Several common methods of making vesicles (except an emulsion method) capture intended lipid ratios

Abstract

Researchers choose different methods of making giant unilamellar vesicles to satisfy different constraints of their experimental designs. A challenge that arises when researchers use a variety of methods is that each method may produce vesicles with a different average lipid ratio, even if all experiments use lipids from a common stock mixture. Here, we use mass spectrometry to investigate ratios of lipids in vesicle solutions made by five common methods: electroformation on indium tin oxide slides, electroformation on platinum wires, gentle hydration, emulsion transfer, and extrusion. We made vesicles from either five-component or binary mixtures of lipids chosen to span a wide range of physical properties: di(18:1)PC, di(16:0)PC, di(18:1)PG, di(12:0)PE, and cholesterol. For a mixture of all five of these lipids, ITO electroformation, Pt electroformation, gentle hydration, and extrusion methods result in only minor shifts in lipid ratios (≤5 mol %) relative to a common stock solution. In contrast, emulsion transfer results in ∼80% less cholesterol than expected from the stock solution, which is counterbalanced by a surprising overabundance of saturated PC-lipid relative to all other phospholipids. Experiments using binary mixtures of saturated and unsaturated PC-lipids and cholesterol largely support results from the five-component mixture. In general, our results imply that experiments that increment lipid ratios in small steps will produce data that are highly sensitive to the technique used and to sample-to-sample variations. For example, sample-to-sample variations are ∼±2 mol % for five-component vesicles produced by a single technique. In contrast, experiments that explore larger increments in lipid ratio or that seek to explain general trends and new phenomena will be less sensitive to sample-to-sample variation and the method used.