26 research outputs found
Behavior of Bilayer Leaflets in Asymmetric Model Membranes: Atomistic Simulation Studies
Spatial
organization within lipid bilayers is an important feature
for a range of biological processes. Leaflet compositional asymmetry
and lateral lipid organization are just two of the ways in which membrane
structure appears to be more complex than initially postulated by
the fluid mosaic model. This raises the question of how the phase
behavior in one bilayer leaflet may affect the apposing leaflet and
how one begins to construct asymmetric model systems to investigate
these interleaflet interactions. Here we report on all-atom molecular
dynamics simulations (a total of 4.1 ÎĽs) of symmetric and asymmetric
bilayer systems composed of liquid-ordered (Lo) or liquid-disordered
(Ld) leaflets, based on the nanodomain-forming POPC/DSPC/cholesterol
system. We begin by analyzing an asymmetric bilayer with leaflets
derived from simulations of symmetric Lo and Ld bilayers. In this
system, we observe that the properties of the Lo and Ld leaflets are
similar to those of the Lo and Ld leaflets in corresponding symmetric
systems. However, it is not obvious that mixing the equilibrium structures
of their symmetric counterparts is the most appropriate way to construct
asymmetric bilayers nor that these structures will manifest interleaflet
couplings that lead to domain registry/antiregistry. We therefore
constructed and simulated four additional asymmetric bilayer systems
by systematically adding or removing lipids in the Ld leaflet to mimic
potential density fluctuations. We find that the number of lipids
in the Ld leaflet affects its own properties, as well as those of
the apposing Lo leaflet. Collectively, the simulations reveal the
presence of weak acyl chain interdigitation across bilayer leaflets,
suggesting that interdigitation alone does not contribute significantly
to the interleaflet coupling in nonphase-separated bilayers of this
chemical composition. However, the properties of both leaflets appear
to be sensitive to changes in in-plane lipid packing, possibly providing
a mechanism for interleaflet coupling by modulating local density
and/or curvature fluctuations
Molecular Structures of Fluid Phosphatidylethanolamine Bilayers Obtained from Simulation-to-Experiment Comparisons and Experimental Scattering Density Profiles
Following our previous efforts in
determining the structures of
commonly used PC, PG, and PS bilayers, we continue our studies of
fully hydrated, fluid phase PE bilayers. The newly designed parsing
scheme for PE bilayers was based on extensive MD simulations, and
is utilized in the SDP analysis of both X-ray and neutron (contrast
varied) scattering measurements. Obtained experimental scattering
form factors are directly compared to our simulation results, and
can serve as a benchmark for future developed force fields. Among
the evaluated structural parameters, namely, area per lipid <i>A</i>, overall bilayer thickness <i>D</i><sub>B</sub>, and hydrocarbon region thickness 2<i>D</i><sub>C</sub>, the PE bilayer response to changing temperature is similar to previously
studied bilayers with different headgroups. On the other hand, the
reduced hydration of PE headgroups, as well as the strong hydrogen
bonding between PE headgroups, dramatically affects lateral packing
within the bilayer. Despite sharing the same glycerol backbone, a
markedly smaller area per lipid distinguishes PE from other bilayers
(i.e., PC, PG, and PS) studied to date. Overall, our data are consistent
with the notion that lipid headgroups govern bilayer packing, while
hydrocarbon chains dominate the bilayer’s response to temperature
changes
<sup>1</sup>H NMR Shows Slow Phospholipid Flip-Flop in Gel and Fluid Bilayers
We
measured the transbilayer diffusion of 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphocholine (DPPC) in large unilamellar
vesicles, in both the gel (<i>L</i><sub>β′</sub>) and fluid (<i>L</i><sub>α</sub>) phases. The choline
resonance of headgroup-protiated DPPC exchanged into the outer leaflet
of headgroup-deuterated DPPC-<i>d</i>13 vesicles was monitored
using <sup>1</sup>H NMR spectroscopy, coupled with the addition of
a paramagnetic shift reagent. This allowed us to distinguish between
the inner and outer bilayer leaflet of DPPC, to determine the flip-flop
rate as a function of temperature. Flip-flop of fluid-phase DPPC exhibited
Arrhenius kinetics, from which we determined an activation energy
of 122 kJ mol<sup>–1</sup>. In gel-phase DPPC vesicles, flip-flop
was not observed over the course of 250 h. Our findings are in contrast
to previous studies of solid-supported bilayers, where the reported
DPPC translocation rates are at least several orders of magnitude
faster than those in vesicles at corresponding temperatures. We reconcile
these differences by proposing a defect-mediated acceleration of lipid
translocation in supported bilayers, where long-lived, submicron-sized
holes resulting from incomplete surface coverage are the sites of
rapid transbilayer movement
Interactions between Ether Phospholipids and Cholesterol As Determined by Scattering and Molecular Dynamics Simulations
Cholesterol and ether lipids are ubiquitous in mammalian
cell membranes,
and their interactions are crucial in ether lipid mediated cholesterol
trafficking. We report on cholesterol’s molecular interactions
with ether lipids as determined using a combination of small-angle
neutron and X-ray scattering, and all-atom molecular dynamics (MD)
simulations. A scattering density profile model for an ether lipid
bilayer was developed using MD simulations, which was then used to
simultaneously fit the different experimental scattering data. From
analysis of the data the various bilayer structural parameters were
obtained. Surface area constrained MD simulations were also performed
to reproduce the experimental data. This iterative analysis approach
resulted in good agreement between the experimental and simulated
form factors. The molecular interactions taking place between cholesterol
and ether lipids were then determined from the validated MD simulations.
We found that in ether membranes cholesterol primarily hydrogen bonds
with the lipid headgroup phosphate oxygen, while in their ester membrane
counterparts cholesterol hydrogen bonds with the backbone ester carbonyls.
This different mode of interaction between ether lipids and cholesterol
induces cholesterol to reside closer to the bilayer surface, dehydrating
the headgroup’s phosphate moiety. Moreover, the three-dimensional
lipid chain spatial density distribution around cholesterol indicates
anisotropic chain packing, causing cholesterol to tilt. These insights
lend a better understanding of ether lipid-mediated cholesterol trafficking
and the roles that the different lipid species have in determining
the structural and dynamical properties of membrane associated biomolecules
Morphological Characterization of DMPC/CHAPSO Bicellar Mixtures: A Combined SANS and NMR Study
Spontaneously forming structures
of a system composed of dimyristoyl
phosphatidylcholine (DMPC) and 3-[(3-cholamidopropyl)Âdimethylammonio]-2-hydroxy-1-propanesulfonate
(CHAPSO) were studied by small-angle neutron scattering (SANS), <sup>31</sup>P NMR, and stimulated echo (STE) pulsed field gradient (PFG) <sup>1</sup>H NMR diffusion measurements. Charged lipid dimyristoyl phosphatidylglycerol
(DMPG) was used to induce different surface charge densities. The
structures adopted were investigated as a function of temperature
and lipid concentration for samples with a constant molar ratio of
long-chain to short-chain lipids (= 3). In the absence of DMPG, zwitterionic
bicellar mixtures exhibited a phase transition from discoidal bicelles,
or ribbons, to multilamellar vesicles either upon dilution or with
increased temperature. CHAPSO-containing mixtures showed a higher
thermal stability in morphology than DHPC-containing mixtures at the
corresponding lipid concentrations. In the presence of DMPG, discoidal
bicelles (or ribbons) were also found at low temperature and lower
lipid concentration mixtures. At high temperature, perforated lamellae
were observed in high-concentration mixtures (≥7.5 wt %) whereas
uniform unilamellar vesicles and bicelles formed in low-concentration
mixtures (≤2.5 wt %), respectively, when the mixtures were
moderately and highly charged. From the results, spontaneous structural
diagrams of the zwitterionic and charged systems were constructed
Tocopherol Activity Correlates with Its Location in a Membrane: A New Perspective on the Antioxidant Vitamin E
We show evidence of an antioxidant
mechanism for vitamin E which
correlates strongly with its physical location in a model lipid bilayer.
These data address the overlooked problem of the physical distance
between the vitamin’s reducing hydrogen and lipid acyl chain
radicals. Our combined data from neutron diffraction, NMR, and UV
spectroscopy experiments all suggest that reduction of reactive oxygen
species and lipid radicals occurs specifically at the membrane’s
hydrophobic–hydrophilic interface. The latter is possible when
the acyl chain “snorkels” to the interface from the
hydrocarbon matrix. Moreover, not all model lipids are equal in this
regard, as indicated by the small differences in vitamin’s
location. The present result is a clear example of the importance
of lipid diversity in controlling the dynamic structural properties
of biological membranes. Importantly, our results suggest that measurements
of aToc oxidation kinetics, and its products, should be revisited
by taking into consideration the physical properties of the membrane
in which the vitamin resides
Morphology-Induced Defects Enhance Lipid Transfer Rates
Molecular
transfer between nanoparticles has been considered to
have important implications regarding nanoparticle stability. Recently,
the interparticle spontaneous lipid transfer rate constant for discoidal
bicelles was found to be very different from spherical, unilamellar
vesicles (ULVs). Here, we investigate the mechanism responsible for
this discrepancy. Analysis of the data indicates that lipid transfer
is entropically favorable, but enthalpically unfavorable with an activation
energy that is independent of bicelle size and long- to short-chain
lipid molar ratio. Moreover, molecular dynamics simulations reveal
a lower lipid dissociation energy cost in the vicinity of interfaces
(“defects”) induced by the segregation of the long-
and short-chain lipids in bicelles; these defects are not present
in ULVs. Taken together, these results suggest that the enhanced lipid
transfer observed in bicelles arises from interfacial defects as a
result of the hydrophobic mismatch between the long- and short-chain
lipid species. Finally, the observed lipid transfer rate is found
to be independent of nanoparticle stability
Bilayer Thickness Mismatch Controls Domain Size in Model Membranes
The observation of lateral phase separation in lipid bilayers has
received considerable attention, especially in connection to lipid
raft phenomena in cells. It is widely accepted that rafts play a central
role in cellular processes, notably signal transduction. While micrometer-sized
domains are observed with some model membrane mixtures, rafts much
smaller than 100 nmî—¸beyond the reach of optical microscopyî—¸are
now thought to exist, both in vitro and in vivo. We have used small-angle
neutron scattering, a probe free technique, to measure the size of
nanoscopic membrane domains in unilamellar vesicles with unprecedented
accuracy. These experiments were performed using a four-component
model system containing fixed proportions of cholesterol and the saturated
phospholipid 1,2-distearoyl-<i>sn</i>-glycero-3-phosphocholine
(DSPC), mixed with varying amounts of the unsaturated phospholipids
1-palmitoyl-2-oleoyl-<i>sn</i>-glycero-3-phosphocholine
(POPC) and 1,2-dioleoyl-<i>sn</i>-glycero-3-phosphocholine
(DOPC). We find that liquid domain size increases with the extent
of acyl chain unsaturation (DOPC:POPC ratio). Furthermore, we find
a direct correlation between domain size and the mismatch in bilayer
thickness of the coexisting liquid-ordered and liquid-disordered phases,
suggesting a dominant role for line tension in controlling domain
size. While this result is expected from line tension theories, we
provide the first experimental verification in free-floating bilayers.
Importantly, we also find that changes in bilayer thickness, which
accompany changes in the degree of lipid chain unsaturation, are entirely
confined to the disordered phase. Together, these results suggest
how the size of functional domains in homeothermic cells may be regulated
through changes in lipid composition
The in vivo structure of biological membranes and evidence for lipid domains
<div><p>Examining the fundamental structure and processes of living cells at the nanoscale poses a unique analytical challenge, as cells are dynamic, chemically diverse, and fragile. A case in point is the cell membrane, which is too small to be seen directly with optical microscopy and provides little observational contrast for other methods. As a consequence, nanoscale characterization of the membrane has been performed ex vivo or in the presence of exogenous labels used to enhance contrast and impart specificity. Here, we introduce an isotopic labeling strategy in the gram-positive bacterium <i>Bacillus subtilis</i> to investigate the nanoscale structure and organization of its plasma membrane in vivo. Through genetic and chemical manipulation of the organism, we labeled the cell and its membrane independently with specific amounts of hydrogen (H) and deuterium (D). These isotopes have different neutron scattering properties without altering the chemical composition of the cells. From neutron scattering spectra, we confirmed that the <i>B</i>. <i>subtilis</i> cell membrane is lamellar and determined that its average hydrophobic thickness is 24.3 ± 0.9 Ångstroms (Å). Furthermore, by creating neutron contrast within the plane of the membrane using a mixture of H- and D-fatty acids, we detected lateral features smaller than 40 nm that are consistent with the notion of lipid rafts. These experiments—performed under biologically relevant conditions—answer long-standing questions in membrane biology and illustrate a fundamentally new approach for systematic in vivo investigations of cell membrane structure.</p></div
Comparison between the in-plane scans of DPPC-d62 bilayers with 32.5 mol% cholesterol using a) the conventional high energy resolution (small Δ<i>E</i>) setup and b) the low energy resolution (large Δ<i>E</i>) setup.
<p>The data are denoted by circles with the fit shown as a solid line. A disordered structure was observed in a), while the sharp features in b) are indicative of the presence of highly ordered lipid domains. A top view of the corresponding molecular structures are shown in the insets to the Figure using the same symbols as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066162#pone-0066162-g001" target="_blank">Figure 1</a> b); the quasi-Bragg reflections are indicated by vertical dashed lines and their associated Miller indices, [<i>hkl</i>]. Peaks resulting from the silicon substrates and the aluminum sample chamber (as described in the Materials and Methods Section) are highlighted in grey, but not accounted for in the fit.</p