Spherical Nanovesicles Transform into a Multitude of Nonspherical Shapes

Abstract

Nanovesicles are closed, bubblelike surfaces with a diameter between 20 and 200 nm, formed by lipid bilayers and biomembranes. Electron microscopy (EM) studies have shown that these vesicles can attain both spherical and nonspherical shapes. One disadvantage of EM methods is that they provide only a single snapshot of each vesicle. Here, we use molecular dynamics simulations to monitor the morphological transformations of individual nanovesicles. We start with the assembly of spherical vesicles that enclose a certain volume of water and contain a certain total number of lipids. When we reduce their volume, the spherical vesicles are observed to transform into a multitude of nonspherical shapes such as oblates and stomatocytes as well as prolates and dumbbells. This surprising polymorphism can be controlled by redistributing a small fraction of lipids between the inner and outer leaflets of the bilayer membranes. As a consequence, the inner and the outer leaflets experience different mechanical tensions. Small changes in the vesicle volume reduce the overall bilayer tension by 2 orders of magnitude, thereby producing tensionless bilayers. In addition, we show how to determine, for a certain total number of lipids, the unique spherical vesicle for which both leaflet tensions vanish individually. We also compute the local spontaneous curvature of the spherical membranes by identifying the first moment of the spherically symmetric stress profiles across the lipid bilayers with the nanoscopic torque as derived from curvature elasticity. Our study can be extended to other types of lipid membranes and sheds new light on cellular nanovesicles such as exosomes, which are increasingly used as biomarkers and drug delivery systems.

Remodeling of Membrane Shape and Topology by Curvature Elasticity and Membrane Tension

“…These shape transformations of nanovesicles are, however, accessible to molecular dynamics simulations. [ 16,17,41–45 ] Here, we focus on two recent simulations [ 16,17 ] which revealed that the leaflet tensions and the concentrations of small solutes, which adsorb onto the membranes, represent important control parameters for the nanovesicle morphologies, see Figure 7 and Section 7.3 further below. In particular, these studies identified parameter values for the formation of membrane necks.…”

“…Budding of nanovesicles as observed in molecular dynamics simulations. [ 16 ] Each snapshot displays half a vesicle, with the cross‐section across the hydrophobic core of the bilayer membrane in grey and the lipid head groups in green. The budding transformations were induced by decreasing the volume v of the two spherical vesicles with v = 1.…”

“…Reproduced under terms of the CC‐BY license. [ 16 ] Copyright 2019, The Authors, published by American Chemical Society.…”

“…These shape transformations of nanovesicles are, however, accessible to molecular dynamics simulations as has been recently demonstrated. [ 16,17 ]…”

Membrane Vesicle Production as a Bacterial Defense Against Stress

“…Notably, when the volume of spherical vesicles is experimentally reduced, mimicking osmotic deflation, the vesicles undergo greatly differing shape transformations. This polymorphism is due to the redistribution of a small fraction of the lipids between the inner and outer leaflets of the bilayered membranes thereby, reduce the overall bilayer tension and, produce tensionless bilayers (Ghosh et al, 2019).…”