Membrane tension increases fusion efficiency of model membranes in the presence of SNAREs

Abstract

The large gap in time scales between membrane fusion occurring in biological systems during neurotransmitter release and fusion observed between model membranes has provoked speculations over a large number of possible factors that might explain this discrepancy. One possible reason is an elevated lateral membrane tension present in the presynaptic membrane. We investigated the tension-dependency of fusion using model membranes equipped with a minimal fusion machinery consisting of syntaxin 1, synaptobrevin and SNAP 25. Two different strategies were realized; one based on supported bilayers and the other one employing sessile giant liposomes. In the first approach, isolated patches of planar bilayers derived from giant unilamellar vesicles containing syntaxin 1 and preassembled SNAP 25 (ΔN-complex) were deposited on a dilatable PDMS sheet. In a second approach, lateral membrane tension was controlled through the adhesion of intact giant unilamellar vesicles on a functionalized surface. In both approaches fusion efficiency increases considerably with lateral tension and we identified a threshold tension of 3.4 mN m−1, at which the number of fusion events is increased substantially.

Directed Growth of Biomimetic Microcompartments

“…[106] Furthermore, fusion methods have been expanded to biomimetic strategies [88,107] in addition to more exotic approaches, involving the use of light (to heat nanoparticles [108] or isomerize azocompounds [109] ) or electric fields [88,110,111] to perturb and porate the bilayer. [105,116,117] However, possibly due to the low fusion efficiency of protein-reconstituted systems or the low protein density, vesicle growth has not been detected. [110] Presumably, the simplest mechanism to induce membrane fusion is based on membrane tension as has been elucidated by molecular simulations.…”

Reciprocal link between cell biomechanics and exocytosis

“…Mathematical simulation of small vesicles (28 nm diameter) fusing to planar membrane shows that high tension increase membrane fusion rate and the tensions on both membranes should both be high enough to induce a successful fusion . A study using model membrane fusion machinery consisting of syntaxin 1, VAMP2 and SNAP25, shows that vesicles have a higher fusion rate to an acceptor membrane having increased in‐plane tension, either by stretching a planar lipid bilayer on a dilatable polydimethylsiloxane (PDMS) sheet or stretching a giant unilamellar vesicle (GUV) by docking onto a flat surface …”

“…60,61 A study using model membrane fusion machinery consisting of syntaxin 1, VAMP2 and SNAP25, shows that vesicles have a higher fusion rate to an acceptor membrane having increased in-plane tension, either by stretching a planar lipid bilayer on a dilatable polydimethylsiloxane (PDMS) sheet or stretching a giant unilamellar vesicle (GUV) by docking onto a flat surface. 62 Therefore, at the nanoscopic scale of SNARE-mediated membrane fusion, membrane tension is likely a force counteracting pore opening. The current state of the art suggests that the number of SNARE complexes would regulate pore opening.…”

Highly Efficient Protein-free Membrane Fusion: A Giant Vesicle Study

“…Surprisingly, despite these advantages, not many studies report the use of GUVs in investigating processes associated with membrane fusion. These relatively few studies include the first direct visualization of a peptide-induced fusion (30) and formation of a hemifusion diaphragm (31), heated nanoparticle-mediated fusion (32), resolving the very fast nature of the fusion neck expansion on the order of cm/s (10) with rates depending on membrane properties (33), the role of regulatory proteins on SNARE-mediated fusion (34), and the effect of charge (35), multivalent ions (36), tension (37), and pH (38) on membrane fusion.…”