Effect of liposome surface modification with water-soluble phospholipid polymer chain-conjugated lipids on interaction with human plasma proteins.

Alternative liposome surface coatings for PEGylation to evade the immune system, particularly the complement system, have garnered significant interest. We previously reported poly(2-methacryloyloxyethyl phosphorylcholine) (MPC)-based lipids (PMPC-lipids) and investigated the surface modification of liposomes. In this study, we synthesize PMPC-lipids with polymerization degrees of 10 (MPC10-lipid), 20 (MPC20-lipid), 50 (MPC50-lipid), and 100 (MPC100-lipid), and coated liposomes with 1, 5, or 10 mol% PMPC-lipids (PMPC-liposomes). Non-modified and PEGylated liposomes are used as controls. We investigate the liposome size, surface charge, polydispersity index, and adsorption of plasma proteins to the liposomes post incubation in human plasma containing N,N,N',N'-ethylenediamine tetraacetic acid (EDTA) or lepirudin by some methods such as sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), western blotting, and automated capillary western blot, with emphasis on the binding of complement protein C3. It is shown that the coating of liposome PMPC-lipids can suppress protein adsorption more effectively with an increase in the molecular weight and molar ratio (1-10 mol%). Apolipoprotein A-I is detected on PMPC-liposomes with a higher molecular weight and higher molar ratio of PMPC-lipids, whereas α2-macroglobulin is detected on non-modified, PEGylated, and PMPC-liposomes with a shorter polymer chain. In addition, a correlation is shown among the PMPC molecular weight, molar ratio, and C3 binding. The MPC10-lipid cannot inhibit C3 binding efficiently, whereas surface modifications with 10 mol% MPC20-lipid and 5 mol% and 10 mol% MPC50-lipid suppress both total protein and C3 binding. Hence, liposome modification with PMPC-lipids can be a possible strategy for avoiding complement activation.

Membrane properties of ether-type phosphatidylcholine bearing partially fluorinated C18-monoacetylenic chains and their applicability to membrane protein reconstitution matrices.

To examine the applicability of fluorinated membrane-forming phospholipids to reconstitution matrices for functional membrane proteins, the membrane properties of a synthetic ether-type phosphatidylcholine (PC) bearing partially fluorinated C18-monoacetylenic (9-octadecynyl) chains, DF8CCH8PC, were compared with those of its non-fluorinated counterpart, DH8CCH8PC. Light-harvesting complex 2 (LH2) and the light-harvesting 1‒reaction center core complex (LH1-RC) isolated from purple photosynthetic bacteria were employed as probe membrane proteins to evaluate the extent to which their reconstitution into DF8CCH8PC membranes could proceed. DF8CCH8PC formed more expanded and more stable fluid monolayers than DH8CCH8PC at the air-water interface at 25 °C; the former PC molecule occupied an area of ca. 0.70 nm2 at a collapse pressure, πc, of 52 mN/m, while the latter occupied an area of ca. 0.55 nm2 at a πc of 45 mN/m. In contrast, the molecular motion detected using fluorescent probes was much more restricted in DF8CCH8PC bilayers than in DH8CCH8PC ones. Although the reconstitution efficiencies of both LH2 and LH1-RC into DF8CCH8PC bilayers were lower than those into DH8CCH8PC bilayers, the membrane proteins incorporated into DF8CCH8PC bilayers showed increased thermostability. The increased thermostability of these proteins in fluorinated PC membranes might be due to the restricted molecular motion in the hydrophobic chains. The results of this study suggest that partially fluorinated PCs can be useful materials for the construction of lipid‒functional membrane protein assemblies including large membrane protein complexes, such as LH1-RC, for biotechnological applications.

Effect of the fluorination degree of partially fluorinated octyl-phosphocholine surfactants on their interfacial properties and interactions with purple membrane as a membrane protein model.

Interfacial properties and membrane protein solubilization activity of a series of partially fluorinated octyl-phosphocholine (PC) surfactants were investigated from the viewpoint of the fluorination degree of the hydrophobic chain. The critical micelle concentration (CMC), surface tension lowering activity, molecular occupied area at the CMC and free energy changes of micellization as well as adsorption to the air-water interface for each PC surfactant were estimated from surface tension measurements at 25 °C. The PCs with higher degree of fluorination exhibited low CMC and high surface activity, while the single trifluoromethyl group at the end of the chain appeared to enhance the hydrophilicity of the surfactant molecule. Under conditions where conventional short-chain surfactants, n-octyl-ß-D-glucoside, Triton X-100 and dioctanoylphosphatidylcholine significantly solubilize purple membranes (PM), none of the fluorinated-PCs solubilized PM. This suggests that fluorinated-PCs are low-invasive enough to maintain the structure of lipids/protein assemblies like PM.

Mechanisms of aggregation and fibril formation of the amyloidogenic N-terminal fragment of apolipoprotein A-I.

The N-terminal (1-83) fragment of the major constituent of plasma high-density lipoprotein, apolipoprotein A-I (apoA-I), strongly tends to form amyloid fibrils, leading to systemic amyloidosis. Here, using a series of deletion variants, we examined the roles of two major amyloidogenic segments (residues 14-22 and 50-58) in the aggregation and fibril formation of an amyloidogenic G26R variant of the apoA-I 1-83 fragment (apoA-I 1-83/G26R). Thioflavin T fluorescence assays and atomic force microscopy revealed that elimination of residues 14-22 completely inhibits fibril formation of apoA-I 1-83/G26R, whereas Δ32-40 and Δ50-58 variants formed fibrils with markedly reduced nucleation and fibril growth rates. CD measurements revealed structural transitions from random coil to ß-sheet structures in all deletion variants except for the Δ14-22 variant, indicating that residues 14-22 are critical for the ß-transition and fibril formation. Thermodynamic analysis of the kinetics of fibril formation by apoA-I 1-83/G26R indicated that both nucleation and fibril growth are enthalpically unfavorable, whereas entropically, nucleation is favorable, but fibril growth is unfavorable. Interestingly, the nucleation of the Δ50-58 variant was entropically unfavorable, indicating that residues 50-58 entropically promote the nucleation step in fibril formation of apoA-I 1-83/G26R. Moreover, a residue-level structural investigation of apoA-I 1-83/G26R fibrils with site-specific pyrene labeling indicated that the two amyloidogenic segments are in close proximity to form an amyloid core structure, whereas the N- and C-terminal tail regions are excluded from the amyloid core. These results provide critical insights into the aggregation mechanism and fibril structure of the amyloidogenic N-terminal fragment of apoA-I.

Effect of Phosphatidylserine and Cholesterol on Membrane-mediated Fibril Formation by the N-terminal Amyloidogenic Fragment of Apolipoprotein A-I.

Here, we examined the effects of phosphatidylserine (PS) and cholesterol on the fibril-forming properties of the N-terminal 1‒83 fragment of an amyloidogenic G26R variant of apoA-I bound to small unilamellar vesicles. A thioflavin T fluorescence assay together with microscopic observations showed that PS significantly retards the nucleation step in fibril formation by apoA-I 1‒83/G26R, whereas cholesterol slightly enhances fibril formation. Circular dichroism analyses demonstrated that PS facilitates a structural transition from random coil to α-helix in apoA-I 1‒83/G26R with great stabilization of the α-helical structure upon lipid binding. Isothermal titration calorimetry measurements revealed that PS induces a marked increase in capacity for binding of apoA-I 1‒83/G26R to the membrane surface, perhaps due to electrostatic interactions of positively charged amino acids in apoA-I with PS. Such effects of PS to enhance lipid interactions and inhibit fibril formation of apoA-I were also observed for the amyloidogenic region-containing apoA-I 8‒33/G26R peptide. Fluorescence measurements using environment-sensitive probes indicated that PS induces a more solvent-exposed, membrane-bound conformation in the amyloidogenic region of apoA-I without affecting membrane fluidity. Since cell membranes have highly heterogeneous lipid compositions, our findings may provide a molecular basis for the preferential deposition of apoA-I amyloid fibrils in tissues and organs.

Heparin promotes fibril formation by the N-terminal fragment of amyloidogenic apolipoprotein A-I.

Glycosaminoglycans are known to be associated with extracellular amyloid deposits of various amyloidogenic proteins. In this study, we found that the glycosaminoglycan heparin greatly accelerates the elongation step in fibril formation by the N-terminal 1-83 fragment of human apolipoprotein A-I (apoA-I), especially in the amyloidogenic W50R variant. Using fragment peptides, we demonstrate that heparin significantly promotes ß-transition and fibril formation of the highly amyloidogenic region spanning residues 44-65 and colocalizes with fibrils formed by the W50R variant. These results suggest the possible role of glycosaminoglycans in fibril formation by amyloidogenic apoA-I variants.

Formation of stable nanodiscs by bihelical apolipoprotein A-I mimetic peptide.

Nanodiscs are composed of scaffold protein or peptide such as apolipoprotein A-I (apoA-I) and phospholipids. Although peptide-based nanodiscs have an advantage to modulate the size of nanodiscs by changing phospholipid/peptide ratios, they are usually less stable than apoA-I-based nanodiscs. In this study, we designed a novel nanodisc scaffold peptide (NSP) that has proline-punctuated bihelical amphipathic structure based on apoA-I mimetic peptides. NSP formed α-helical structure on 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) nanodiscs prepared by cholate dialysis method. Dynamic light scattering measurements demonstrated that diameters of NSP nanodiscs vary depending upon POPC/NSP ratios. Comparison of thermal unfolding of nanodiscs monitored by circular dichroism measurements demonstrated that NSP forms much more stable nanodiscs with POPC than monohelical peptide, 4F, exhibiting comparable stability to apoA-I-POPC nanodiscs. Intrinsic Trp fluorescence measurements showed that Trp residues of NSP exhibit more hydrophobic environment than that of 4 F on nanodiscs, suggesting the stronger interaction of NSP with phospholipids. Thus, the bihelical structure of NSP appears to increase the stability of nanodiscs because of the enhanced interaction of peptides with phospholipids. In addition, NSP as well as 4F spontaneously solubilized POPC vesicles into nanodiscs without using detergent. These results indicate that bihelical NSP forms nanodiscs with comparable stability to apoA-I and has an ability to control the size of nanodiscs simply by changing phospholipid/peptide ratios.

Amyloidogenic Mutation Promotes Fibril Formation of the N-terminal Apolipoprotein A-I on Lipid Membranes.

The N-terminal amino acid 1-83 fragment of apolipoprotein A-I (apoA-I) has a strong propensity to form amyloid fibrils at physiological neutral pH. Because apoA-I has an ability to bind to lipid membranes, we examined the effects of the lipid environment on fibril-forming properties of the N-terminal fragment of apoA-I variants. Thioflavin T fluorescence assay as well as fluorescence and transmission microscopies revealed that upon lipid binding, fibril formation by apoA-I 1-83 is strongly inhibited, whereas the G26R mutant still retains the ability to form fibrils. Such distinct effects of lipid binding on fibril formation were also observed for the amyloidogenic prone region-containing peptides, apoA-I 8-33 and 8-33/G26R. This amyloidogenic region shifts from random coil to α-helical structure upon lipid binding. The G26R mutation appears to prevent this helix transition because lower helical propensity and more solvent-exposed conformation of the G26R variant upon lipid binding were observed in the apoA-I 1-83 fragment and 8-33 peptide. With a partially α-helical conformation induced by the presence of 2,2,2-trifluoroethanol, fibril formation by apoA-I 1-83 was strongly inhibited, whereas the G26R variant can form amyloid fibrils. These findings suggest a new possible pathway for amyloid fibril formation by the N-terminal fragment of apoA-I variants: the amyloidogenic mutations partially destabilize the α-helical structure formed upon association with lipid membranes, resulting in physiologically relevant conformations that allow fibril formation.

Effect of the fluorination degree of hydrophobic chains on the monolayer behavior of unsaturated diacylphosphatidylcholines bearing partially fluorinated 9-octadecynoyl (stearoloyl) groups at the air-water interface.

The effect of the fluorination degree of hydrophobic chains on the monolayer behavior of unsaturated diacylphosphatidylcholines (PCs) was examined by employing a series of PCs bearing partially fluorinated 9-octadecynoyl (stearoloyl) groups (DFnStPCs, n: the number of fluorinated carbon atoms in a stearoloyl group; n=1, 2, 4, 8), including their hydrophobic parts--partially fluorinated stearolic acids (FnStAs)--at the air-water interface. π-A isotherm measurements and Brewster angle microscope observations revealed: (i) all DFnStPCs including FnStAs form monolayers of liquid character at 25 °C; (ii) they form more expanded monolayers than their non-fluorinated counterparts, distearoloyl-PC (DStPC) and stearolic acid, while the monolayer stability increases with n; (iii) compared with DStPC and DF8StPC, DFnStPCs (n=1, 2, 4) in the low-π region tend to show a weakening in their self-aggregation property and an increase in the work required for monolayer compression; (iv) although DF8StPC forms the most expanded monolayer, the behavior of DF8StPC resembles that of DStPC rather than that of DFnStPCs (n=1, 2, 4). The monolayer behavior of DFnStPCs (n=1, 2, 4) is explained by postulating a flatly-lying conformation of hydrophobic chains, in which three polar parts (ester group, triple bond, CF2-CH2 linkage) in chains are immersed in the subphase at large areas. DStPC and DF8StPC lacking a CF2-CH2 linkage, however, do not likely adopt such a conformation.

Effect of perfluoroalkyl chain length on monolayer behavior of partially fluorinated oleic acid molecules at the air-water interface.

A series of oleic acid (OA) analogs containing terminal perfluoroalkyl groups (CF3, C2F5, n-C3F7, n-C4F9 or n-C8F17) was synthesized to clarify how the fluorinated chain length affects the stability and molecular packing of liquid-expanded OA monolayers at the air-water interface. Although the substitution of terminal CF3 group for CH3 in OA had no effect on monolayer stability, further fluorination led to a gradual increase in monolayer stability at 25 °C. Surface pressure-area isotherm revealed that partially fluorinated OA analogs form more expanded monolayers than OA at low surface pressures, and that the monolayer behavior of OA analogs with the even-carbon numbered fluorinated chain is almost the same as that of OA upon monolayer compression, whereas the behavior of OA analogs with the odd-carbon numbered fluorinated chain significantly differs from that of OA. These results indicate: (i) the terminal short part (at least C2 residue) in OA predominantly determines the liquid-expanded monolayer stability; (ii) the molecular packing state of OA may be perturbed by the substitution of a short odd-carbon numbered fluorinated chain; (iii) hence, OA analogs with even-carbon numbered chain are considered to be preferable as hydrophobic building blocks for the synthesis of fluorinated phospholipids.

Physicochemical studies of bacteriorhodopsin reconstituted in partially fluorinated phosphatidylcholine bilayers.

A membrane protein bacteriorhodopsin (bR) that is successfully reconstituted in liposome of a novel partially fluorinated analog of dimyristoylphosphatidylcholine (DMPC) with the perfluorobutyl segments in the myristoyl groups, diF4H10-PC, has been investigated by some spectroscopic and X-ray diffraction techniques to clarify effects of substitution of nine hydrogen atoms by fluorine atoms on structural and physical properties of the membrane protein by comparison with the previous results on proteoliposome of bR and DMPC. Below the gel-to-liquid crystalline phase transition of diF4H10-PC bilayer, bR molecules adopt the two-dimensional lattice structure of trimers as the structural unit and show a photocycle very similar to that of native purple membrane like reconstituted bR in DMPC liposome in the gel phase. Even upon heating up to temperatures well above the phase transition, the nativelike functional reconstitution and higher structural stability of bR molecules in diF4H10-PC liposome are retained, which strikingly contrasts with lipid phase transition-induced disaggregation of protein molecules and light-induced denaturation in DMPC liposome. Greater membrane rigidity and low affinity between bR and fluorinated lipid molecules are proposed as a driving force for keeping nativelike properties of bR molecules in diF4H10-PC liposome even in the fluid phase.

Dynamic molecular behavior of semi-fluorinated oleic, elaidic and stearic acids in the liquid state.

Ordinary fatty acids such as oleic, elaidic and stearic acids exist as their hydrogen-bonded dimers in their liquids and in non-polar solvents. Infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy have revealed that semi-fluorinated (SF) acids containing a perfluorooctyl group (C8F17) as a terminal segment exist also as hydrogen-bonded dimers, which are the units of inter- and intramolecular movements in their liquids and CCl4. The dynamic molecular properties, such as self-diffusion coefficients and intramolecular movements of SF-oleic, SF-elaidic, and SF-stearic acids were compared with those of corresponding ordinary fatty acids (H-acids). From the high equilibrium spreading pressures (ESPs) for SF-acids compared with those for their corresponding H-acids, it was expected that the inter-acyl chain interaction is weaker for the SF-acid than for the H-acid: the SF-fatty acids should have higher molecular mobility than the corresponding ordinary H-acids in the liquid state. However, the self-diffusion coefficients obtained for SF-acids were smaller than those for the corresponding H-acids; the apparent activation energies for the self-diffusion process (translational movement) of SF-acids were larger than those for the corresponding H-acids. Namely, the motion of SF-acid molecules in a liquid phase is rather restricted compared with H-acid in spite of lower inter-acyl chain interaction of SF-acid. This unexpected result suggests that the molecular motion of SF-acid in a liquid phase is not directly governed by inter-acyl interaction, but may be interpreted as a reptation movement of an acid molecule, which is related to intramolecular movement. In fact, low intramolecular movements for SF-acid were confirmed by ¹³C-NMR T1 measurements.

Dynamic interaction between oppositely charged vesicles: aggregation, lipid mixing, and disaggregation.

We investigated dynamic interactions between oppositely charged small unilamellar vesicles using positively charged vesicles containing 1,2-dioleoyl-3-trimethylammonium-propane or 3beta-[N-(N('),N(')-dimethylaminoethane)-carbamoyl] cholesterol and negatively charged vesicles containing L-alpha-phosphatidyl-DL-glycerol. Aggregation, lipid bilayer mixing, contents mixing and contents leakage were systematically examined using optical density measurements, fluorescence resonance energy transfer assays, fluorescence quenching assays, light-scattering analyses, and freeze-fracture transmission electron microscopy. The oppositely charged vesicles aggregated immediately. Lipid mixing was observed, but there was no mixing of the contents. The vesicle aggregates disaggregated spontaneously after several minutes. The surface potential of the disaggregated vesicles was neutralized. From these results, we infer that the lipids in the external monolayers were exchanged between the oppositely charged vesicles while the internal monolayers remained intact. The two types of cationic lipids used exhibited different speeds of disaggregation.

Molecular dynamics study of bipolar tetraether lipid membranes.

Membranes composed of bipolar tetraether lipids have been studied by a series of 25-ns molecular dynamics simulations to understand the microscopic structure and dynamics as well as membrane area elasticity. By comparing macrocyclic and acyclic tetraether and diether archaeal lipids, the effect of tail linkage of the two phytanyl-chained lipids on the membrane properties is elucidated. Tetraether lipids show smaller molecular area and lateral mobility. For the latter, calculated diffusion coefficients are indeed one order-of-magnitude smaller than that of the diether lipid. These two tetraether membranes are alike in many physical properties except for membrane area elasticity. The macrocyclic tetraether membrane shows a higher elastic area expansion modulus than its acyclic counterpart by a factor of three. Free energy profiles of a water molecule crossing the membranes show no major difference in barrier height; however, a significant difference is observed near the membrane center due to the lack of the slip-plane in tetraether membranes.

Comparative molecular dynamics study of ether- and ester-linked phospholipid bilayers.

The lipid membranes found in archaea have high bilayer stability and low permeability. The molecular structure of their constituent lipids is characterized by ether-linked, branched hydrophobic chains, whereas the conventional lipids obtained from eukaryotic or eubacterial sources have ester linked straight chains. In order to elucidate the influence of the ether linkage, instead of an ester one, on the physical properties of the lipid bilayers, we have carried out comparative 10 ns molecular dynamics simulations of diphytanyl phosphatidylcholine (ether-DPhPC) and diphytanoyl phosphatidylcholine (ester-DPhPC) bilayers in water, respectively. We analyze bilayer structures, hydration of the lipids, membrane dipole potentials, and free energy profiles of water and oxygen across the bilayers. We observe that the membrane dipole potential for the ether-DPhPC bilayer, which arises mainly from the ether linkage, is about half of that of the ester-DPhPC. The calculated free energy barrier for a water molecule in the ether-DPhPC bilayer system is slightly higher than that in the ester-DPhPC counterpart, which is in accord with experimental data.