flexible, which allows them to bend, reshape and fuse in response to mechanical and chemical stimuli within their natural microenvironments. Variations in calcium ion concentration are of particular importance for chemical stimulation, as calcium ions play a major role in many cellular processes, including signaling, proliferation, cell division, migration, and exocytosis. Previous studies with lipid membranes showed that binding of calcium ions to the membrane results in dehydration of the lipid head groups, ordering of the hydrocarbon tails, and in the increase of membrane rigidity and tension. At the same time, direct membrane remodeling upon stimulation with calcium ions remains largely unknown. In this thesis, giant unilamellar vesicles (GUVs) and nanotubes were used as model systems of cellular membranes. These lipid membranes were subjected to variations in local calcium ion concentration close to the membrane surface using the microinjection technique and imaged with fluorescence microscopy. The results of our studies on the membrane model systems provide evidence for formation of highly curved membrane structures such as membrane tubular protrusions in GUVs (Paper I, II), and lipid aggregates (bulges) in lipid nanotubes (Paper III), as consequences of controlled calcium ion exposure, which cannot be obtained under bulk conditions. It is also possible to move these highly curved membrane structures by repositioning the source of the calcium ion gradients along the GUV surfaces. Our findings demonstrate how elevated calcium ion concentration close to the GUV surface can trigger membrane remodeling, and how calcium gradients can be used to manipulate and guide lipid-based systems. It is further shown that calcium ion gradients can trigger directed movement and reorientation of phase-separated free-floating GUVs towards the calcium ion source (Paper IV), suggesting possible aspects of protocell migration due to changes in the chemical microenvironment. Lastly, in Paper V, we developed a technique to produce artificial intracellular vesicles (AIVs) inside a GUV. The formation of AIVs occurred upon localized microinjection of calcium solution using a glass micropipette, which was placed in direct contact with the outer surface of the GUV membrane. The AIVs can be used to study mechanisms of exo- and endocytosis, and due to the capability to entrap high concentrations of proteins within the AIVs, the findings bear potential for drug delivery applications.
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