Fusion modifies the material properties of membranes

Membrane fusion is a ubiquitous process in biology and potentially an alternative to endocytic internalization pathways in drug delivery. Due to its high complexity, assays have been developed to disentangle its key aspects, although many of its molecular mechanisms remain elusive. Here, we use a combination of multicolour and time-resolved confocal fluorescence microscopy to study the effects of membrane mechanics on fusion and conversely, how fusion alters membrane mechanics. As a minimal fusion system, we used cationic large unilamellar fusogenic liposomes (LUVs) that rapidly fuse with giant unilamellar vesicles (GUVs) as evidenced by the probes being delivered into the GUVs. Fusion is favoured in fluid membranes and hampered upon an increase in packing. Extensive fusion of a large number of LUVs leads to the creation of area leaflet asymmetry in GUVs, resulting in budding formation driven by spontaneous curvature. In these conditions, the membrane becomes perforated as a result of a decrease in edge tension and the size of the formed pores seems to scale with LUV concentration. Here, the GUVs also display a range of permeability states. In live human embryonic kidney (HEK) cells, lipid mixing and intracellular delivery of LUV encapsulated water-soluble probes are observed, consistent with fusion of the liposomes with the plasma membrane. Furthermore, when exposed to liposomes, cell membrane permeabilization and cell death is observed at liposomal concentrations an order of magnitude higher than pore formation in the GUVs, suggesting an increased resilience of cells compared to GUVs. Fusion also seems to fluidize the membrane and effects are observed within minutes. Overall, the results shed new light onto the effects of liposome-membrane interactions.