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27.10.2017

A better artificial shell for cells

Max Planck researchers managed to create lipid vesicles that mimic the multifaceted natural cell membranes and thus may soon be used to study the inner workings of cells.

Cell membranes have to fulfill a multitude of requirements and are thus notoriously hard to mimic. Max Planck researchers now managed to artificially create lipid vesicles that could become the new model system to study cellular processes.
Copyright: 
Weiss et al, Nature Materials 2017

Cell membranes are an essential part of all animal or plant cells. They facilitate waste and nutrient exchange between the interior and the exterior of the cell while also protecting it from external influences and confining the contents of the cell to a stable internal environment. All of this is achieved by a seemingly simple double layer of fatty acid molecules. In contrast, while artificial cells are also provided with a shell of fat molecules, until now, they have usually been too unstable and non-porous. Consequently, cellular mechanisms cannot yet be modelled in these artificial cells. In the journal Nature Materials scientists from the MaxSynBio network, the Max Planck Society and the Universities of Heidelberg, Jena, Magdeburg, and Bordeaux describe how they were able to create lipid vesicles that could soon function as a model system for studying the internal processes of natural cells.

Polymer droplets: oily on the outside, aqueous on the inside

To mimic the structure and composition of natural cells, the researchers used droplets made from long-chain organic molecules known as amphiphilic polymers, which act like surfactants. These droplets consist of an outer layer of perfluorinated polyether and an inner layer of water soluble polyethylene glycol to which gold nanoparticles have been attached. The difference in solubility between the inner and outer layer means that the droplets float in an oil-containing medium, while retaining an aqueous solution in their interior. “The lipid vesicles that this produces are mechanically and chemically stable, allowing us to inject proteins into them, as in natural cells,” says Joachim Spatz from the Max Planck Institute for Medical Research in Heidelberg.

Vesicles can be populated with proteins

Using a micro-injection system, the researchers were able to inject tiny lipid vesicles into the polymer droplets. Adding magnesium causes the vesicles inside the droplets to dissipate and merge to form a single lipid layer on the inside of the droplet. Subsequently, the researchers were able to inject precisely controlled quantities of cellular proteins into the polymer-lipid vesicles by using a picoinjection system especially developed for this purpose. “Using this technique, we are able to populate up to 1000 vesicles per second with proteins – cytoskeletal proteins like actin and tubulin or the transmembrane protein integrin. This means we can quickly obtain enough vesicles for biological or medical analysis,” explains Spatz. The scientists then remove the surfactant shell and transfer the lipid vesicles to an aqueous solution. The vesicles can, for example, then be made to interact with natural cells.

However, the new technique is not just limited to helping develop artificial cells. It also offers a simple model system that is quick to manufacture and can be used to study interactions with signalling molecules on other cells or viruses.

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