Accumulation of macular xanthophylls in unsaturated membrane domains

The distribution of macular xanthophylls, lutein and zeaxanthin, between domains formed in membranes made from an equimolar ternary mixture of dioleoylphosphatidylcholine/sphingomyelin/cholesterol, called a raft-forming mixture, was investigated. In these membranes, two domains are formed: the raft domain enriched in saturated lipids and cholesterol (detergent-resistant membranes, DRM), and the bulk domain enriched in unsaturated lipids (detergent-soluble membranes, DSM). These membrane domains have been separated using cold Triton X-100 extraction from membranes containing 1 mol% of either lutein or zeaxanthin. The results indicated that xanthophylls are substantially excluded from DRM and remain concentrated in DSM. Concentrations of xanthophylls in DRM and DSM calculated as the mole ratio of either xanthophyll to phospholipid were 0.005 and 0.03, respectively, and calculated as the mole ratio of either xanthophyll to total lipid (phospholipid + cholesterol) were 0.003 and 0.025, respectively. Thus, xanthophylls are over eight times more concentrated in DSM than in DRM. No significant difference in the distribution of lutein and zeaxanthin was found. It was also demonstrated using saturation-recovery EPR that at 1 mol%, neither lutein nor zeaxanthin affect the formation of membrane domains. The location of xanthophylls in domains formed from unsaturated lipids is ideal if they are to act as a lipid antioxidant, which is the most accepted mechanism through which lutein and zeaxanthin protect the retina from age-related macular diseases.     

Structural aspects of the antioxidant activity of lutein in a model of photoreceptor membranes

It was shown that in membranes containing raft domains, the macular xanthophylls lutein and zeaxanthin are not distributed uniformly, but are excluded from saturated raft domains and about ten times more concentrated in unsaturated bulk lipids. The selective accumulation of lutein and zeaxanthin in direct proximity to unsaturated lipids, which are especially susceptible to lipid peroxidation, could be very important as far as their antioxidant activity is concerned. Therefore, the protective role of lutein against lipid peroxidation was investigated in membranes made of raft-forming mixtures and in models of photoreceptor outer segment membranes and compared with their antioxidant activity in homogeneous membranes composed of unsaturated lipids. Lipid peroxidation was induced by photosensitized reactions using rose Bengal and monitored by an MDA-TBA test, an iodometric assay, and oxygen consumption (using EPR spectroscopy and the mHCTPO spin label as an oxygen probe). The results show that lutein protects unsaturated lipids more effectively in membranes made of raft-forming mixtures than in homogeneous membranes. This suggests that the selective accumulation of macular xanthophylls in the most vulnerable regions of photoreceptor membranes may play an important role in enhancing their antioxidant properties and ability to prevent age-related macular diseases (such as age-related macular degeneration (AMD)).     

Can macular xanthophylls replace cholesterol in formation of the liquid-ordered phase in lipid-bilayer membranes?

Lateral organization of membranes made from binary mixtures of dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) and macular xanthophylls (lutein or zeaxanthin) was investigated using the saturation-recovery (SR) EPR spin-labeling discrimination by oxygen transport (DOT) method in which the bimolecular collision rate of molecular oxygen with the nitroxide spin label is measured. This work was undertaken to examine whether or not lutein and zeaxanthin, macular xanthophylls that parallel cholesterol in its function as a regulator of both membrane fluidity and hydrophobicity, can parallel other structural functions of cholesterol, including formation of the liquid-ordered phase in membranes. The DOT method permits discrimination of different membrane phases when the collision rates (oxygen transport parameter) differ in these phases. Additionally, membrane phases can be characterized by the oxygen transport parameter in situ without the need for separation, which provides information about the dynamics of each phase. In gel-phase membranes, two coexisting phases were discriminated in the presence of macular xanthophylls - namely, the liquid-ordered-like and solid-ordered-like phases. However, in fluid-phase membranes, xanthophylls only induce the solitary liquid-ordered-like phase, while at similar concentrations, cholesterol induces coexisting liquid-ordered and liquid-disordered phases. No significant differences between the effects of lutein and zeaxanthin were found.     

Lutein and zeaxanthin as protectors of lipid membranes against oxidative damage: the structural aspects.

Two main xanthophyll pigments are present in the membranes of macula lutea of the vision apparatus of primates, including humans: lutein and zeaxanthin. Protection against oxidative damage of the lipid matrix and screening against excess radiation are the most likely physiological functions of these xanthophyll pigments in macular membranes. A protective effect of lutein and zeaxanthin against oxidative damage of egg yolk lecithin liposomal membranes induced by exposure to UV radiation and incubation with 2, 2'-azobis(2-methypropionamidine)dihydrochloride, a water-soluble peroxidation initiator, was studied. Both lutein and zeaxanthin were found to protect lipid membranes against free radical attack with almost the same efficacy. The UV-induced lipid oxidation was also slowed down by lutein and zeaxanthin to a very similar rate in the initial stage of the experiments (5-15 min illumination) but zeaxanthin appeared to be a better photoprotector during the prolonged UV exposure. The decrease in time of a protective efficacy of lutein was attributed to the photooxidation of the carotenoid itself. Both lutein and zeaxanthin were found to slightly modify mechanical properties of the liposomes in a very similar fashion as concluded on the basis of H(1) NMR and diffractometric measurements of pure egg yolk membranes and membranes pigmented with the xanthophylls. Linear dichroism analysis of the mean orientation of the dipole transition moment of the xanthophylls incorporated to the lipid multibilayers revealed essentially different orientation of zeaxanthin and lutein in the membranes. Zeaxanthin was found to adopt roughly vertical orientation with respect to the plane of the membrane. The relatively large orientation angle between the transition dipole and the axis normal to the plane of the membrane found in the case of lutein (67 degrees in the case of 2 mol% lutein in EYPC membranes) was interpreted as a representation of the existence of two orthogonally oriented pools of lutein, one following the orientation of zeaxanthin and the second parallel with respect to the plane of the membrane. The differences in the protective efficacy of lutein and zeaxanthin in lipid membranes were attributed to a different organization of zeaxanthin-lipid and lutein-lipid membranes.     

Organisation of xanthophyll pigments lutein and zeaxanthin in lipid membranes formed with dipalmitoylphosphatidylcholine

Carotenoid pigments and in particular xanthophylls play several physiological functions in plant and animal membranes. Xanthophylls are present in biological membranes in the form of pigment-protein complexes but also as direct components of lipid phase. The biological activity of carotenoids in membranes depends on a molecular organisation of pigments in lipid bilayers, in particular the localisation, orientation and aggregational state. In the present work the organisation of lutein- and zeaxanthin-containing lipid membranes was analysed with the application of electronic absorption spectroscopy. Both xanthophyll pigments incorporated to the dipalmitoylphosphatidylcholine (DPPC) unilamellar liposomes form H-type molecular aggregates, manifested by the hypsochromic shift of the main absorption band of carotenoids. The aggregation of lutein and zeaxanthin in DPPC membranes was observed even at relatively low concentrations of a pigment in the lipid phase (1-5 mol%). Gaussian analysis of the absorption spectra of lutein and zeaxanthin in DPPC membranes in terms of the exciton splitting theory revealed the formation of different molecular structures of pigments interpreted as dimers, trimers, tetramers and large aggregates. The fraction of lutein and zeaxanthin in the monomeric form was found to depend on the physical state of the lipid phase. Pronounced monomerisation of lutein and zeaxanthin was observed as accompanying the transition from the P(beta)' phase to the L(alpha) phase of DPPC, mostly at the expense of the trimeric and tetrameric forms. The fraction of monomers of lutein is always lower by 10-30% than that of zeaxanthin under the same experimental conditions. Different organisational forms of lutein and zeaxanthin in the model system studied are discussed in terms of possible physiological functions of these pigments in the membranes of the retina: zeaxanthin in the protection of the lipid phase against oxidative damage and lutein in absorbing short wavelength radiation penetrating retina membranes.     

Effect of chronic endothelin blockade on PKC isoform distribution in mesenteric arteries from diabetic rats

Rigid plaques containing protein particles in plasma membrane build on the apical surface of the mammalian urothelium. We have previously shown that dietary fats modified the fatty acid profile as well as the fluorescence anisotropy of rat urothelial plasma membranes. In this study, we have further examined the proportion of phosphatidylcholine, phosphatidylethanolamine, cerebrosides, sulfatides and cholesterol in detergent resistant (DRM) and soluble (DSM) plasma membrane fractions as well as the properties of the particles. Four groups of weaned rats were fed for 12 weeks on a commercial diet (control), or on a formula containing 5% (w/w) of corn oil, fish oil or olein. The control DRM behaved as a distinctive domain since it was enriched in cholesterol and glycosphingolipids. DSM showed higher levels of phosphatidylcholine and phosphatidylethanolamine with respect to DRM. On the other hand, the lipid distributions were affected by the diets. Homogeneous lipid distributions between DSM and DRM were found in olein membranes, suggesting a decreased potential formation of lipid domains. In addition, properties of the uroplakins were altered by dietary treatments. Thus, uroplakins (UP) Ia, Ib, II and III observed by SDS-PAGE, were in lower proportions (mainly olein) than in controls. Moreover, a higher proportion of UPIII was cross-linked to UPIII and UPIb in olein treatment than in control. Meanwhile, only cross-linking to UPIII or UPIb was altered in corn and fish diets, respectively. These results suggest a role of the lipids in the establishment of the uroplakin interactions. Thus, specific dietary fats may have important functional implications. (Mol Cell Biochem 271: 69–75, 2005)     

Metabolism of lutein and zeaxanthin in rhesus monkeys: identification of (3R,6'R)- and (3R,6'S)-3'-dehydro-lutein as common metabolites and comparison to humans

Lutein and zeaxanthin are xanthophylls that can be found highly concentrated in the macula of the retina. They are thought to protect the macula through their role as blue-light filters and because of their antioxidant and singlet oxygen quenching properties. Examination of metabolites unique to lutein and zeaxanthin such as 3'-dehydro-lutein, and of their stereochemistry may provide insight to the mechanism by which they are formed and by which they exert protection. To evaluate the formation of such metabolites, eleven monkeys were raised on a xanthophyll-free diet, and supplemented with pure lutein or pure zeaxanthin (2.2 mg/kg body weight/d). The period of supplementation ranged between 12 and 92 weeks. At study start and throughout the study, serum samples were taken and analyzed for xanthophylls using different HPLC systems. Xanthophyll metabolites were identified using UV/VIS and HR-MS detection. Lutein and zeaxanthin metabolites were found in detectable amounts with 3'-dehydro-lutein being a common metabolite of both. Using chiral-phase HPLC, two diastereomers, (3R,6'R)-3'-dehydro-lutein and (3R,6'S)-3'-dehydro-lutein, were identified and shown to be present in nearly equimolar amounts. A pathway for their formation from either lutein or zeaxanthin is proposed. These findings were comparable to results obtained with human plasma.     

Detergent-free domain isolated from Xenopus egg plasma membrane with properties similar to those of detergent-resistant membranes

Microdomains known as "rafts" have been isolated from many cell types as detergent-resistant membranes (DRMs) and are enriched in sphingolipids and cholesterol. However, there has been considerable controversy over whether such domains are found in native membranes or are artificially generated by the purification procedure. This controversy is based at least in part on the fact that raft membranes were first detected following detergent extraction in the cold. We isolated two plasma membrane fractions, without detergent treatment, using a discontinuous sucrose density gradient. One fraction was designated "light" and the other "heavy." These fractions were compared with DRMs, which were isolated in the presence of 1% Triton X-100. We found that Xenopus DRMs are enriched with sphingomyelin and cholesterol and exhibit a phase state similar to the liquid-ordered phase. Comparison of DRM complexes with the light and heavy plasma membrane fractions revealed some physical and biochemical similarities between the light fraction of the plasma membrane and the DRM complexes, based on (1) the phosphatidylcholine/sphingomyelin ratio and (2) the protein composition visualized on a two-dimensional gel. These two fractions are also quite similar in their thermotropic phase behavior, and their high levels of ganglioside GM1. We conclude that the light membrane fraction isolated in a detergent-free environment has many of the characteristics normally associated with DRMs.     

Competitive carotenoid and cholesterol incorporation into liposomes: effects on membrane phase transition, fluidity, polarity and anisotropy

Pure 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC) or mixed DPPC:1,2-dipalmitoyl phosphatidyletanolamine (DPPE):1,2-dipalmitoyl diphosphatidylserine (DPPS) (17:5:3) liposomes were incorporated with 5 mol% dietary carotenoids (beta-carotene, lutein and zeaxanthin) or with cholesterol (16 and 48 mol%) in the absence or presence of 15 mol% carotenoids, respectively. The carotenoid incorporation yields ranged from 0.42 in pure to 0.72 in mixed phospholipid liposomes. They decreased significantly, from 3 to 14%, in the corresponding cholesterol-doped liposomes, respectively. Highest incorporation yields were achieved by zeaxanthin and lutein in phospholipid liposomes while in cholesterol-containing liposomes, lutein was highest incorporated. The effects on membrane structure and dynamics were determined by differential scanning calorimetry, steady-state fluorescence and anisotropy measurements. Polar carotenoids and cholesterol cause similar, dose-dependent effects: ordering and rigidification revealed by broadening of the transition peak, and increase of anisotropy. Membrane hydrophobicity is determined by cholesterol content and carotenoid polarity. In cholesterol-doped liposomes, beta-carotene is less incorporated than in cholesterol-free liposomes. Our observations suggest effects of carotenoids, even at much lower effective concentrations than cholesterol (8 to 80-fold), on membrane structure and dynamics. Although they are minor constituents of animal membranes, carotenoids may act as modulators of membrane phase transition, fluidity, polarity and permeability, and therefore, can influence the membrane physiology and pathology.     

Structure and cholesterol dynamics of caveolae/raft and nonraft plasma membrane domains

Despite recognition that the plasma membrane (PM) is comprised of lipid raft domains that are key organizing sites of multiple signaling pathways and other cell functions, limited information is available regarding the structure and function in sterol dynamics of these microdomains. To begin to resolve these issues, MDCK membranes were subfractionated by three different techniques to produce (i) detergent-resistant membranes (DRM) and detergent-soluble membranes (DSM), (ii) nondetergent caveolae/rafts (NDCR), and (iii) nondetergent, affinity-purified caveolae/rafts (ACR) and noncaveolae/nonrafts (NR). ACR exhibited the least cross contamination with other PM domains or intracellular membranes, in marked contrast to DRM that contained the highest level of cross contaminants. Spectral properties of dehydroergosterol (DHE), a naturally occurring fluorescent sterol, showed that ACR, NDCR, and NR did not contain crystalline sterol, consistent with the lack of crystalline sterol in PM of intact cells. In contrast, DRM contained significant levels of crystalline sterol. Fluorescence polarization of membrane probes showed that ACR were the least fluid and had the highest transbilayer fluidity gradient, the most liquid ordered phase, and the sterol dynamics most responsive to sterol carrier protein-2 (SCP-2). In contrast, DRM had structural properties similar to those of NR, anomalous (very fast) spontaneous sterol dynamics, and sterol dynamics that were unresponsive to SCP-2. Differences between the structural and functional properties of DRM and those of the nondetergent preparations (ACR and NDCR) were not due to the presence of detergent. A nondetergent, affinity-purified (ACR) lipid domain fraction isolated from MDCK cells for the first time revealed unique structural (noncrystalline sterol, liquid-ordered, high transbilayer fluidity gradient) and functional (cholesterol dynamics) properties of lipid rafts as compared to nonrafts (NR). In summary, this study showed membrane microdomains (rafts/caveolae) isolated by three different methodologies have unique structural, functional, and organizational characteristics.     

Determination of the stoichiometry and strength of binding of xanthophylls to the photosystem II light harvesting complexes

Xanthophylls have a crucial role in the structure and function of the light harvesting complexes of photosystem II (LHCII) in plants. The binding of xanthophylls to LHCII has been investigated, particularly with respect to the xanthophyll cycle carotenoids violaxanthin and zeaxanthin. It was found that most of the violaxanthin pool was loosely bound to the major complex and could be removed by mild detergent treatment. Gentle solubilization of photosystem II particles and thylakoids allowed the isolation of complexes, including a newly described oligomeric preparation, enriched in trimers, that retained all of the in vivo violaxanthin pool. It was estimated that each LHCII monomer can bind at least one violaxanthin. The extent to which different pigments can be removed from LHCII indicated that the relative strength of binding was chlorophyll b > neoxanthin > chlorophyll a > lutein > zeaxanthin > violaxanthin. The xanthophyll binding sites are of two types: internal sites binding lutein and peripheral sites binding neoxanthin and violaxanthin. In CP29, a minor LHCII, both a lutein site and the neoxanthin site can be occupied by violaxanthin. Upon activation of the violaxanthin de-epoxidase, the highest de-epoxidation state was found for the main LHCII component and the lowest for CP29, suggesting that only violaxanthin loosely bound to LHCII is available for de-epoxidation.     

Temperature-dependent localization of GPI-anchored intestinal alkaline phosphatase in model rafts

In plasma membranes, most of glycosylphosphatidylinositol (GPI)-anchored proteins would be associated with rafts, a category of ordered microdomains enriched in sphingolipids and cholesterol (Ch). They would be also concentrated in the detergent resistant membranes (DRMs), a plasma membrane fraction extracted at low temperature. Preferential localization of GPI-anchored proteins in these membrane domains is essentially governed by their high lipid order, as compared to their environment. Changes in the temperature are expected to modify the membrane lipid order, suggesting that they could affect the distribution of GPI-anchored proteins between membrane domains. Validity of this hypothesis was examined by investigating the temperature-dependent localization of the GPI-anchored bovine intestinal alkaline phophatase (BIAP) into model raft made of palmitoyloleoylphosphatidylcholine/sphingomyelin/cholesterol (POPC/SM/Chl) supported membranes. Atomic force microscopy (AFM) shows that the inserted BIAP is localized in the SM/Chl enriched ordered domains at low temperature. Above 30 degrees C, BIAP redistributes and is present in both the 'fluid' POPC enriched and the ordered SM/Chl domains. These data strongly suggest that in cells the composition of plasma membrane domains at low temperature differs from that at physiological temperature.     

Location of macular xanthophylls in the most vulnerable regions of photoreceptor outer-segment membranes

Lutein and zeaxanthin are two dietary carotenoids that compose the macular pigment of the primate retina. Another carotenoid, meso-zeaxanthin, is formed from lutein in the retina. A membrane location is one possible site where these dipolar, terminally dihydroxylated carotenoids, named macular xanthophylls, are accumulated in the nerve fibers and photoreceptor outer segments. Macular xanthophylls are oriented perpendicular to the membrane surface, which ensures their high solubility, stability, and significant effects on membrane properties. It was recently shown that they are selectively accumulated in membrane domains that contain unsaturated phospholipids, and thus are located in the most vulnerable regions of the membrane. This location is ideal if they are to act as lipid antioxidants, which is the most accepted mechanism through which lutein and zeaxanthin protect the retina from age-related macular degeneration. In this mini-review, we examine published data on carotenoid-membrane interactions and present our hypothesis that the specific orientation and location of macular xanthophylls maximize their protective action in membranes of the eye retina.     

Carotenoid binding sites in LHCIIb. Relative affinities towards major xanthophylls of higher plants.

The major light-harvesting complex of photosystem II can be reconstituted in vitro from its bacterially expressed apoprotein with chlorophylls a and b and neoxanthin, violaxanthin, lutein, or zeaxanthin as the only xanthophyll. Reconstitution of these one-carotenoid complexes requires low-stringency conditions during complex formation and isolation. Neoxanthin complexes (containing 30-50% of the all-trans isomer) disintegrate during electrophoresis, exhibit a largely reduced resistance against proteolytic attack; in addition, energy transfer from Chl b to Chl a is easily disrupted at elevated temperature. Complexes reconstituted in the presence of either zeaxanthin or lutein contain nearly two xanthophylls per 12 chlorophylls and are more resistant against trypsin. Lutein-LHCIIb also exhibits an intermediate maintenance of energy transfer at higher temperature. Violaxanthin complexes approach a xanthophyll/12 chlorophyll ratio of 3, similar to the ratio in recombinant LHCIIb containing all xanthophylls. On the other hand, violaxanthin-LHCIIb exhibits a low thermal stability like neoxanthin complexes, but an intermediate accessibility towards trypsin, similar to lutein-LHCIIb and zeaxanthin-LHCIIb. Binary competition experiments were performed with two xanthophylls at varying ratios in the reconstitution. Analysis of the xanthophyll contents in the reconstitution products yielded information about relative carotenoid affinities of three assumed binding sites. In lutein/neoxanthin competition experiments, two binding sites showed a strong preference (> 200-fold) for lutein, whereas the third binding site had a higher affinity (25-fold) to neoxanthin. Competition between lutein and violaxanthin gave a similar result, although the specificities were lower: two binding sites have a 36-fold preference for lutein and one has a fivefold preference for violaxanthin. The lowest selectivity was between lutein and zeaxanthin: two binding sites had a fivefold higher affinity for lutein and one has a threefold higher affinity to zeaxanthin.     

Configuration and dynamics of xanthophylls in light-harvesting antennae of higher plants. Spectroscopic analysis of isolated light-harvesting complex of photosystem II and thylakoid membranes

Resonance Raman excitation spectroscopy combined with ultra low temperature absorption spectral analysis of the major xanthophylls of higher plants in isolated antenna and intact thylakoid membranes was used to identify carotenoid absorption regions and study their molecular configuration. The major electronic transitions of the light-harvesting complex of photosystem II (LHCIIb) xanthophylls have been identified for both the monomeric and trimeric states of the complex. One long wavelength state of lutein with a 0-0 transition at 510 nm was detected in LHCIIb trimers. The short wavelength 0-0 transitions of lutein and neoxanthin were located at 495 and 486 nm, respectively. In monomeric LHCIIb, both luteins absorb around 495 nm, but slight differences in their protein environments give rise to a broadening of this band. The resonance Raman spectra of violaxanthin and zeaxanthin in intact thylakoid membranes was determined. The broad 0-0 absorption transition for zeaxanthin was found to be located in the 503-511 nm region. Violaxanthin exhibited heterogeneity, having two populations with one absorbing at 497 nm (0-0), 460 nm (0-1), and 429 nm (0-2), and the other major pool absorbing at 488 nm (0-0), 452 nm (0-1), and 423 nm (0-2). The origin of this heterogeneity is discussed. The configuration of zeaxanthin and violaxanthin in thylakoid membranes was different from that of free pigments, and both xanthophylls (notably, zeaxanthin) were found to be well coordinated within the antenna proteins in vivo, arguing against the possibility of their free diffusion in the membrane and supporting our recent biochemical evidence of their association with intact oligomeric light-harvesting complexes (Ruban, A. V., Lee, P. J., Wentworth, M., Young, A. J., and Horton, P. (1999) J. Biol. Chem. 274, 10458-10465).     

Characterization of fatty acids and proteins associated with the xanthophyll-enriched membrane fraction isolated from the thylakoid membranes of irradiance-stressed Dunaliella salina

It has been previously reported that a considerable amount of lutein and zeaxanthin could be fractionated, upon mild detergent treatment, from the thylakoid membranes of irradiance-stressed unicellular green alga, Dunaliella salina, into a yellow pellet fraction. Such membrane pellet was found to be devoid of chlorophylls and any known proteins of photosynthesis but rather contained a significant amount of unknown polypeptides. It was speculated that this xanthophyll-rich membrane pellet might originate from incomplete solubilization of the photoinhibited thylakoids by weak surfactants, due to extra rigidity imposed by the xanthophylls being directly imbedded into the lipid bilayer. In this study, we further characterized this membrane fraction by studying its associated proteins and fatty acid composition. Analysis by gas chromatography–mass spectrometry indicated that this yellow pellet membrane was enriched in saturated fatty acids, supporting the rigidity notion of the pellet. Protein identification by MALDI-TOF MS further revealed that at least 20 water-soluble proteins were found in association with this pellet. These proteins may originate from unspecific contamination of abundant polypeptides co-precipitated with the membrane upon fractionation. Possible explanations regarding the nature of this xanthophyll-rich membrane are also discussed.     

Structure, composition, and peptide binding properties of detergent soluble bilayers and detergent resistant rafts

Lipid bilayers composed of unsaturated phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol are thought to contain microdomains that have similar detergent insolubility characteristics as rafts isolated from cell plasma membranes. We chemically characterized the fractions corresponding to detergent soluble membranes (DSMs) and detergent resistant membranes (DRMs) from 1:1:1 PC:SM:cholesterol, compared the binding properties of selected peptides to bilayers with the compositions of DSMs and DRMs, used differential scanning calorimetry to identify phase transitions, and determined the structure of DRMs with x-ray diffraction. Compared with the equimolar starting material, DRMs were enriched in both SM and cholesterol. Both transmembrane and interfacial peptides bound to a greater extent to DSM bilayers than to DRM bilayers, likely because of differences in the mechanical properties of the two bilayers. Thermograms from 1:1:1 PC:SM:cholesterol from 3 to 70 degrees C showed no evidence for a liquid-ordered to liquid-disordered phase transition. Over a wide range of osmotic stresses, each x-ray pattern from equimolar PC:SM:cholesterol or DRMs contained a broad wide-angle band at 4.5 A, indicating that the bilayers were in a liquid-crystalline phase, and several sharp low-angle reflections that indexed as orders of a single lamellar repeat period. Electron density profiles showed that the total bilayer thickness was 57 A for DRMs, which was approximately 5 A greater than that of 1:1:1 PC:SM:cholesterol and 10 A greater than the thickness of bilayers with the composition of DSMs. These x-ray data provide accurate values for the widths of raft and nonraft bilayers that should be important in understanding mechanisms of protein sorting by rafts.     

Correlated AFM and NanoSIMS imaging to probe cholesterol-induced changes in phase behavior and non-ideal mixing in ternary lipid membranes

Cholesterol is believed to be an important component in compositionally distinct lipid domains in the cellular plasma membrane, which are referred to as lipid rafts. Insight into how cholesterol influences the interactions that contribute to plasma membrane organization can be acquired from model lipid membranes. Here we characterize the lipid mixing and phase behavior exhibited by (15)N-dilaurolyphosphatidycholine ((15)N-DLPC)/deuterated distearoylphosphatiylcholine (D(70)-DSPC) membranes with various amounts of cholesterol (0, 3, 7, 15 or 19mol%) at room temperature. The microstructures and compositions of individual membrane domains were determined by imaging the same membrane locations with both atomic force microscopy (AFM) and high-resolution secondary ion mass spectrometry (SIMS) performed with a Cameca NanoSIMS 50. As the cholesterol composition increased from 0 to 19mol%, the circular ordered domains became more elongated, and the amount of (15)N-DLPC in the gel-phase domains remained constant at 6-7mol%. Individual and micron-sized clusters of nanoscopic domains enriched in D(70)-DSPC were abundant in the 19mol% cholesterol membrane. AFM imaging showed that these lipid domains had irregular borders, indicating that they were gel-phase domains, and not non-ideally mixed lipid clusters or nanoscopic liquid-ordered domains.     

Detergents induce raft-like domains budding and fission from giant unilamellar heterogeneous vesicles: a direct microscopy observation

The effect of detergents on giant unilamellar vesicles (GUVs) composed of phosphatidylcholine, sphingomyelin and cholesterol and containing liquid-ordered phase (l(o)) domains was investigated. Such domains have been used as models for the lipid rafts present in biological membranes. The studied detergents included lyso-phosphatidylcholine, the product of phospholipase A2 activity, as well as Triton X-100 and Brij 98, i.e. detergents used to isolate lipid rafts as DRMs. Local external injection of each of the three detergents at subsolubilizing amounts promoted exclusion of l(o) domains from the GUV as small vesicles. The budding and fission processes associated with this vesiculation were interpreted as due to two distinct effects of the detergent. In this framework, the budding is caused by the initial incorporation of the detergent in the outer membrane leaflet which increases the spontaneous curvature of the bilayer. The fission is related to the inverted-cone molecular shape of the detergent which stabilizes positively curved structures, e.g. pores involved in vesicle separation. On the other hand, we observed in GUVs neither domain formation nor domain coalescence to be induced by the addition of detergents. This supports the idea that isolation of DRM from biological membranes by detergent-induced extraction is not an artifact. It is also suggested that the physico-chemical mechanisms involved in l(o) domain budding and fission might play a role in rafts-dependant endocytosis in cells.     

Differential sensitivity to detergents of actin cytoskeleton from nerve endings

Detergent-resistant membranes (DRM), an experimental model used to study lipid rafts, are typically extracted from cells by means of detergent treatment and subsequent ultracentrifugation in density gradients, Triton X-100 being the detergent of choice in most of the works. Since lipid rafts are membrane microdomains rich in cholesterol, depletion of this component causes solubilization of DRM with detergent. In previous works from our group, the lack of effect of cholesterol depletion on DRM solubilization with Triton X-100 was detected in isolated rat brain synaptosomes. In consequence, the aim of the present work is to explore reasons for this observation, analyzing the possible role of the actin cytoskeleton, as well as the use of an alternative detergent, Brij 98, to overcome the insensitivity to Triton X-100 of cholesterol-depleted DRM. Brij 98 yields Brij-DRM that are highly dependent on cholesterol, since marker proteins (Flotillin-1 and Thy-1), as well as actin, appear solubilized after MCD treatment. Pretreatment with Latrunculin A results in a significant increase in Flotillin-1, Thy-1 and actin solubilization by Triton X-100 after cholesterol depletion. Studies with transmission electron microscopy show that combined treatment with MCD and Latrunculin A leads to a significant increase in solubilization of DRM with Triton X-100. Thus, Triton-DRM resistance to cholesterol depletion can be explained, at least partially, thanks to the scaffolding action of the actin cytoskeleton, without discarding differential effects of Brij 98 and Triton X-100 on specific membrane components. In conclusion, the detergent of choice is important when events that depend on the actin cytoskeleton are going to be studied.