Model Lipid Bilayer - Vesicles

Vesicles

A vesicle is a lipid bilayer rolled up into a spherical shell, enclosing a small amount of water and separating it from the water outside the vesicle. Because of this fundamental similarity to the cell membrane, vesicles have been used extensively to study the properties of lipid bilayers. Another reason vesicles have been used so frequently is that they are relatively easy to make. If a sample of dehydrated lipid is exposed to water it will spontaneously form vesicles. These initial vesicles are typically multilamellar (many-walled) and are of a wide range of sizes from tens of nanometers to several micrometres. Methods such as sonication or extrusion through a membrane are needed to break these initial vesicles into smaller, single-walled vesicles of uniform diameter known as small unilamellar vesicles (SUVs). SUVs typically have diameters between 50 and 200 nm. Alternatively, rather than synthesizing vesicles it is possible to simply isolate them from cell cultures or tissue samples. Vesicles are used to transport lipids, proteins and many other molecules within the cell as well as into or out of the cell. These naturally isolated vesicles are composed of a complex mixture of different lipids and proteins so, although they offer greater realism for studying specific biological phenomena, simple artificial vesicles are preferred for studies of fundamental lipid properties.

Since artificial SUVs can be made in large quantities they are suitable for bulk material studies such as x-ray diffraction to determine lattice spacing and differential scanning calorimetry to determine phase transitions. Dual polarisation interferometry can measure unilamelar and multilamelar structures and insertion into and disruption of the vesicles in a label free assay format. Vesicles can also be labeled with fluorescent dyes to allow sensitive FRET-based fusion assays. In spite of this fluorescent labeling it is often difficult to perform detailed imaging on SUVs simply because they are so small. To combat this problem researchers have developed the giant unilamellar vesicle (GUV). GUVs are large enough (several tens of micrometres) to study with traditional fluorescence microscopy. Many of the studies of lipid rafts in artificial lipid systems have been performed with GUVs for this reason. Compared to supported bilayers, GUVs present a more “natural” environment since there is no nearby solid surface to induce defects or denature proteins. However, GUVs are relatively fragile, time consuming to make and can only be produced in limited yield compared to SUVs.

To circumvent these problems a microfluidic assembly line approach to GUVs was reported.