Relationship between Sterol/Steroid Structure and Participation in Ordered Lipid Domains (Lipid Rafts):  Implications for Lipid Raft Structure and Function

Abstract

The formation and stability of ordered lipid domains (rafts) in model membrane vesicles were studied using a series of sterols and steroids structurally similar to cholesterol. In one assay, insolubility in Triton X-100 was assessed in bilayers composed of sterol/steroid mixed with dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine, or a 1:1 mixture of these phospholipids. In a second assay fluorescence quenching was used to determine the degree of ordered domain formation in bilayers containing sterol/steroid and a 1:1 mixture of DPPC and a quencher-carrying phosphatidylcholine. Both methods showed that several single modifications of the cholesterol structure weaken, but do not fully abolish, the ability of sterols and steroids to promote ordered domain formation when mixed with DPPC. Some of these modifications included a shift of the double bond from the 5-6 carbons (cholesterol) to 4-5 carbons (allocholesterol), derivatization of the 3-OH (cholesterol methyl ether, cholesteryl formate), and alteration of the 3-hydroxy to a keto group (cholestanone). An oxysterol involved in atherosclerosis, 7-ketocholesterol, formed domains with DPPC that were as thermally stable as those with cholesterol although not as tightly packed as judged by fluorescence anisotropy. It was also found that 7-ketocholesterol has fluorescence quenching properties making it a useful spectroscopic probe. Lathosterol, which has a 7-8 carbon double bond in place of the 5-6 double bond of cholesterol, formed rafts with DPPC that were at least as detergent-resistant as, and even more thermally stable than, rafts containing cholesterol. Because lathosterol is an intermediate in cholesterol biosynthesis, we conclude it is unlikely that sterol biosynthesis continues past lathosterol in order to create a raft-favoring lipid.

Ordered Membrane Domain-Forming Properties of the Lipids of Borrelia burgdorferi

“…Co-existing ordered and disordered domains were detected by FRET in living Borrelia. In agreement with sterol effects on lipid rafts composed of mammalian lipids (36,37), strongly raft-supporting sterols maintained domain formation ability in Borrelia, whereas raft-inhibiting sterols disrupted raft formation (35). Another property of Borrelia domains that was found to be shared with ordered lipid rafts formed by eukaryotic lipids was selective accumulation of lipids with saturated acyl chains in ordered domains (35).…”

Cholesterol is a determinant of the structures of discoidal high density lipoproteins formed by the solubilization of phospholipid membranes by apolipoprotein A-I

“…7-ketocholesterol, which are less efficient in forming ordered lipid phases than cholesterol are 2 to 35 fold less effective than cholesterol in enhancing the rate [29,30]. However, lathosterol and β-sitosterol which are measurably better than cholesterol in forming ordered lipid phases did not significantly enhance the rate [48,49]. The rate limiting step in rHDL formation is the insertion α-helical segments of apoA-I into lattice defects that occur at s o and l d phase boundaries.…”

Cholesterol homeostasis and the escape tendency (activity) of plasma membrane cholesterol

“…While the studies just mentioned document the particular ability of cholesterol to promote the formation of condensed liquid-ordered phases, these publications also provide evidence that various sterols (biosynthetic intermediates and otherwise) are as good as and sometimes better than cholesterol at building rafts [26,49,50,88,89,91]. In this set are ergosterol, 25-hydroxycholesterol, lathosterol, epicholesterol, 7-dehydrocholesterol and dihydrocholesterol.…”