Topological connection between vesicles and nanotubes in single-molecule lipid membranes driven by head-tail interactions

TL;DR

This study demonstrates the formation of topologically-connected vesicles and nanotubes in single-molecule lipid membranes driven by head-tail interactions of an anisotropic glucolipid, without external directional forces, above its melting temperature.

Contribution

It reveals a novel mechanism for vesicle-nanotube connectivity in single-molecule membranes based on lipid configuration, expanding understanding of membrane topology.

Findings

Connected vesicles and nanotubes observed between 60°C and 90°C

Glucolipid can adopt head-head and head-tail configurations

Connectivity persists without external directional forces

Abstract

Lipid nanotube-vesicle networks are important channels for intercellular communication and transport of matter. Experimentally observed in neighboring mammalian cells, but also reproduced in model membrane systems, a broad consensus exists on their formation and stability. Lipid membranes must be composed of at least two molecular components, each stabilizing low (generally a phospholipid) and high curvatures. Strong anisotropy or enhanced conical shape of the second amphiphile is crucial for the formation of nanotunnels. Anisotropic driving forces generally favor nanotube protrusions from vesicles. In the present work, we report the unique case of topologically-connected nanotubes-vesicles obtained in the absence of directional forces, in single-molecule membranes, composed of an anisotropic bolaform glucolipid, above its melting temperature, Tm. Cryo-TEM and fluorescence confocal microscopy show the interconnection between vesicles and nanotubes in a single-phase region, between 60{\textdegree} and 90{\textdegree}C under diluted conditions. Solid-state NMR demonstrates that the glucolipid can assume two distinct configurations, head-head and head-tail. These arrangements, seemingly of comparable energy above the Tm, could explain the existence and stability of the topologically-connected vesicles and nanotubes, which are generally not observed for classical single-molecule phospholipid-based membranes above their Tm.