Cholesterol‐Containing Liposomes Decorated With Au Nanoparticles as Minimal Tunable Fusion Machinery

Abstract

Membrane fusion is essential for the basal functionality of eukaryotic cells. In physiological conditions, fusion events are regulated by a wide range of specialized proteins, operating with finely tuned local lipid composition and ionic environment. Fusogenic proteins, assisted by membrane cholesterol and calcium ions, provide the mechanical energy necessary to achieve vesicle fusion in neuromediator release. Similar cooperative effects must be explored when considering synthetic approaches for controlled membrane fusion. We show that liposomes decorated with amphiphilic Au nanoparticles (AuLips) can act as minimal tunable fusion machinery. AuLips fusion is triggered by divalent ions, while the number of fusion events dramatically changes with, and can be finely tuned by, the liposome cholesterol content. We combine quartz‐crystal‐microbalance with dissipation monitoring (QCM‐D), fluorescence assays, and small‐angle X‐ray scattering (SAXS) with molecular dynamics (MD) at coarse‐grained (CG) resolution, revealing new mechanistic details on the fusogenic activity of amphiphilic Au nanoparticles (AuNPs) and demonstrating the ability of these synthetic nanomaterials to induce fusion regardless of the divalent ion used (Ca2+ or Mg2+). The results provide a novel contribution to developing new artificial fusogenic agents for next‐generation biomedical applications that require tight control of the rate of fusion events (e.g., targeted drug delivery).

Nanoparticle-induced biomembrane fusion: unraveling the effect of core size on stalk formation

“…Indeed, a recent study by Tahir et al demonstrated that MUS:OT NPs induce fusion between pure phosphatidylcholines (PC) liposomes, 17 and our group showed that the liposome cholesterol content can nely tune MUS:OT NP-induced fusion events. 18 Molecular Dynamics (MD) simulations, thanks to their high spatiotemporal resolution, have been successfully employed to shed light on the molecular mechanisms involved in the different stages of the fusion process, either spontaneous [19][20][21][22] or mediated by fusion agents. 17,[23][24][25] The presence of high free energy barriers makes the use of atomistic models challenging and calls for coarse-grained models and/or enhanced sampling techniques.…”

“…9 Here we build on the original denition of the x ch chain coordinate, modifying it to account for the presence of the fusogenic agent, a MUS:OT Au NP embedded in one of the two lipid bilayers. Our previous MD simulations based on a coarsegrained force eld revealed that the stalk rim constantly forms over a single Au NP, 18 consequently to the formation of a hydrophobic contact between a lipid tail of the top bilayer and transient hydrophobic defects on the NP, water-facing surface. This mechanism is consistent with the one described in the literature for NP-free membranes, with the only difference being that the contact is a lipid-NP one and not a lipid-lipid one.…”

“…Indeed, a recent study by Tahir et al demonstrated that MUS:OT NPs induce fusion between pure phosphatidylcholines (PC) liposomes, 17 and our group showed that the liposome cholesterol content can finely tune MUS:OT NP-induced fusion events. 18 …”

“…Thanks to coarse-grained modelling, spontaneous NP-induced stalk formation has been observed in unbiased MD, but only increasing the temperature much above the physiological one. 18 Therefore, the derivation of the free energy profile of stalk formation requires enhanced sampling, and the challenge is the definition of a collective variable (CV) that approximates well the reaction coordinate. Finding a CV suitable to describe stalk formation is arduous because of the high number of molecules involved and their high conformational freedom, and a poor CV could easily lead to hysteresis in the free energy calculation along the stalk-formation and stalk-destruction pathways.…”

Nanoparticle induced fusion of lipid membranes

“…[9][10][11][12] Most of these studies use functionalized gold nanoparticles. Though these nanoparticles can induce stalk formation, they are not capable to promote pore opening by themselves, and they use external factors such as addition of calcium 9,10 or heating of the nanoparticle 12 to continue the fusion process. Silica nanoparticles have also been used for membrane fusion, and while they can facilitate stalk and hemifusion diaphragm formation, they do not catalyze the formation of a fusion pore.…”