Giant unilamellar vesicles as a model system for studying ion transport

Cellular activity depends on constant flux of ions across biological membranes. Artificial membrane models like planar lipid bilayers and liposomes are ideal for studying membrane transport phenomena as they are free of the structural complexity of cells and allow examination of transport processes under tightly controlled conditions. Over the last decades, artificial membrane-based techniques like single-channel recording and fluorescent monitoring of transport through bulk lipid vesicle suspensions have revealed many molecular mechanisms of transport. Recently, giant unilamellar vesicles (GUVs), cell-sized liposomes, have emerged as an important tool for studying cellular processes, including ion transport. The principal advantage of GUVs derives from their micron scale, which enables ease of visualisation and manipulation using microscopy and microhandling. For that reason, GUVs have also become the state-of-the-art for recapitulating a host of cell structures and functions for the purpose of developing artificial cells. Taken together, GUVs represent a promising biomimetic system to elucidate ion transport mechanisms and unravel the association between ion fluxes and various cellular processes such as neuronal transduction, nutrient uptake, electrochemical gradient development. Nevertheless, despite their great potential as a model system, the use of GUVs in ion transport studies is still limited. The aim of this review is to outline recent GUV-based ion transport studies, describe the current techniques for measuring ion transport in GUVs, compare the utility of GUVs relative to other available techniques such as single-channel current recording, and explore the potential of using GUVs to investigate complex ion transport processes.