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Abstract
Membrane fusion is a ubiquitous process in biology and is a prerequisite for many
intracellular delivery protocols relying on the use of liposomes as drug carriers.
Here, we investigate in detail the process of membrane fusion and the role of opposite
charges in a protein-free lipid system based on cationic liposomes (LUVs, large unilamellar
vesicles) and anionic giant unilamellar vesicles (GUVs) composed of different palmitoyloleoylphosphatidylcholine
(POPC)/palmitoyloleoylphosphatidylglycerol (POPG) molar ratios. By using a set of
optical-microscopy- and microfluidics-based methods, we show that liposomes strongly
dock to GUVs of pure POPC or low POPG fraction (up to 10 mol%) in a process mainly
associated with hemifusion and membrane tension increase, commonly leading to GUV
rupture. On the other hand, docked LUVs quickly and very efficiently fuse with negative
GUVs of POPG fractions at or above 20 mol%, resulting in dramatic GUV area increase
in a charge-dependent manner; the vesicle area increase is deduced from GUV electrodeformation.
Importantly, both hemifusion and full fusion are leakage-free. Fusion efficiency is
quantified by the lipid transfer from liposomes to GUVs using fluorescence resonance
energy transfer (FRET), which leads to consistent results when compared to fluorescence-lifetime-based
FRET. We develop an approach to deduce the final composition of single GUVs after
fusion based on the FRET efficiency. The results suggest that fusion is driven by
membrane charge and appears to proceed up to charge neutralization of the acceptor
GUV.