Influence of cholesterol on equilibrium and dynamic bilayer structure of unsaturated acyl chain phosphatidylcholine vesicles as determined from higher order analysis of fluorescence anisotropy decay

Different lateral packing stress in acyl chains alters KcsA orientation and structure in lipid membranes

The molecular structures of the various intrinsic lipids in membranes regulate lipid-protein interactions. These different lipid structures with unique volumes produce different lipid molecular packing stresses/lateral stresses in lipid membranes. Most studies examining lipid packing effects have used phosphatidylcholine and phosphatidylethanolamine (PE), which are the main phospholipids of eukaryotic cell membranes. In contrast, Gram-negative or Gram-positive bacterial membranes are composed primarily of phosphatidylglycerol (PG) and PE, and the physical and thermodynamic properties of each acyl chain in PG at the molecular level remain unresolved. In this study, we used 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG, 16:0-18:1 PG) and 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (PAPG, 16:0-20:4 PG) to prepare lipid bilayers (liposome) with the rod-type fluorescence probe DPH. We measured the lipid packing conditions by determining the rotational freedom of DPH in POPG or PAPG bilayers. Furthermore, we investigated the effect of different monoacyl chains on a K+ channel (KcsA) structure when embedded in POPG or PAPG membranes. The results revealed that differences in the number of double bonds and carbon chain length in the monoacyl chain at sn-2 affected the physicochemical properties of the membrane and the structure and orientation of KcsA.

Membrane mediated mechanical stimuli produces distinct active-like states in the AT1 receptor

The Angiotensin II Type 1 (AT1) receptor is one of the most widely studied GPCRs within the context of biased signaling. While the AT1 receptor is activated by agonists such as the peptide AngII, it can also be activated by mechanical stimuli such as membrane stretch or shear in the absence of a ligand. Despite the importance of mechanical activation of the AT1 receptor in biological processes such as vasoconstriction, little is known about the structural changes induced by external physical stimuli mediated by the surrounding lipid membrane. Here, we present a systematic simulation study that characterizes the activation of the AT1 receptor under various membrane environments and mechanical stimuli. We show that stability of the active state is highly sensitive to membrane thickness and tension. Structural comparison of membrane-mediated vs. agonist-induced activation shows that the AT1 receptor has distinct active conformations. This is supported by multi-microsecond free energy calculations that show unique landscapes for the inactive and various active states. Our modeling results provide structural insights into the mechanical activation of the AT1 receptor and how it may produce different functional outcomes within the framework of biased agonism.

Composition effects on photooxidative membrane destabilization by TiO2 nanoparticles

Membrane interactions and photooxidative membrane destabilization of titanium dioxide (TiO₂) nanoparticles were investigated, focusing on the effects of membrane composition, notably phospholipid headgroup charge and presence of cholesterol. For this, we employed a battery of state-of-the-art methods for studies of bilayers formed by zwitterionic palmitoyloleoylphosphatidylcholine (POPC) containing also polyunsaturated palmitoylarachidonoylphosphocholine (PAPC), as well as its mixtures with anionic palmitoyloleoylphosphatidylglycerol (POPG) and cholesterol. It was found that the TiO₂ nanoparticles display close to zero charge at pH 7.4, resulting in aggregation. At pH 3.4, in contrast, the 6 nm TiO₂ nanoparticles are well dispersed due to a strongly positive ζ-potential. Mirroring this pH dependence, TiO₂ nanoparticles were observed to bind to negatively charged lipid bilayers at pH 3.4, but much less so at pH 7.4. While nanoparticle binding has some destabilizing effect alone, illumination with ultraviolet (UV) light accentuates membrane destabilization, a result of oxidative stress caused by generated reactive oxygen species (ROS). Neutron reflectivity (NR), quartz crystal microbalance (QCM), and small-angle X-ray scattering (SAXS) results all demonstrate that membrane composition strongly influences membrane interactions and photooxidative destabilization of lipid bilayers. In particular, the presence of anionic POPG makes the bilayers more sensitive to oxidative destabilization, whereas a stabilizing effect was observed in the presence of cholesterol. Also, structural aspects of peroxidation were found to depend strongly on membrane composition, notably the presence of anionic phospholipids. The results show that membrane interactions and UV-induced ROS generation act in concert and need to be considered together to understand effects of lipid membrane composition on UV-triggered oxidative destabilization by TiO₂ nanoparticles, e.g., in the context of oxidative damage of bacteria and cells.

Fluorescent probes sensitive to changes in the cholesterol-to-phospholipids molar ratio in human platelet membranes during atherosclerosis

Environment-sensitive fluorescent probes were used for the spectroscopic visualization of pathological changes in human platelet membranes during cerebral atherosclerosis. It has been estimated that the ratiometric probes 2-(2'-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole and 2-phenyl-phenanthr[9,10]oxazole can detect changes in the cholesterol-to-phospholipids molar ratio in human platelet membranes during the disease.

Cholesterol in the rod outer segment: A complex role in a "simple" system

The rod outer segment (ROS) of retinal photoreceptor cells consists of disk membranes surrounded by the plasma membrane. It is a relatively uncomplicated system in which to investigate cholesterol distribution and its functional consequences in biologically relevant membranes. The light sensitive protein, rhodopsin is the major protein in both membranes, but the lipid compositions are significantly different in the disk and plasma membranes. Cholesterol is high in the ROS plasma membrane. Disk membranes are synthesized at the base of the ROS and are also high in cholesterol. However, cholesterol is rapidly depleted as the disks are apically displaced. During this apical displacement the disk phospholipid fatty acyl chains become progressively more unsaturated, which creates an environment unfavorable to cholesterol. Membrane cholesterol has functional consequences. The high cholesterol found in the plasma membrane and in newly synthesized disks inhibits the activation of rhodopsin. As disks are apically displaced and cholesterol is depleted rhodopsin becomes more responsive to light. This effect of cholesterol on rhodopsin activation has been shown in both native and reconstituted membranes. The modulation of activity can be at least partially explained by the effect of cholesterol on bulk lipid properties. Cholesterol decreases the partial free volume of the hydrocarbon region of the bilayer and thereby inhibits rhodopsin conformational changes required for activation. However, cholesterol binds to rhodopsin and may directly affect the protein also. Furthermore, cholesterol stabilizes rhodopsin to thermal denaturation. The membrane must provide an environment that allows rhodopsin conformational changes required for activation while also stabilizing the protein to thermal denaturation. Cholesterol thus plays a complex role in modulating the activity and stability of rhodopsin, which have implications for other G-protein coupled receptors.

Probing the Microenvironment in the Confined Pores of Mesoporous Silica Nanoparticles

The microenvironment inside of the pores of mesoporous silica nanoparticles is probed using spectroscopic techniques. The probe molecules are sealed inside of the pores by a nanovalve system that is capable of controlling the access to the pore and ensuring the exclusive probing of the pore environment without any interference from the probe molecules on the outer surface of the particles or from the surrounding solution. Rigidochromism studies are used to evaluate the rigidity of the solvent matrix inside of the pore, and dynamic fluorescence anisotropy experiments are employed to determine the rotational diffusion freedom of the probe molecule. The data show that those probe molecules are neither completely free to move nor tightly attached to the pore wall, and their mobility is changed by altering the charge of the pore wall.

Quantifying the differential effects of DHA and DPA on the early events in visual signal transduction

A range of evidence from animal, clinical and epidemiological studies indicates that highly polyunsaturated acyl chains play important roles in development, cognition, vision and other aspects of neurological function. In a number of these studies n3 polyunsaturated fatty acids (PUFAs) appear to be more efficacious than n6 PUFAs. In a previous study of retinal rod outer segments obtained from rats raised on either an n3 adequate or deficient diet, we demonstrated that the replacement of 22:6n3 by 22:5n6 in the n3 deficient rats led to functional deficits in each step in the visual signaling process (Niu et al., 2004). In this study, we examined rhodopsin and phosphodiesterase function and acyl chain packing properties in membranes consisting of phosphatidylcholines with sn-1=18:0, and sn-2=22:6n3, 22:5n6, or 22:5n3 in order to determine if differences in function are due to the loss of one double bond or due to differences in double bond location. At 37 °C the n6 lipid shifted the equilibrium between the active metarhodopsin II (MII) state and inactive metarhodopsin I (MI) state towards MI. In addition, 22:5n6 reduced the rates of MII formation and MII-transducin complex formation by 2- and 6-fold, respectively. At a physiologically relevant level of rhodopsin light stimulation, the activity of phosphodiesterase was reduced by 50% in the 22:5n6 membrane, relative to either of the n3 membranes. Activity levels in the two n3 membranes were essentially identical. Ensemble acyl chain order was assessed with time-resolved fluorescence measurements of the membrane probe diphenylhexatriene (DPH). Analysis in terms of the orientational distribution of DPH showed that acyl chain packing in the two n3 membranes is quite similar, while in the 22:5n6 membrane there was considerably less packing disorder in the bilayer midplane. These results demonstrate that the n3 bond configuration uniquely optimizes the early steps in signaling via a mechanism which may involve acyl chain packing deep in the bilayer.

The bilayer enhances rhodopsin kinetic stability in bovine rod outer segment disk membranes

Rhodopsin is a kinetically stable protein constituting >90% of rod outer segment disk membrane protein. To investigate the bilayer contribution to rhodopsin kinetic stability, disk membranes were systematically disrupted by octyl-β-D-glucopyranoside. Rhodopsin kinetic stability was examined under subsolubilizing (rhodopsin in a bilayer environment perturbed by octyl-β-D-glucopyranoside) and under fully solubilizing conditions (rhodopsin in a micelle with cosolubilized phospholipids). As determined by DSC, rhodopsin exhibited a scan-rate-dependent irreversible endothermic transition at all stages of solubilization. The transition temperature (T(m)) decreased in the subsolubilizing stage. However, once the rhodopsin was in a micelle environment there was little change of the T(m) as the phospholipid/rhodopsin ratio in the mixed micelles decreased during the fully solubilized stage. Rhodopsin thermal denaturation is consistent with the two-state irreversible model at all stages of solubilization. The activation energy of denaturation (E(act)) was calculated from the scan rate dependence of the T(m) and from the rate of rhodopsin thermal bleaching at all stages of solubilization. The E(act) as determined by both techniques decreased in the subsolubilizing stage, but remained constant once fully solubilized. These results indicate the bilayer structure increases the E(act) to rhodopsin denaturation.

The phospholipid monolayer associated with perilipin-enriched lipid droplets is a highly organized rigid membrane structure

The significance of lipid droplets (LD) in lipid metabolism, cell signaling, and membrane trafficking is increasingly recognized, yet the role of the LD phospholipid monolayer in LD protein targeting and function remains unknown. To begin to address this issue, two populations of LD were isolated by ConA sepharose affinity chromatography: 1) functionally active LD enriched in perilipin, caveolin-1, and several lipolytic proteins, including ATGL and HSL; and 2) LD enriched in ADRP and TIP47 that contained little to no lipase activity. Coimmunoprecipitation experiments confirmed the close association of caveolin and perilipin and lack of interaction between caveolin and ADRP, in keeping with the separation observed with the ConA procedure. The phospholipid monolayer structure was evaluated to reveal that the perilipin-enriched LD exhibited increased rigidity (less fluidity), as shown by increased cholesterol/phospholipid, Sat/Unsat, and Sat/MUFA ratios. These results were confirmed by DPH-TMA, NBD-cholesterol, and NBD-sphingomyelin fluorescence polarization studies. By structure and organization, the perilipin-enriched LD most closely resembled the adipocyte PM. In contrast, the ADRP/TIP47-enriched LD contained a more fluid monolayer membrane, reflecting decreased polarizations and lipid order based on phospholipid fatty acid analysis. Taken together, results indicate that perilipin and associated lipolytic enzymes target areas in the phospholipid monolayer that are highly organized and rigid, similar in structure to localized areas of the PM where cholesterol and fatty acid uptake and efflux occur.

Cholesterol depletion of hepatoma cells impairs hepatitis B virus envelopment by altering the topology of the large envelope protein

Previous reports have shown that cholesterol depletion of the membrane envelope of the hepatitis B virus (HBV) impairs viral infection of target cells. A potential function of this lipid in later steps of the viral life cycle remained controversial, with secretion of virions and subviral particles (SVP) being either inhibited or not affected, depending on the experimental approach employed to decrease the intracellular cholesterol level. This work addressed the role of host cell cholesterol on HBV replication, assembly, and secretion, using an alternative method to inhibition of the enzymes involved in the biosynthesis pathway. Growing HBV-producing cells with lipoprotein-depleted serum (LPDS) resulted in an important reduction of the amount of cholesterol within 24 h of treatment (about 40%). Cell exposure to chlorpromazine, an inhibitor of the clathrin-mediated pathway used by the low-density lipoprotein receptor for endocytosis, also impacted the cholesterol level; however, this level of inhibition was not achievable when the synthesis inhibitor lovastatin was used. HBV secretion was significantly inhibited in cholesterol-depleted cells (by ∼80%), while SVP release remained unaffected. The viral DNA genome accumulated in LPDS-treated cells in a time-dependent manner. Specific immunoprecipitation of nucleocapsids and mature virions revealed an increased amount of naked nucleocapsids, while synthesis of the envelope proteins occurred as normally. Following analysis of the large envelope protein conformation in purified microsomes, we concluded that cholesterol is important in maintaining the dual topology of this polypeptide, which is critical for viral envelopment.

The role of the lipid matrix for structure and function of the GPCR rhodopsin

Photoactivation of rhodopsin in lipid bilayers results within milliseconds in a metarhodopsin I (MI)-metarhodopsin II (MII) equilibrium that is very sensitive to the lipid composition. It has been well established that lipid bilayers that are under negative curvature elastic stress from incorporation of lipids like phosphatidylethanolamines (PE) favor formation of MII, the rhodopsin photointermediate that is capable of activating G protein. Furthermore, formation of the MII state is favored by negatively charged lipids like phosphatidylserine and by lipids with longer hydrocarbon chains that yield bilayers with larger membrane hydrophobic thickness. Cholesterol and rhodopsin-rhodopsin interactions from crowding of rhodopsin molecules in lipid bilayers shift the MI-MII equilibrium towards MI. A variety of mechanisms seems to be responsible for the large, lipid-induced shifts between MI and MII: adjustment of the thickness of lipid bilayers to rhodopsin and adjustment of rhodopsin helicity to the thickness of bilayers, curvature elastic deformations in the lipid matrix surrounding the protein, direct interactions of PE headgroups and polyunsaturated hydrocarbon chains with rhodopsin, and direct or lipid-mediated interactions between rhodopsin molecules. This article is part of a Special Issue entitled: Membrane protein structure and function.

Development of fluorophore dynamics imaging as a probe for lipid domains in model vesicles and cell membranes

The ability to detect raft structures in membranes continues to present a problem, especially in the membranes of live cells. Rafts, generally considered to be small (< 200 nm) sphingolipid-rich regions, are commonly modelled using lipid vesicle systems where the ability of fluorophore-labelled lipids to preferentially locate into domains (basically large rafts) is investigated. Instead, in this study the motional properties of different fluorophores were determined using two-photon excitation and time-correlated single-photon counting coupled with diffraction-limited imaging with polarizing optics in scanning mode to obtain nanosecond rotational correlation time images. To develop the method, well-characterized domain-containing models consisting of giant unilamellar vesicles comprising mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, sphingomyelin and cholesterol were used with the fluorophores diphenylhexatriene, 1-palmitoyl-2-{6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]hexanoyl}-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl). Accordingly, images of rotational correlation times of the probes revealed domain structures for all three probes consistent with other studies using different approaches. Rotational correlation time images of living cell membranes were also observed. The method has the advantage that not only does it enable domains to be visualised or imaged in a unique manner but that it can also potentially provide useful information on the lipid dynamics within the structures.

Connexin channels and phospholipids: association and modulation

BACKGROUND

For membrane proteins, lipids provide a structural framework and means to modulate function. Paired connexin hemichannels form the intercellular channels that compose gap junction plaques while unpaired hemichannels have regulated functions in non-junctional plasma membrane. The importance of interactions between connexin channels and phospholipids is poorly understood.

RESULTS

Endogenous phospholipids most tightly associated with purified connexin26 or connexin32 hemichannels or with junctional plaques in cell membranes, those likely to have structural and/or modulatory effects, were identified by tandem electrospray ionization-mass spectrometry using class-specific interpretative methods. Phospholipids were characterized by headgroup class, charge, glycerol-alkyl chain linkage and by acyl chain length and saturation. The results indicate that specific endogenous phospholipids are uniquely associated with either connexin26 or connexin32 channels, and some phospholipids are associated with both. Functional effects of the major phospholipid classes on connexin channel activity were assessed by molecular permeability of hemichannels reconstituted into liposomes. Changes to phospholipid composition(s) of the liposome membrane altered the activity of connexin channels in a manner reflecting changes to the surface charge/potential of the membrane and, secondarily, to cholesterol content. Together, the data show that connexin26 and connexin32 channels have a preference for tight association with unique anionic phospholipids, and that these, independent of headgroup, have a positive effect on the activity of both connexin26 and connexin32 channels. Additionally, the data suggest that the likely in vivo phospholipid modulators of connexin channel structure-function that are connexin isoform-specific are found in the cytoplasmic leaflet. A modulatory role for phospholipids that promote negative curvature is also inferred.

CONCLUSION

This study is the first to identify (endogenous) phospholipids that tightly associate with connexin channels. The finding that specific phospholipids are associated with different connexin isoforms suggests connexin-specific regulatory and/or structural interactions with lipid membranes. The results are interpreted in light of connexin channel function and cell biology, as informed by current knowledge of lipid-protein interactions and membrane biophysics. The intimate involvement of distinct phospholipids with different connexins contributes to channel structure and/or function, as well as plaque integrity, and to modulation of connexin channels by lipophilic agents.

Interaction of an amphipathic peptide with phosphatidycholine/phosphatidylethanolamine mixed membranes

The effect of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) in mixed membranes with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) on interaction with a class A amphipathic peptide, Ac-DWLKAFYDKVAEKLKEAF-NH(2) (Ac-18A-NH(2)), was investigated. The fluorescence lifetime of 2-(9-anthroyloxy)stearic acid and (2)H NMR spectra were used to evaluate the penetration of water molecules into the membrane interface and the order of lipid acyl chains, respectively. The results demonstrated that DOPE in the mixed membranes decreased the fluorescence lifetime and increased the acyl-chain order, and that Ac-18A-NH(2) affected them more for membranes with higher DOPE fractions. The partition coefficient (K(p)) of the peptide to the mixed membranes was increased with the increase in the DOPE mole fractions. From the temperature dependence of the K(p) values, the binding of Ac-18A-NH(2) to POPC/DOPE mixed membranes was found to be entropy-driven. The formation of an alpha-helix at the membrane's surface is supposed to induce positive curvature strain, which decreases the headgroup hydration and acyl-chain order of lipids. Thus, the binding of Ac-18A-NH(2) to membranes is entropically more favorable at higher DOPE fractions since the peptide's insertion into the membrane can decrease the order parameter and unfavorable headgroup hydration, which explains the enhanced peptide binding.

Effect of aminophospholipid glycation on order parameter and hydration of phospholipid bilayer

The effect of aminophospholipid glycation on lipid order and lipid bilayer hydration was investigated using time-resolved fluorescence spectroscopy. The changes of lipid bilayer hydration were estimated both from its effect on the fluorescence lifetime of The 1-[4-(trimethylammonium)-phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH) and 1,6-diphenylhexa-1,3,5-triene (DPH) and using solvatochromic shift studies with 1-anilinonaphthalene-8-sulfonic acid. The head-group and acyl chain order were determined from time-resolved fluorescence anisotropy measurements of the TMA-DPH and DPH. The suspensions of small unilamellar vesicles (with phosphatidylethanolamine/phosphatidylcholine molar ratio 1:2.33) were incubated with glyceraldehyde and it was found that aminophospholipids react with glyceraldehyde to form products with the absorbance and the fluorescence properties typical for protein advanced glycation end products. The lipid glycation was accompanied by the progressive oxidative modification of unsaturated fatty acid residues. It was found that aminophospholipid glycation increased the head-group hydration and lipid order in both regions of the membrane. The lipid oxidation accompanying the lipid glycation affected mainly the lipid order, while the effect on the lipid hydration was small. The increase in the lipid order was presumably the result of two effects: (1) the modification of head-groups of phosphatidylethanolamine by glycation; and (2) the degradation of unsaturated fatty acid residues by oxidation.

Comparison of excitation and emission ratiometric fluorescence methods for quantifying the membrane dipole potential

We are interested in developing fluorescence methods for quantifying lateral variations in the dipole potential across cell surfaces. Previous work in this laboratory showed that the ratio of fluorescence intensities of the voltage-sensitive dye di-8-ANEPPS using excitation wavelengths at 420 and 520 nm correlates well with measurements of the dipole potential. In the present work we evaluate the use of di-8-ANEPPS and an emission ratiometric method for measuring dipole potentials, as Bullen and Saggau (Biophys. J. 65 (1999) 2272-2287) have done to follow changes in the membrane potential in the presence of an externally applied field. Emission ratiometric methods have distinct advantages over excitation methods when applied to fluorescence microscopy because only a single wavelength is needed for excitation. We found that unlike the excitation ratio, the emission ratio does not correlate with the dipole potential of vesicles made from different lipids. A difference in the behaviour of the emission ratio in saturated compared to unsaturated lipid vesicles was noted. Furthermore, the emission ratio did not respond in the same way as the excitation ratio when cholesterol, 6-ketocholestanol, 7-ketocholesterol, and phloretin were added to dimyristoylphosphatidylcholine (DMPC) vesicles. We attribute the lack of correlation between the emission ratio and the dipole potential to simultaneous changes in membrane fluidity caused by changes in membrane composition, which do not occur when the electric field is externally applied as in the work of Bullen and Saggau. Di-8-ANEPPS can, thus, only be used via an excitation ratiometric method to quantify the dipole potential.

Unassisted translocation of large polypeptide domains across phospholipid bilayers

Although transmembrane proteins generally require membrane-embedded machinery for integration, a few can insert spontaneously into liposomes. Previously, we established that the tail-anchored (TA) protein cytochrome b(5) (b5) can posttranslationally translocate 28 residues downstream to its transmembrane domain (TMD) across protein-free bilayers (Brambillasca, S., M. Yabal, P. Soffientini, S. Stefanovic, M. Makarow, R.S. Hegde, and N. Borgese. 2005. EMBO J. 24:2533–2542). In the present study, we investigated the limits of this unassisted translocation and report that surprisingly long (85 residues) domains of different sequence and charge placed downstream of b5's TMD can posttranslationally translocate into mammalian microsomes and liposomes at nanomolar nucleotide concentrations. Furthermore, integration of these constructs occurred in vivo in translocon-defective yeast strains. Unassisted translocation was not unique to b5 but was also observed for another TA protein (protein tyrosine phosphatase 1B) whose TMD, like the one of b5, is only moderately hydrophobic. In contrast, more hydrophobic TMDs, like synaptobrevin's, were incapable of supporting unassisted integration, possibly because of their tendency to aggregate in aqueous solution. Our data resolve long-standing discrepancies on TA protein insertion and are relevant to membrane evolution, biogenesis, and physiology.

Effects of an Amphipathic α-Helical Peptide on Lateral Pressure and Water Penetration in Phosphatidylcholine and Monoolein Mixed Membranes

The physicochemical properties of mixed membranes of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and a nonlamellar-forming lipid, 1-monoolein (MO), and the effects of an amphipathic alpha-helical peptide, 18A (DWLKAFYDKVAEKLKEAF), on the membranes were investigated by fluorescence measurements and 31P NMR. The intramolecular excimer formation of dipyrenylphosphatidylcholines showed that the increased lateral pressure near the bilayer center by MO is reduced by the lamellar-cubic phase transition at an MO mole fraction of 0.7, while the lateral pressure near the polar-apolar interface increases even through the phase transition. The fluorescence lifetime of 2-(9-anthroyloxy)stearic acid revealed that water penetration into the interface region increases with the MO fraction. The insertion of the 18A peptide into the membrane interface region decreased both the lateral pressure near the interface and water penetration, and shifted the lamellar-cubic phase transition to a higher MO fraction. This suggests that 18A induces a positive curvature strain and lowers the lateral pressure and water penetration. Furthermore, the increase in the MO fraction in POPC/MO LUV promoted partitioning of 18A to the membranes. This preferential binding to the MO-containing membranes is presumably ascribed to the propensity of 18A to reduce the membrane strain.

Lipid composition effect on permeability across PAMPA

The parallel artificial membrane permeability assay (PAMPA) system has promise to rapidly screen drug candidate passive permeability, but has been poorly described in terms of its lipid membrane structure and function. The objective was to investigate the role of PAMPA lipid composition on the permeability of five model compounds. PAMPA was used and employed individual phospholipids that varied in phosphate head group and acyl chain unsaturation. Transport of benzoic acid, taurocholic acid, metoprolol, sucrose, and mannitol was measured. Membrane fluidity was assessed by 1,3-diphenylhexatriene fluorescence anisotropy. Results indicate that compound permeability across PAMPA differed in their sensitivity to membrane lipid composition, where compounds with appreciable permeability (i.e. at least 0.2 x 10(-6)cm/s) were possibly sensitive to membrane fluidity and apparent ion pair effects. Benzoic acid permeability ranged 51-fold across membrane types, suggesting acyl chain effect on membrane fluidity. Mannitol, sucrose, and taurocholic acid permeabilities were low and independent of lipid composition. Metoprolol permeability ranged 17-fold and exhibited a markedly high permeability across 1,2-dioleoyl-sn-glycero-3-[phospho-L-serine] due to apparent ion pair-facilitated transport. Compound permeability was lowest across the phosphatidylcholines, which is consistent with phosphatidylcholine exhibiting relatively high membrane rigidity. In contrast to results from phosphatidylethanolamines and phosphatidylserines, acyl chain unsaturation had no effect on permeability across phosphatidylcholines. In conclusion, while much remains unknown about PAMPA structure and subsequent PAMPA permeability, results here from five solutes suggest that, for solutes with appreciable permeability, lipid composition modulated drug permeability through possible membrane fluidity and apparent ion pair influences.

Ion pair-mediated transport of metoprolol across a three lipid-component PAMPA system

Parallel Artificial Membrane Permeability Assay (PAMPA) is a method to screen drug candidates for membrane permeability. The objective was to characterize the transport of a model weak base, metoprolol, across a three lipid-component PAMPA system (denoted A-PAMPA, for anionic-PAMPA) and challenge ion pairing as a mechanism for metoprolol transport. A-PAMPA was designed to mimic the lipid composition of the enterocyte's plasma membrane and included 1,2-dioleoyl-sn-glycero-3-[phospho-l-serine] (PS18:1) as an anionic lipid-component. Metoprolol flux was measured across A-PAMPA, as well as across three other PAMPA systems. Permeability studies were conducted under various conditions, with varying pH, ionic strength, and presence/absence of competing cations. Permeabilities of mannitol and benzoic acid, as model neutral and anionic solutes, were also measured. PAMPA membrane fluidity was inferred from anisotropy measurements in liposomes. Ion pairing between metoprolol and PS18:1 was assessed via NMR. Metoprolol transport across A-PAMPA was dominated by an ion pair-mediated mechanism (i.e. metoprolol-PS18:1 complex), rather than a membrane fluidity-mediated mechanism. Compared to other PAMPA systems, metoprolol permeability across A-PAMPA and PS18:1 was high. Permeability and anisotropy values suggested PS18:1 selectively facilitated metoprolol transport, while neutral lipid did not. Additional studies supporting ion pairing of metoprolol across A-PAMPA showed that a) metoprolol transport was self-inhibited across A-PAMPA but not across neutral lipid PAMPA; b) competing cations reduced metoprolol permeability across A-PAMPA but not across neutral lipid PAMPA; and c) NMR spectrum of a mixture of metoprolol and PS18:1 showed a broadening of some metoprolol peaks, presumably due to metoprolol interaction with anionic lipid. Metoprolol transport across a three lipid-component PAMPA system that contained anionic lipid was facilitated by apparent ion pairing.

Lipid raft components cholesterol and sphingomyelin increase H+/OH- permeability of phosphatidylcholine membranes

H+/OH- permeation through lipid bilayers occurs at anomalously high rates and the determinants of proton flux through membranes are poorly understood. Since all life depends on proton gradients, it is important to develop a greater understanding of proton leak phenomena. We have used stopped-flow fluorimetry to probe the influence of two lipid raft components, chol (cholesterol) and SM (sphingomyelin), on H+/OH- and water permeability. Increasing the concentrations of both lipids in POPC (palmitoyl-2-oleoyl phosphatidylcholine) liposomes decreased water permeability in a concentration-dependent manner, an effect that correlated with increased lipid order. Surprisingly, proton flux was increased by increasing the concentration of chol and SM. The chol effect was complex with molar concentrations of 17.9, 33 and 45.7% giving 2.8-fold (P<0.01), 2.2-fold (P<0.001) and 5.1-fold (P<0.001) increases in H+/OH- permeability from a baseline of 2.4x10(-2) cm/s. SM at 10 mole% effected a 2.8-fold increase (P<0.01), whereas 20 and 30 mole% enhanced permeability by 3.6-fold (P<0.05) and 4.1-fold respectively (P<0.05). Supplementing membranes containing chol with SM did not enhance H+/OH- permeability. Of interest was the finding that chol addition to soya-bean lipids decreased H+/OH- permeability, consistent with an earlier report [Ira and Krishnamoorthy (2001) J. Phys. Chem. B 105, 1484-1488]. We speculate that the presence of proton carriers in crude lipid extracts might contribute to this result. We conclude that (i) chol and SM specifically and independently increase rates of proton permeation in POPC bilayers, (ii) domains enriched in these lipids or domain interfaces may represent regions with high H+/OH- conductivity, (iii) H+/OH- fluxes are not governed by lipid order and (iv) chol can inhibit or promote H+/OH- permeability depending on the total lipid environment. Theories of proton permeation are discussed in the light of these results.

A New Fluorescent Squaraine Probe for the Measurement of Membrane Polarity

The present study was undertaken to evaluate the sensitivity of newly synthesized squaraine dye 1 to the changes in lipid bilayer physical properties and compared it with the well-known dye 2. Partitioning of the dye 1 into lipid bilayer was found to be followed by significant increase of its fluorescence intensity and red-shift of emission maximum, while intensity of the dye 2 fluorescence increased only slightly on going from aqueous to lipidic environment. This suggests that dye 1 is more sensitive to the changes in membrane properties as compared to dye 2. Partition coefficients of the dye 1 have been determined for the model membranes composed of zwitterionic phospholipid phosphatidylcholine (PC) and its mixtures with positively charged detergent cetyltrimethylammonium bromide (CTAB), anionic phospholipid cardiolipin (CL), and sterol (Chol). The spectral responses of the dye 1 in different liposome media proved to correlate with the increase of bilayer polarity induced by Chol and CL or its decrease caused by CTAB. It was concluded that dye 1 can be used as fluorescent probe for examining membrane-related processes.

Cholesterol reduces membrane electroporation and electric deformation of small bilayer vesicles

Electric fields, similar in the order of magnitude of the natural membrane fields of cellular lipid/protein membranes, and chemical relaxation spectrometry can be used as tools to quantify the rigidifying effect of cholesterol in membranes. Small unilamellar vesicles of radius a=50+/-3 nm, prepared form phosphatidylcholine, phosphatidylserine and phosphatidyl-glycerol in the molar ratio 1:1:1 and containing the optical lipid probe molecule 2-(3-diphenyl-hexatrienyl) propanoyl)-1-palmitoyl-sn-glycerol-3-phosphocholine (beta-DPH pPC), serve as examples for curved lipid membranes. The data of electrooptical turbidity and absorbance relaxations at the wavelength lambda=365 nm are analysed in terms of membrane bending rigidity kappa and membrane stretching modulus K. Both kappa and K increase with increasing mole fraction x of cholesterol up to x=0.5. The cholesterol induced denser packing of the lipids reduces the extent of both membrane electroporation (ME) and electroelongation of the vesicles. Further on, cholesterol in the lipid phase and sucrose in the aqueous suspension reduce the extent of membrane undulation and electro-stretching.

The role of cholesterol in rod outer segment membranes

The photoreceptor rod outer segment (ROS) provides a unique system in which to investigate the role of cholesterol, an essential membrane constituent of most animal cells. The ROS is responsible for the initial events of vision at low light levels. It consists of a stack of disk membranes surrounded by the plasma membrane. Light capture occurs in the outer segment disk membranes that contain the photopigment, rhodopsin. These membranes originate from evaginations of the plasma membrane at the base of the outer segment. The new disks separate from the plasma membrane and progressively move up the length of the ROS over the course of several days. Thus the role of cholesterol can be evaluated in two distinct membranes. Furthermore, because the disk membranes vary in age it can also be investigated in a membrane as a function of the membrane age. The plasma membrane is enriched in cholesterol and in saturated fatty acids species relative to the disk membrane. The newly formed disk membranes have 6-fold more cholesterol than disks at the apical tip of the ROS. The partitioning of cholesterol out of disk membranes as they age and are apically displaced is consistent with the high PE content of disk membranes relative to the plasma membrane. The cholesterol composition of membranes has profound consequences on the major protein, rhodopsin. Biophysical studies in both model membranes and in native membranes have demonstrated that cholesterol can modulate the activity of rhodopsin by altering the membrane hydrocarbon environment. These studies suggest that mature disk membranes initiate the visual signal cascade more effectively than the newly synthesized, high cholesterol basal disks. Although rhodopsin is also the major protein of the plasma membrane, the high membrane cholesterol content inhibits rhodopsin participation in the visual transduction cascade. In addition to its effect on the hydrocarbon region, cholesterol may interact directly with rhodopsin. While high cholesterol inhibits rhodopsin activation, it also stabilizes the protein to denaturation. Therefore the disk membrane must perform a balancing act providing sufficient cholesterol to confer stability but without making the membrane too restrictive to receptor activation. Within a given disk membrane, it is likely that cholesterol exhibits an asymmetric distribution between the inner and outer bilayer leaflets. Furthermore, there is some evidence of cholesterol microdomains in the disk membranes. The availability of the disk protein, rom-1 may be sensitive to membrane cholesterol. The effects exerted by cholesterol on rhodopsin function have far-reaching implications for the study of G-protein coupled receptors as a whole. These studies show that the function of a membrane receptor can be modulated by modification of the lipid bilayer, particularly cholesterol. This provides a powerful means of fine-tuning the activity of a membrane protein without resorting to turnover of the protein or protein modification.

Bimodal distribution and fluorescence response of environment-sensitive probes in lipid bilayers

A remarkable heterogeneity is often observed in the spectroscopic properties of environment-sensitive fluorescence probes in phospholipid bilayers. To explain its origin, we provided a detailed investigation of the fluorescence excitation and emission spectra of 4'-dimethylamino-3-hydroxyflavone (probe F) in bilayer vesicles with the variations of fatty acid composition, polar heads, temperature, and cholesterol content. Probe F, due to excited-state intramolecular proton transfer, exhibits two bands in emission that are differently sensitive to intermolecular interactions-thereby allowing us to distinguish universal (dipole-dipole) and specific (H-bonding) interactions within the bilayer. Spectroscopic, quenching, and anisotropy data suggest the presence of two forms of probe F at different locations in the bilayer: an H-bond free form located below sn(1)-carbonyls and an H-bonded form located at the polar membrane interface. We provide a quantitative analysis of the distribution of the probe between these two locations as well as the polarity of these locations, and show that both the distribution and the polarity contribute to the probe response. Moreover, analysis of literature data on other environment-sensitive probes (Prodan, Laurdan, Nile Red, NBD lipids, etc.) in lipid bilayers allows us to suggest that the bimodal distribution in the lipid bilayer is probably a general feature of low-polar molecules with polar groups capable of H-bonding interactions.

Understanding Sterol-Membrane Interactions Part I: Hartree-Fock versus DFT Calculations of13C and1H NMR Isotropic Chemical Shifts of Sterols in Solution and Analysis of Hydrogen-Bonding Effects

1H and 13C NMR chemical shifts are exquisitely sensitive probes of the local environment of the corresponding nuclei. Ultimately, direct determination of the chemical shifts of sterols in their membrane environment has the potential to reveal their molecular interactions and dynamics, in particular concerning the hydrogen-bonding partners of their OH groups. However, this strategy requires an accurate and efficient means to quantify the influence of the various interactions on chemical shielding. Herein the validity of Hartree-Fock and DFT calculations of the 13C and 1H NMR chemical shifts of cholesterol and ergosterol are compared with one another and with experimental chemical shifts measured in solution at 500 MHz. A computational strategy (definition of basis set, simpler molecular models for the sterols themselves and their molecular complexes) is proposed and compared with experimental data in solution. It is shown in particular that the effects of hydrogen bonding with various functional groups (water as a hydrogen-bond donor and acceptor, acetone) on NMR chemical shifts in CDCl3 solution can be accurately reproduced with this computational approach.

Simultaneous probing of hydration and polarity of lipid bilayers with 3-hydroxyflavone fluorescent dyes

The penetration of water into the hydrophobic interior leads to polarity and hydration profiles across lipid membranes which are fundamental in the maintenance of membrane architecture as well as in transport and insertion processes into the membrane. The present paper is an original attempt to evaluate simultaneously polarity and hydration properties of lipid bilayers by a fluorescence approach. We applied two 3-hydroxyflavone probes anchored in lipid bilayers at a relatively precise depth through their attached ammonium groups. They are present in two forms: either in H-bond-free form displaying a two-band emission due to an excited state intramolecular proton transfer reaction (ESIPT), or in H-bonded form displaying a single-band emission with no ESIPT. The individual emission profiles of these forms were obtained by deconvolution of the probes' fluorescence spectra. The polarity of the probe surrounding the bilayer was estimated from the two-band spectra of the H-bond-free form, while the local hydration was estimated from the relative contribution of the two forms. Our results confirm that by increasing the lipid order (phase transition from fluid to gel phase, addition of cholesterol or decrease in the lipid unsaturation), the polarity and to a lesser extent, the hydration of the bilayers decrease simultaneously. In contrast, when fluidity (i.e. lipid order) is kept invariant, increase of temperature and of bilayer curvature leads to a higher bilayer hydration with no effect on the polarity. Furthermore, no correlation was found between dipole potential and the hydration of the bilayers.

Cholesterol and ergosterol influence nystatin surface aggregation: relation to pore formation

Nystatin interaction with liposomes mimicking fungal and mammalian membranes (ergosterol- and cholesterol-containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) large unilamellar vesicles, respectively) was studied by fluorescence spectroscopy. The activity of this antibiotic was also measured using a pyranine fluorescence detected K+/H+ exchange assay. Nystatin mean fluorescence lifetime varied with the antibiotic concentration and ergosterol content (0-30 mol%) of the lipid vesicles. It sharply increased from 5 to 37 ns upon reaching 100 molecules per liposome, reporting nystatin oligomerization in the membrane. Concomitantly, spectral alterations typical of excitonic coupling were detected and there was a pronounced increase in the initial rate of pore formation by nystatin. These findings suggest that nystatin exerts its antibiotic activity via a two-stage mechanism: at low antibiotic concentrations, surface-adsorbed monomeric antibiotic molecules perturb the lipid packing, changing the permeability properties of the ergosterol-rich liposomes. Upon reaching a critical threshold, nystatin mode of action switches to the classical model of transmembrane aqueous channel formation. In the presence of cholesterol-containing POPC liposomes, neither nystatin spectroscopic properties, nor the kinetics of K+ efflux varied with the antibiotic concentration suggesting that in this case the first stage of antibiotic mode of action always prevails or the assemblies formed by nystatin and cholesterol are very loose.

Correlation of membrane order and dynamics derived from time-resolved fluorescence measurements with solute permeability

The relevance of order and dynamics of phospholipid bilayer membranes as detected with fluorescent probe molecules embedded in the membranes for describing their permeability properties was studied. Order parameters (S) and rotational diffusion coefficients (Dperpendicular) of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) in unilamellar vesicles were determined by time-resolved fluorescence spectroscopy. Vesicles consisting of combinations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine, 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine, egg sphingomyelin and cholesterol were studied at 288, 298, and 308 degrees K. Permeability coefficients (P) of the model permeant D-[14C]mannitol were determined. A model was proposed for correlating P with both S and Dperpendicular, where S is linked to the average free surface area per lipid molecule and Dperpendicular reflects lipid thermal motion and, thus, redistribution rate of free surface area of the bilayer. P values ranging from 0.9 to 12.4 x 10(-11) cm/s were well described by the model. This supports the notion that permeation depends on membrane structural and dynamic properties. While changes in both S and Dperpendicular, at relative significance varying with the situation, appeared responsible for the effect of lipid composition on permeability, the effect of temperature on P was related primarily to Dperpendicular. P correlated better with S and Dperpendicular obtained with TMA-DPH rather than DPH. The location of the fluorescent probe molecules within the membranes is discussed as the cause for this difference.

Sphingomyelin and cholesterol: from membrane biophysics and rafts to potential medical applications

The preferential sphingomyelin-cholesterol interaction which results from the structure and the molecular properties of these two lipids seems to be the physicochemical basis for the formation and maintenance of cholesterol/sphingolipid-enriched nano- and micro-domains (referred to as membrane "rafts") in the plane of plasma and other organelle (i.e., Golgi) membranes. This claim is supported by much experimental evidence and also by theoretical considerations. However, although there is a large volume of information about these rafts regarding their lipid and protein composition, their size, and their dynamics, there is still much to be clarified on these issues, as well as on how rafts are formed and maintained. It is well accepted now that the lipid phase of the rafts is the liquid ordered (LO) phase. However, other (non-raft) parts of the membrane may also be in the LO phase. There are indications that the raft LO phase domains are more tightly packed than the non-raft LO phase, possibly due to intermolecular hydrogen bonding involving sphingolipid and cholesterol. This also explains why the former are detergent-resistant membranes (DRM), while the non-raft LO phase domains are detergent-soluble (sensitive) membranes (DSM). Recent findings suggest that protein-protein interactions such as cross-linking can be controlled by protein distribution between raft and non-raft domains, and, as well, these interactions affect raft size distribution. The cholesterol/sphingomyelin-enriched rafts seem to be involved in many biological processes, mediated by various receptors, as exemplified by various lipidated glycosylphosphatidylinositol (GPI)- and acyl chain-anchored proteins that reside in the rafts. The rafts serve as signaling platforms in the cell. Various pathogens (viruses and toxins) utilize the raft domains on the host cell membrane as a port of entry, site of assembly (viruses), and port of exit (viral budding). Existence and maintenance of cholesterol-sphingomyelin rafts are dependent on the level of membrane cholesterol and sphingomyelin. This explains why reduction of cholesterol level--either through reverse cholesterol transport, using cholesterol acceptors such as beta-cyclodextrin, or through cholesterol biosynthesis inhibition using statins--interferes with many processes which involve rafts and can be applied to treating raft-related infections and diseases. Detailed elucidation of raft structure and function will improve understanding of biological membrane composition-structure-function relationships and also may serve as a new avenue for the development of novel treatments for major diseases, including viral infections, neurodegenerative diseases (Alzheimer's), atherosclerosis, and tumors.

Enhancement of G protein-coupled signaling by DHA phospholipids

The effect of phospholipid acyl chain and cholesterol composition on G protein-coupled signaling was studied in native rod outer segment (ROS) disk and reconstituted membranes by measuring several steps in the visual transduction pathway. The cholesterol content of disk membranes was varied from 4 to 38 mol% cholesterol with methyl-beta-cyclodextrin. The visual signal transduction system [rhodopsin, G protein (G(t)), and phosphodiesterase (PDE)] was reconstituted with membranes containing various levels of phospholipid acyl chain unsaturation, with and without cholesterol. ROS membranes from rats raised on n-3 fatty acid-deficient and -adequate diets were also studied. The ability of rhodopsin to form the active metarhodopsin II conformation and bind G(t) was diminished by a reduction in the level of DHA (22:6n-3) acyl chains or an increase in membrane cholesterol. DHA acyl chain containing phospholipids minimized the inhibitory effects of cholesterol on the rate of rhodopsin-G(t) coupling. The activity of PDE, which is a measure of the integrated signal response, was reduced in membranes lacking or deficient in DHA acyl chains. PDE activity in membranes containing docosapentaenoic acid (DPA, 22:5n-6) acyl chains, which replace DHA in n-3 fatty acid deficiency, was 50% lower than in DHA-containing membranes. Our results indicate that efficient and rapid propagation of G protein-coupled signaling is optimized by DHA phospholipid acyl chains.

A 2D-ELDOR study of the liquid ordered phase in multilamellar vesicle membranes

2D-ELDOR spectroscopy has been employed to study the dynamic structure of the liquid-ordered (Lo) phase versus that of the liquid-crystalline (Lc) phase in multibilayer phospholipid vesicles without (Lc) and with (Lo) cholesterol, using end-chain and headgroup labels and spin-labeled cholestane. The spectra are in most cases found to be dramatically different for these two phases. Thus, visual inspection of the 2D-ELDOR spectra provides a convenient way to distinguish the two phases in membranes. Detailed analysis shows these observations are due to increased ordering in the Lo phase and modified reorientation rates. In the Lo phase, acyl chains undergo a faster rotational diffusion and higher ordering than in the Lc phase, whereas spin-labeled cholestane exhibits slower rotational diffusion and higher ordering. On the other hand, the choline headgroup in the Lo phase exhibits faster motion and reduced but realigned ordering versus the Lc phase. The microscopic translational diffusion rates in the Lo phase are significantly reduced in the presence of cholesterol. These results are compared with previous studies, and a consistent model is provided for interpreting them in terms of the differences in the dynamic structure of the Lo and Lc phases.

Manipulation of cholesterol levels in rod disk membranes by methyl-beta-cyclodextrin: effects on receptor activation

The effect of cholesterol on rod outer segment disk membrane structure and rhodopsin activation was investigated. Disk membranes with varying cholesterol concentrations were prepared using methyl-beta-cyclodextrin as a cholesterol donor or acceptor. Cholesterol exchange followed a simple equilibrium partitioning model with a partition coefficient of 5.2 +/- 0.8 in favor of the disk membrane. Reduced cholesterol in disk membranes resulted in a higher proportion of photolyzed rhodopsin being converted to the G protein-activating metarhodopsin II (MII) conformation, whereas enrichment of cholesterol reduced the extent of MII formation. Time-resolved fluorescence anisotropy measurements using 1,6-diphenyl-1,3,5-hexatriene showed that increasing cholesterol reduced membrane acyl chain packing free volume as characterized by the parameter f(v). The level of MII formed showed a positive linear correlation with f(v) over the range of 4 to 38 mol % cholesterol. In addition, the thermal stability of rhodopsin increased with mol % of cholesterol in disk membranes. No evidence was observed for the direct interaction of cholesterol with rhodopsin in either its agonist- or antagonist-bound form. These results indicate that cholesterol mediates the function of the G protein-coupled receptor, rhodopsin, by influencing membrane lipid properties, i.e. reducing acyl chain packing free volume, rather than interacting specifically with rhodopsin.

Inhibition of protein translocation across the endoplasmic reticulum membrane by sterols

Cholesterol and related sterols are known to modulate the physical properties of biological membranes and can affect the activities of membrane-bound protein complexes. Here, we report that an early step in protein translocation across the endoplasmic reticulum (ER) membrane is reversibly inhibited by cholesterol levels significantly lower than those found in the plasma membrane. By UV-induced chemical cross-linking we further show that high cholesterol levels prevent cross-linking between ribosome-nascent chain complexes and components of the Sec61 translocon, but have no effect on cross-linking to the signal recognition particle. The inhibiting effect on translocation is different between different sterols. Our data suggest that the protein translocation machinery may be sensitive to changes in cholesterol levels in the ER membrane.

Large solutes induce structural perturbations in proteins and membranes

Structural perturbations in biopolymers with hydrophobic interiors i.e. specific proteins and dimyristoylphosphatidylcholine (DMPC) vesicles were investigated as a function of solute concentrations in the medium. 1,6-diphenyl-1,3,5-hexatriene (DPH) was used as fluorescent probe. Response of DPH was comparable to that of intrinsic tryptophan in BSA in terms of steady state and time resolved fluorescence. The solutes induced a decrease in steady state anisotropy as well as rotational correlation time (computed from lifetime measurements) for DPH in both proteins and membranes. Enhanced access of the quencher potassium iodide to tryptophan in bovine serum albumin (BSA) and ovalbumin, and enhanced terbium leakage in DMPC vesicles induced by various solutes concomitant with decreased anisotropy/correlation time were consistent with structural perturbations of the nature of defects or voids in these polymers.

Lipid phase separation in phospholipid bilayers and monolayers modeling the plasma membrane

It is postulated that biological membrane lipids are heterogeneously distributed into lipid microdomains. Recent evidence indicates that docosahexaenoic acid-containing phospholipids may be involved in biologically important lipid phase separations. Here we investigate the elastic and thermal properties of a model plasma membrane composed of egg sphingomyelin (SM), cholesterol and 1-stearoyl-2-docosahexaenoyl-sn-glycerophosphoethanolamine (SDPE). Two techniques are employed, pressure-area isotherms on monolayers to examine condensation and interfacial elasticity behavior, and differential scanning calorimetry (DSC) on bilayers to evaluate phase separations. Significant levels of condensation are observed for mixtures of SM and cholesterol. Surface elasticity measurements indicate that cholesterol decreases and SDPE increases the in-plane elasticity of SM monolayers. At X(SDPE)> or =0.15 in SM, a more horizontal region emerges in the pressure-area isotherms indicating 'squeeze out' of SDPE from the monolayers. Addition of cholesterol to equimolar amounts of SM and SDPE further increases the amount of 'squeeze out', supporting the concept of phase separation into a cholesterol- and SM-rich liquid ordered phase and a SDPE-rich liquid disordered phase. This conclusion is corroborated by DSC studies where as little as X(Chol)=0.0025 induces a phase separation between the two lipids.

Osmotic perturbations induce differential movements in the core and periphery of proteins, membranes and micelles

Polymeric structures, namely, micelles, membranes and globular proteins share the property of two distinct regions: a hydrophobic core and a hydrophilic exterior. The dynamics of these regions of the polymeric structures were probed using selective fluorophores 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-anilinonaphthalene-8-sulfonate (ANS), respectively. Perturbation of the polymers by external osmotic pressure, ionic strength and temperature was monitored in the two regions using steady state measurements of fluorescence intensity and anisotropy. While the fluorescence lifetime of DPH and ANS did not change significantly, parallel change in steady state anisotropy values and the rotational correlation time indicated mobility in the probe/probe-domain. Osmotic perturbation of the polymers in electrolyte media led to decreased DPH mobility. Enhanced ellipticity at 222 nm in bovine serum albumin was observed in 1.5 M NaCl and sucrose media. ANS exhibited a decreased anisotropy with progressive dehydration in proteins in NaCl media, in dimyristoylphosphatidylcholine (DMPC) vesicles in sucrose media, and in neutral laurylmaltoside micelles in both NaCl and sucrose media. Thus, ANS showed responses opposite to that of DPH in these systems. A comparison with several domain selective probes indicated that DPH reported findings common to depth probes while ANS reported data common to interfacial probes used for voltage monitoring.

Solvent interconnectedness permits measurement of proximal as well as distant phase transitions in polymer mixtures by fluorescence

We monitored the fluorescence intensity and anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) incorporated in bovine serum albumin (BSA) and dimyristoylphosphatidylcholine (DMPC) vesicle membranes, which in turn were embedded in optically clear gelatin solutions, as a function of temperature. DPH in BSA gave unanticipated large changes in fluorescence intensity and anisotropy at the instant of gelatin gel melting. Both steady state anisotropy and fluorescence intensity reported the gel-sol transition point in gelatin unambiguously, which was independently confirmed as physical-pour point of the gel. In the case of DMPC vesicles, fluorescence intensity indicated the gelatin transition, while the anisotropy indicated DMPC phase transition. This fluorescence methodology uniquely offered a common probe for two distinct transitions in two distinct domains interconnected by the solvent, water.

Alterations in cell lipid metabolism by glycol methacrylate (HEMA)

Components of dental resins such as dimethylaminoethyl methacrylate (DMAEMA) can alter cell lipid composition, presumably by esterase-mediated hydrolysis. The resulting dimethylethanolamine is incorporated into cell phospholipids, while the methacrylic acid may alter several metabolic pathways. We hypothesize that HEMA is cleaved in a similar manner and the released ethylene glycol is incorporated into cell lipids, yielding phosphatidylethylene glycol (PtEG), and the methacrylic acid alters other lipid pathways in a manner similar to that of methacrylic acid released from hydrolysis of DMAEMA. Cultures of hamster buccal pouch (HCP) and rabbit kidney (RK13) epithelial cells were exposed to subtoxic concentrations of HEMA in the presence of [14C]-acetate or [3H]-oleic acid. Other cultures were prelabeled with [14C]-acetate followed by exposure to various concentrations of HEMA. Cell lipids were extracted by the method of Bligh and Dyer and separated by thin layer chromatography on silica gel K-6 plates or SG-81 silica gel loaded chromatography paper. The fate of the ethylene glycol was traced using [14C]-ethylene glycol. Radioactive lipids were located using autoradiography and known standard lipids and quantitated by liquid scintillation spectrometry. In the presence of HEMA several classes of lipids were altered. Among the neutral lipids, the most notable changes involved sterol precursors, triglycerides, fatty acids, and cholesterol esters, while phosphatidylcholine was affected among the phospholipids. The results differed quantitatively between the two cell types. Results also suggest that EG, including that released by hydrolysis of HEMA, is incorporated into cell phospholipids, producing PtEG. The changes in neutral lipid labeling may occur by alteration of lipid synthetic pathways utilizing acetyl Co-A as well as inhibition of enzymes involved in synthesis of cholesterol from sterol precursors and hydrolysis of cholesterol esters. Synthesis of PtEG may take place via phospholipase D-mediated headgroup exchange. Alterations in the cellular lipids may affect cell membrane properties and associated cell functions.

Location and orientation of DODCI in lipid bilayer membranes: effects of lipid chain length and unsaturation

The location and orientation of a linear dye molecule, DODCI, in lipid bilayer membrane were determined by the effect of viscosity and refractive index of the aqueous medium on the fluorescence properties of the dye bound to the membrane. The membrane-bound dye is solubilized in two sites, one near the surface (short fluorescence lifetime) and another in the interior of the membrane (long lifetime). The ratio of the dye in the two locations and the orientation of the dye (parallel or perpendicular to the membrane) are sensitive to the lipid chain length and unsaturation in the alkyl chain. The fraction of the dye in the interior region is higher for short alkyl chains (C12>C14>C16>>C18C20) and in unsaturated lipids (C14:1>C14:0, C16:1>C16:0). These experimental results are consistent with the general principle that the penetration of an amphiphilic organic molecule in the interior region of the membrane is more when the structure of th bilayer is more fluid-like.

Sphingomyelin-cholesterol interactions in biological and model membranes

Cholesterol and sphingomyelin are both important plasma membrane constituents in cells. It is now becoming evident that these two lipid classes affect each other's metabolism in the cell to an extent that was not previously appreciated. It is the aim of this review to present recent data in the literature concerning both molecular and membrane properties of the two lipid classes, how they interact in membranes (both biological and model), and the consequences their mutual interaction have on different functional and metabolic processes in cells and lipoproteins.

Effect of aging on plasma membrane fluidity of rat aortic endothelial cells

To assess age-related changes in the physical properties of vascular endothelial cell (EC) plasma membranes, we measured membrane fluidity with 1,6-diphenyl-1,3,5-hexatriene (DPH), 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene, and 10-(1-pyrene)dodecanoic acid, and investigated the parameters affecting membrane fluidity of endothelial cells (ECs) cultured from the thoracic aortas of young (5-week-old) and aged (100-week-old) rats. Plasma membrane fluidity of aged rat ECs was significantly lower than that of young rat ECs, as assessed by increased 1,6-diphenyl-1,3,5-hexatriene fluorescence polarization and by decreased pyrene excimer formation, although 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene did not demonstrate a change in membrane fluidity with aging. Compared with those in young rat ECs, cholesterol concentrations in aged rat ECs were significantly higher, whereas phospholipid concentrations were unchanged; consequently, the cholesterol/phospholipid molar ratio was significantly higher in aged rat ECs. Lipid peroxide levels measured with thiobarbituric acid reactive substances were significantly higher in EC plasma membranes of aged rats. These results indicate that age-related increases in cholesterol and lipid peroxide in vascular EC plasma membranes reduce membrane fluidity.