Domain Formation in Model Membranes Studied by Pulsed-Field Gradient-NMR: The Role of Lipid Polyunsaturation

Cell membranes contain a large number of different lipid species. Such a multicomponent mixture exhibits a complex phase behavior with regions of structural and compositional heterogeneity. Especially domains formed in ternary mixtures, composed of saturated and unsaturated lipids together with cholesterol, have received a lot of attention as they may resemble raft formation in real cells. Here we apply a simulation model to assess the molecular nature of these domains at the nanoscale, information that has thus far eluded experimental determination. We are able to show the spontaneous separation of a saturated phosphatidylcholine (PC)/ unsaturated PC/cholesterol mixture into a liquid-ordered and a liquid-disordered phase with structural and dynamic properties closely matching experimental data. The near-atomic resolution of the simulations reveals remarkable features of both domains and the boundary domain interface. Furthermore, we predict the existence of a small surface tension between the monolayer leaflets that drives registration of the domains. At the level of molecular detail, raft-like lipid mixtures show a surprising face with possible implications for many cell membrane processes.A ccording to a recent definition, rafts are small (Ͻ200 nm) heterogeneous, highly dynamic, sterol-and sphingolipidenriched domains that compartmentalize cellular processes (1) and are believed to play an important role in cellular function (2). Although direct observation of rafts in vivo remains complicated, raft-like mixtures in model membranes can form domains that have been visualized directly for an increasing number of experimental systems and conditions (3-7). At cholesterol levels representative of biological membranes (10-30%), mixtures of saturated and unsaturated lipids separate into macroscopic domains of a liquid-ordered (L o ) phase and a liquiddisordered (L d ) phase. The first, raft-like phase is enriched in both cholesterol and the saturated lipid; the second, non-raft phase consists mainly of the unsaturated lipid and is depleted of cholesterol. In order to not confuse the reader concerning the meaning and implication of the term raft, here and throughout the remainder of this article we use the term ''raft-like'' phase or domain to denote the L o phase observed in model membranes. Interestingly, isolated plasma membranes have recently been shown to be capable of forming such domains as well (8, 9). Yet it should be stressed that in real cell membranes raft formation may not resemble macroscopic phase separation. For instance, other recent experiments on plasma membranes demonstrate micrometer-scale composition fluctuations arising from critical demixing behavior (10). The focus of the current work is on phase segregation in model membranes.To interpret the experimental measurements performed on model membranes, knowledge of the structure and dynamics of the domains at the molecular level is essential. Here we report molecular dynamics simulations of the spontaneous formation of raft-like domains in ternar...

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“…(5,15,21), which report factors of 2 to 10 depending on system details. In our simulations, the individual components can be easily traced.…”

Section: Resultsmentioning

confidence: 99%

“…Linoleic acid is a representative of the important class of 6 fatty acids. Experimentally, it has been shown that the presence of polyunsaturated tails increases the tendency to form domains (15,16).…”

mentioning

confidence: 99%

The molecular face of lipid rafts in model membranes

Risselada

Marrink

2008

Proc. Natl. Acad. Sci. U.S.A.

511

632

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“…Fast exchange between domains has similarly been inferred from 2 H NMR spectra for deuterium labeled phospholipid in 18:0-22:6PC/18:0-22:6PE/18:0-22:6PS/cholesterol (4:4:1:1 mol), on the basis of which a radius of <250Å was assigned to 18:0-22:6PC/cholesterol clusters (Huster et al, 1998), and for deuterium labeled cholesterol in 16:0-18:1PC/brain SM/cholesterol (1:1:1 mol) (Aussenac et al, 2003). Intriguingly, recent pulsed field gradient (pfg) NMR measurements on PC/egg SM/cholesterol (3:3:2 mol) membranes aligned between glass plates suggest that the presence of PUFA can trigger the formation of large (>m) from small domains (Filippov et al, 2007). These experiments using PC with a 18:0 acid sn-1 chain revealed two lipid lateral diffusion coefficients, assigned to l o and l d phases, when 20:4 or 22:6, but not 18:1 and 18:2, acid was at the sn-2 position.…”

Section: Dha Domainsmentioning

confidence: 99%

Docosahexaenoic acid domains: the ultimate non-raft membrane domain

Wassall

Stillwell

2008

Chemistry and Physics of Lipids

189

156

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“…[22,27,39] According to this idea, less free volume of bilayers (tighter packing of lipids) must lead to less translational mobility of lipid molecules and to higher activation energy of the diffusion process. Free volumes of DPPC and PSM have been determined previously by computer simulation [16] and by bilayer compressibility.…”

Section: Discussionmentioning

confidence: 99%

“…The PFG NMR method has proven to be very efficient in detecting structural and dynamic heterogeneities in lipid bilayers. [23,25,27,31] …”

Section: Introductionmentioning

confidence: 99%

Lateral diffusion in sphingomyelin bilayers

Филиппов

Rudakova

Munavirov

2010

Magnetic Reson in Chemistry

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Sphingomyelin (SM) is an important lipid of eukaryotic cellular membranes and neuronal tissues. We studied lateral diffusion in macroscopically oriented bilayers of synthetic palmitoylsphingomyelin (PSM) and natural sphingomyelins of egg yolk (eSM), bovine brain (bSM) and bovine milk (mSM) by pulsed field gradient NMR (PFG NMR) in the temperature range 45-60 °C. We found that the mean values of lateral diffusion coefficients (LDCs) of SMs are 1.9-fold lower compared with those of dipalmitoylphosphatidylcholine (DPPC), which is similar in molecular structure. This discrepancy could be explained by the characteristics of intermolecular SM interactions. The LDCs of different SMs differ: egg SM is most similar to PSM; both of them have a 10% higher LDC value compared with the other two natural SMs. Besides, all natural SMs show a complicated form of the spin-echo diffusion decay (DD), which is an indicator of a distribution of LDC values in bilayers. This peculiarity is explained by the broad distributions of hydrocarbon chain lengths of the natural SMs studied here, especially mSM and bSM. We confirmed the relationship between chain length and LDC in the bilayers by computer analysis of a set of (1)H NMR spectra obtained by scanning the value of the pulsed field gradient. There is a correlation between lower LDC values and SM molecules with longer acyl chains. The most probable mechanisms by which long-chain SM molecules decrease their lateral diffusion relative to the average value are protrusion into the other side of the bilayer or lateral separation into areas that diverge with their LDCs.

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Domain Formation in Model Membranes Studied by Pulsed-Field Gradient-NMR: The Role of Lipid Polyunsaturation

Cited by 71 publications

References 41 publications

The molecular face of lipid rafts in model membranes

The molecular face of lipid rafts in model membranes

Docosahexaenoic acid domains: the ultimate non-raft membrane domain

Lateral diffusion in sphingomyelin bilayers

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