Macroscopic Response and Manipulation of Lipid Nanotubes with Calcium Ion Gradient

Calcium ions are involved in many cellular processes such as signaling, gene transcription, muscle contraction, cell division, and proliferation. Their interaction with cellular components and lipid membranes in particular, has been a subject of an extensive research. Herein we present results of investigation of Ca2+ interaction with lipid membranes, focusing on tubular protrusions. We demonstrate that calcium ion gradient, established along the lipid nanotubes, induces movement of lipids towards the area of highest calcium concentration. Upon initial introduction of calcium to the vicinity of the nanotube, we observe an intensity change at the site of application, indicative of lipid transport, resulting in expansion of the tube. Continued application of calcium or termination of calcium supply induces transformation of this expanded lipid nanotube tube region into prolate non-hollow lipid bulge. This bulge can be relocated along the lipid nanotube in a controlled manner by relocating Ca2+ source location. The speed of relocation of the bulge depends on speed of ion point source as well as ion concentration. Furthermore, we present the possibility of cargo entrapment (e.g. fluorescent beads) in these lipid bulges. The controlled relocation of bulges with entrapped cargo, presents perspectives for directed and contactless transport in surfactant systems in response to positive ions. In addition, interactions of calcium with tubular protrusions pulled from cellular plasma membrane were studied. We observed similar processes of lipid movement towards highest Ca2+ concentration as in artificial lipid system. However, bulge structure and lipid movement kinetics appear to be distinct from biomimetic lipid model system, likely due to different lipid membrane composition.