Lipid rafts as a membrane organizing principle
Cell membranes have developed a tremendous complexity of lipids and proteins geared to perform the functions cells require. To coordinate these functions, the bilayer has evolved the propensity to segregate its constituents laterally. This capability is based on dynamic liquid-liquid immiscibility and underlies the raft concept of membrane sub-compartmentalization. Key to understanding the principles underlying liquid-liquid de-mixing in cell membranes is the mutual weak interactions between sterols, sphingolipids and raft proteins. The potential for sphingolipid-cholesterol self-assembly combines with protein specificity to dynamically regulate protein segregation within the membrane plane. This mechanism is employed in regulating endocytic or exocytic membrane transport, in transducing specific signals across the plasma membrane or to perform different biochemical reactions dependent on the proteins involved. The regulation of the 2-dimensional separation of lipids and proteins in membranes into dynamic liquid membrane rafts, separating from the surrounding bilayer, is dependent on the propensity for liquid phase separation. Cellular plasma membranes seem to be poised close to a phase transition, facilitating dynamic sub-compartmentalization with little energetic cost. Liquid phase transitions are not confined to cell membranes. Liquid phase transitions are emerging as a general principle driving cellular organisation.
For membranes to be sub-compartmentalized, the lipid and the protein composition have to be tightly regulated. Together with the Shevchenko lab, we have developed a shotgun mass spectrometry platform that now can analyse hundreds of lipids in only a few minutes with absolute quantification that has now be moved into a start-up company, Lipotype (www.lipotype.com). This effective routine of shotgun lipidomics became possible by introducing a unique workflow that combined improved extraction protocols with cutting edge mass spectrometry and novel software. We have used this technology to analyse blood cells and plasma with the aim to establish multi-parametric lipid signatures that are of diagnostic value.