Fluorescence evidence for cholesterol regular distribution in phosphatidylcholine and in sphingomyelin lipid bilayers

Abstract

Our previous studies indicated that sterols (including cholesterol and dehydroergosterol) can be regularly distributed into hexagonal superlattices in the plane of liquid-crystalline phosphatidylcholine bilayers. It was suggested that regular and irregular regions coexist in the membrane. In the present study, we report supporting evidence for our sterol regular distribution model. We have examined the fractional concentration dependencies of dehydroergosterol (a naturally occurring cholesterol analogue) fluorescence intensity and lifetime in various phosphatidylcholine and sphingomyelin bilayers. Fluorescence intensity and lifetime dips have been observed at specific sterol mole fractions. At those mole fractions, the acrylamide quenching rate constant of dehydroergosterol fluorescence reaches a local maximum. Those mole fractions match the critical sterol mole fractions at which sterol molecules are expected to be regularly distributed into hexagonal superlattices. The results support the idea that the sterols in the regular region are embedded in the bilayer less deep than those in the irregular regions. We have also examined the fractional cholesterol concentration dependencies of diphenylhexatriene (DPH) fluorescence intensity, lifetime, and polarization in DMPC vesicles. DPH fluorescence intensity and polarization also exhibit distinct dips and peaks, respectively, at critical sterol mole fractions for hexagonal superlattices. However, DPH lifetime changes little with sterol mole fraction. As a comparison, the fluorescence properties of DHE and DPH behave differently in response to the formation of sterol regular distribution. Furthermore, finding evidence for sterol regular distribution in both phosphatidylcholine and sphingomyelin membranes raises the possibility that sterol regular distribution may occur within phospholipid/cholesterol enriched domains of real biological membranes.

Cholesterol and ergosterol superlattices in three-component liquid crystalline lipid bilayers as revealed by dehydroergosterol fluorescence

“…Our sterol regular distribution model agrees with the well-known cholesterol condensing effect on lipid acyl chains (Huang and Mason, 1982;Almeida et al, 1992;see discussion in Chong, 1994) and is not incompatible with currently proposed phase diagrams (Ipsen et al, 1987;Vist and Davis, 1990;Matteo et al, 1995; McMullen and McElhaney, 1995; see discussion in Chong et al, 1996a). The model is not incompatible with models of aligned sterol domains (Rogers et al, 1979) or homogeneous distributions (Hyslop et al, 1990) because in our model, regular and irregular regions coexist and sterols are aligned (figure 2 A-C in or homogeneously distributed (figure 2, A and B, in in the regular region.…”

Cholesterol-Induced Modifications in Lipid Bilayers: A Simulation Study

“…Our results provide a possible molecular basis for the condensed complex model of Radhakrishnan and McCon-nell (Keller and McConnell, 1999;Radhakrishnan and Mc-Connell, 1999a,b) and possibly for suggestions of "superlattice" formation in lipid:Chol bilayers (Chong, 1994;Chong et al, 1996). In the simulations, each cholesterol preferentially is hydrogen bonded to at least one lipid molecule.…”

Fluorescence Techniques Using Dehydroergosterol to Study Cholesterol Trafficking

“…The usefulness of DHE and cholestatrienol (CTE) as fluorescent cholesterol analogs in model membranes in a series of investigations spanning several decades demonstrated the usefulness of these probes to monitor cholesterol structural (polarization, limiting anisotropy, order) and dynamic (lifetime, rotational rate) properties in membranes (12,65,70,95,96,108,(110)(111)(112)(113)(114)(115)(116)(117)(118)(119)(120). The lifetime studies resolved for the first time at least two DHE domains in model membranes, one less sensitive to the aqueous than the other, and that these domains were dependent on the lipid composition, temperature, and other properties of the membrane (54,119).…”