24 October 2003. In the complex milieu of the cell membrane, some lipids often go against the flow and separate from the main pack to form their own small patch. Such lipid rafts, as they are known, are often packed with cholesterol and have been associated with specialized membrane processes, such as the cleavage of amyloid-β precursor protein (see ARF related news story). However, little is known about how these rafts behave in time and space.
In yesterday's Nature, Tobias Baumgart, working under the direction of Watt Webb at Cornell University, Ithaca, New York, together with Samuel Hess at the National Institute of Child Health and Human Development, Bethesda, Maryland, shine some light on the fluid dynamics of biomembranes. Baumgart and colleagues used two-photon microscopy to visualize lipids that separate into coexisting liquid phases, or domains, in giant unilamellar vesicles. The latter, made of just three lipids—sphingomyelin, cholesterol, and dioleoylphosphatidylcholine (DOPC)—provide a relatively simple membrane model, which nonetheless recapitulates the phase separations, or rafting, seen in more complex membranes.
Baumgart used two different fluorescent dyes to visualize the phase separations. Using perylene, which dissolves in the sphingomyelin/cholesterol phase, and rho-DPPE, which partitions in the DOPC phase, he exquisitely demonstrates stark boundaries between domains with a resolution high enough to reveal how the curvature at the boundary interfaces reverses. In addition, the two-photon microscope uncovers domains that appear to be connected together in a complex order that can adopt a variety of patterns, including parallel stripes and equatorial bands that seem to circle the vesicles. Two-dimensional images taken at five micrometer intervals and stacked to build three-dimensional pictures, show that the lipids fall into hexagonal arrays of circular domains that are also suggestive of a high degree of spatial order.
The results suggest that lipid rafts formed in biological membranes may adopt non-random patterns that belie the aimless floating that the name "raft" suggests. The experimental approach used by the authors may, as they note, be useful to test the influence of additives on lipid domains and probe fundamental membrane processes, such as those that depend on rafting.—Tom Fagan.
Baumgart T, Hess ST, Webb WW. Imaging coexisting fluid domains in biomembrane models coupling curvature and line tension. Nature. 2003 Oct 23;425(6960):821-4. Abstract