Membrane properties of structurally modified ceramides : effects on lipid lateral distribution and sphingomyelin-interactions in artificial bilayer membranes

摘要

Ceramides comprise a class of sphingolipids that exist only in small amountsin cellular membranes, but which have been associated with important roles incellular signaling processes. The influences that ceramides have on the physicalproperties of bilayer membranes reach from altered thermodynamical behaviorto significant impacts on the molecular order and lateral distribution ofmembrane lipids. Along with the idea that the membrane physical state couldinfluence the physiological state of a cell, the membrane properties of ceramideshave gained increasing interest. Therefore, membrane phenomena related toceramides have become a subject of intense study both in cellular as well as inartificial membranes. Artificial bilayers, the so called model membranes, aresubstantially simpler in terms of contents and spatio-temporal variation thanactual cellular membranes, and can be used to give detailed information aboutthe properties of individual lipid species in different environments.This thesis focuses on investigating how the different parts of the ceramidemolecule, i.e., the N-linked acyl chain, the long-chain sphingoid base and themembrane-water interface region, govern the interactions and lateraldistribution of these lipids in bilayer membranes. With the emphasis onceramide/sphingomyelin(SM)-interactions, the relevance of the size of the SMheadgroup for the interaction was also studied. Ceramides with methylbranchedN-linked acyl chains, varying length sphingoid bases, or methylated 2N(amide-nitrogen) and 3O (C3-hydroxyl) at the interface region, as well as SMswith decreased head group size, were synthesized and their bilayer propertiesstudied by calorimetric and fluorescence spectroscopic techniques. In brief, theresults showed that the packing of the ceramide acyl chains was more sensitive tomethyl-branching in the mid part than in the distal end of the N-linked chain,and that disrupting the interfacial structure at the amide-nitrogen, as opposed tothe C3-hydroxyl, had greater effect on the interlipid interactions of ceramides.Interestingly, it appeared that the bilayer properties of ceramides could be moresensitive to small alterations in the length of the long-chain base than what waspreviously reported for the N-linked acyl chain. Furthermore, the data indicatedthat the SM-head group does not strongly influence the interactions betweenSMs and ceramides.The results in this thesis illustrate the pivotal role of some essential parts ofthe ceramide molecules in determining their bilayer properties. The thesisprovides increased understanding of the molecular aspects of ceramides thatpossibly affect their functions in biological membranes, and could relate todistinct effects on cell physiology.