Dynamic motions of 1,6-diphenyl-1,3,5-hexatriene in interdigitated C(18):C(10)phosphatidylcholine bilayers

Polar Lipid Fraction E from Sulfolobus acidocaldarius and Dipalmitoylphosphatidylcholine Can Form Stable yet Thermo-Sensitive Tetraether/Diester Hybrid Archaeosomes with Controlled Release Capability

Archaeosomes have drawn increasing attention in recent years as novel nano-carriers for therapeutics. The main obstacle of using archaeosomes for therapeutics delivery has been the lack of an efficient method to trigger the release of entrapped content from the otherwise extremely stable structure. Our present study tackles this long-standing problem. We made hybrid archaeosomes composed of tetraether lipids, called the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius, and the synthetic diester lipid dipalmitoylphosphatidylcholine (DPPC). Differential polarized phase-modulation and steady-state fluorometry, confocal fluorescence microscopy, zeta potential (ZP) measurements, and biochemical assays were employed to characterize the physical properties and drug behaviors in PLFE/DPPC hybrid archaeosomes in the presence and absence of live cells. We found that PLFE lipids have an ordering effect on fluid DPPC liposomal membranes, which can slow down the release of entrapped drugs, while PLFE provides high negative charges on the outer surface of liposomes, which can increase vesicle stability against coalescence among liposomes or with cells. Furthermore, we found that the zeta potential in hybrid archaeosomes with 30 mol% PLFE and 70 mol% DPPC (designated as PLFE/DPPC(3:7) archaeosomes) undergoes an abrupt increase from −48 mV at 37 °C to −16 mV at 44 °C (termed the ZP transition), which we hypothesize results from DPPC domain melting and PLFE lipid ‘flip-flop’. The anticancer drug doxorubicin (DXO) can be readily incorporated into PLFE/DPPC(3:7) archaeosomes. The rate constant of DXO release from PLFE/DPPC(3:7) archaeosomes into Tris buffer exhibited a sharp increase (~2.5 times), when the temperature was raised from 37 to 42 °C, which is believed to result from the liposomal structural changes associated with the ZP transition. This thermo-induced sharp increase in drug release was not affected by serum proteins as a similar temperature dependence of drug release kinetics was observed in human blood serum. A 15-min pre-incubation of PLFE/DPPC(3:7) archaeosomal DXO with MCF-7 breast cancer cells at 42 °C caused a significant increase in the amount of DXO entering into the nuclei and a considerable increase in the cell’s cytotoxicity under the 37 °C growth temperature. Taken together, our data suggests that PLFE/DPPC(3:7) archaeosomes are stable yet potentially useful thermo-sensitive liposomes wherein the temperature range (from 37 to 42–44 °C) clinically used for mild hyperthermia treatment of tumors can be used to trigger drug release for medical interventions.

1,6-Diphenyl-1,3,5-hexatriene (DPH) as a Novel Matrix for MALDI MS Imaging of Fatty Acids, Phospholipids, and Sulfatides in Brain Tissues

1,6-Diphenyl-1,3,5-hexatriene (DPH) is a commonly used fluorescence probe for studying cell membrane-lipids due to its affinity toward the acyl chains in the phospholipid bilayers. In this work, we investigated its use in matrix-assisted laser desorption/ionization (MALDI) as a new matrix for mass spectrometry imaging (MSI) of mouse and rat brain tissue. DPH exhibits very minimal matrix-induced background signals for the analysis of small molecules (below m/z of 1000). In the negative ion mode, DPH permits the highly sensitive detection of small fatty acids (m/z 200-350) as well as a variety of large lipids up to m/z of 1000, including lyso-phospholipid, phosphatidic acid (PA), phosphoethanolamine (PE), phosphatidylserine (PS), phosphatidylglycerol (PG), phosphatidylinositol (PI), and sulfatides (ST). The analytes were mostly detected as the deprotonated ion [M - H]^-. Our results also demonstrate that sublimated DPH is stable for at least 24 h under the vacuum of our MALDI mass spectrometer. The ability to apply DPH via sublimation coupled with its low volatility allows us to perform tissue imaging of the above analytes at high spatial resolution. The degree of lipid fragmentation was determined experimentally at varying laser intensities. The results illustrated that the use of relatively low laser energy is important to minimize the artificially generated fatty acid signals. On the other hand, the lipid fragmentation obtained at higher laser energies provided tandem MS information useful for lipid structure elucidation.

Adaptation to low body temperature influences pulmonary surfactant composition thereby increasing fluidity while maintaining appropriately ordered membrane structure and surface activity

The interfacial surface tension of the lung is regulated by phospholipid-rich pulmonary surfactant films. Small changes in temperature affect surfactant structure and function in vitro. We compared the compositional, thermodynamic and functional properties of surfactant from hibernating and summer-active 13-lined ground squirrels (Ictidomys tridecemlineatus) with porcine surfactant to understand structure-function relationships in surfactant membranes and films. Hibernating squirrels had more surfactant large aggregates with more fluid monounsaturated molecular species than summer-active animals. The latter had more unsaturated species than porcine surfactant. Cold-adapted surfactant membranes displayed gel-to-fluid transitions at lower phase transition temperatures with reduced enthalpy. Both hibernating and summer-active squirrel surfactants exhibited lower enthalpy than porcine surfactant. LAURDAN fluorescence and DPH anisotropy revealed that surfactant bilayers from both groups of squirrels possessed similar ordered phase characteristics at low temperatures. While ground squirrel surfactants functioned well during dynamic cycling at 3, 25, and 37 degrees C, porcine surfactant demonstrated poorer activity at 3 degrees C but was superior at 37 degrees C. Consequently the surfactant composition of ground squirrels confers a greater thermal flexibility relative to homeothermic mammals, while retaining tight lipid packing at low body temperatures. This may represent the most critical feature contributing to sustained stability of the respiratory interface at low lung volumes. Thus, while less effective than porcine surfactant at 37 degrees C, summer-active surfactant functions adequately at both 37 degrees C and 3 degrees C allowing these animals to enter hibernation. Here further compositional alterations occur which improve function at low temperatures by maintaining adequate stability at low lung volumes and when temperature increases during arousal from hibernation.

Controlling the size distribution of lipid-coated bubbles via fluidity regulation

Lipid-coated bubbles exhibit oscillation responses capable of enhancing the sensitivity of ultrasound imaging by improving contrast. Further improvements in performance enhancement require control of the size distribution of bubbles to promote correspondence between their resonance frequency and the frequency of the ultrasound. Here we describe a size-controlling technique that can shift the size distribution using a currently available agitation method. This technique is based on regulating the membrane dynamic fluidity of lipid mixtures and provides a general size-controlling variable that could also be applied in other fabrication methods. Three materials (1,2-dihexadecanoyl-sn-glycero-3-phosphocholine, 1,2-dioctadecanoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and polyethylene glycol 40 stearate) with distinct initial fluidities and phase behaviors were used to demonstrate the use of fluidity regulation to control bubble sizes. Bubble size distributions of different formulations were determined by electrical impedance sensing, and bubble volumes and surface areas were calculated. To confirm the relationship between regulated fluidity and mean bubble size, the membrane fluidity of each composition was determined by fluorescence anisotropy, with the results indicating linear relations in the compositions with similar main transition temperatures. Compositions with a higher molar proportion of polyethylene glycol 40 stearate showed higher fluidities and larger bubbles. B-mode ultrasound imaging was performed to investigate the echogenicity and lifetime of the fabricated bubbles, with the results indicating that co-mixing a high-transition-temperature charged lipid (i.e., 1,2-dioctadecanoyl-sn-glycero-3-phospho-(1'-rac-glycerol)) extends the tailoring range of this fluidity regulation technique, allowing refined and continuous changes in mean bubble size (from 0.93 to 2.86 μm in steps of ∼0.5 μm), and also prolongs bubble lifetime. The polydispersity of each composition was also determined to evaluate practicality in particular applications. Our study demonstrates a feasible approach to naturally controling bubble size distribution and provides a practical reference for other fabrication systems and ultrasound imaging applications.

Unique profiles of changes in cell membrane fluidity during ethanol-induced yeast-to-pseudohyphal transition in Candida tropicalis

A dimorphic transition from the yeast form to filamentous one in Candida tropicalis pK233 is triggered by the addition of ethanol into the glucose semi-defined liquid medium and the process of filamentation accompanies temporal depolarization of yeast cells. The transition is completely prevented by further supplementation of myo-inositol at the start of cultivation. The addition of ethanol caused an increase in membrane fluidity during the process of depolarization, and then fluidity was gradually lowered to the level equivalent with that of the stationary-phase yeast cells in accordance with filamentation. The increase in membrane fluidity of ethanol-induced cells appeared parallel with reduction in the content of membrane phosphatidylinositol, which was rich in saturated palmitic acid. Introduction of exogenous myo-inositol or 1 M sorbitol into the ethanol-supplemented culture at the start of cultivation restored yeast growth and the reduction of membrane fluidity occurred, coupled with the recovery of the phosphatidylinositol content.

Properties of phosphatidylcholine in the presence of its monofluorinated analogue

In aqueous solution, the monofluorinated phospholipid 1-palmitoyl-2-[16-fluoropalmitoyl]sn-glycero-3-phosphocholine (F-DPPC) interdigitates without the use of inducing agents. To understand the thermal and physical properties of this unique lipid, F-DPPC was combined with the non-fluorinated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-diarachidoyl-sn-glycero-3-phosphocholine (DAPC). Differential scanning calorimetry (DSC) was used to determine the miscibility and thermotropic phase behavior of these binary lipid mixtures. In addition, the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) and a DPH-labeled analogue of DPPC, 2-(3-(diphenylhexatrienyl) propanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine (beta-DPH HPC, aka DPH-PC or DPHpPC), were used to detect interdigitation. In F-DPPC, the fluorescence intensity of both probes decreased a similar amount and to a degree that is consistent with an interdigitated system. We also determined that there are two separate effects of increasing the ratio of F-DPPC in the DPPC/F-DPPC system. With low amounts of F-DPPC, there is little evidence that the system is heavily interdigitated. Instead, we hypothesize that the introduction of F-DPPC provides nucleation sites that alter the kinetics, reversibility, and temperature of the main transition (T(m)). At higher mol% of F-DPPC, we propose that interdigitated F-DPPC-rich domains form to create a phase-segregated system. While DPPC/F-DPPC was highly miscible, the DAPC/F-DPPC system was significantly less miscible. Additionally, we observed that DAPC/F-DPPC samples have reduced solubility in water, which affected the acquisition of fluorescence data. However, our DSC results indicate the existence of DAPC-rich and F-DPPC-rich components. Furthermore, this data support that the mixing was disruptive to lipid packing and that the presence of DAPC hinders the interdigitation of F-DPPC.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Alcohol induction of interdigitation in distearoylphosphatidylcholine: fluorescence studies of alcohol chain length requirements

Although it is now well established that the fully interdigitated phase is induced in saturated like-chain phosphatidylcholines (PCs) by a variety of amphipathic molecules including alcohols, no systematic study of the properties of the inducing molecules has been reported. To elucidate the stereochemical features that lead to the alcohol induction of interdigitation in PCs, we have investigated the induction of interdigitation in distearoylphosphatidylcholine (DSPC) by a series of alcohols. Our previously established DPH (1,6-diphenyl-1,3,5-hexatriene) fluorescence intensity method has been expanded (P. Nambi, E. S. Rowe, and T. M. McIntosh (1988), Biochemistry 27:9175-9182) and used to determine which of the alcohols induce interdigitation and to determine the threshold concentrations for each. We have found that each of the n-alcohols up to heptanol and several branched alcohols are capable of inducing interdigitation in DSPC; octanol and nonanol do not appear to induce interdigitation by these criteria. The threshold concentrations for interdigitation for each of these alcohols up to heptanol were found to be correlated with the membrane: buffer partition coefficients. The mole fraction of bound alcohol at the threshold concentration was similar for each of the alcohols up to pentanol. These results are discussed in terms of a general mechanism of the formation of the interdigitated phase.

Thermodynamic elucidation of solute-induced lipid interdigitation phase: lipid interactions with hydrophobic versus amphipathic species

Comparative thermodynamic studies on the interactions of aqueous dispersions of dipalmitoyl phosphatidylcholine (DPPC) bilayer vesicles with hydrophobic and amphipathic species were conducted to elucidate the nature of the solute-induced interdigitated lipid phase. Cyclohexanol, a strong hydrophobic species, lowers the temperature (tm) of the lipid main phase transition from the gel to the liquid-crystalline phase. Unlike ethanol (an amphipathic species), as reported previously, cyclohexanol does not exert a biphasic effect on tm (lowering tm at lower concentrations and raising tm at higher concentrations). At cyclohexanol greater than or equal to 15.4 mg/ml or 0.154 M, the thermogram of DPPC vesicles exhibits a small transition adjacent to the main phase transition but at a lower temperature. In contrast, ethanol does not promote such a small transition. Furthermore, the enthalpy (delta H) of the transition is increased in the presence of cyclohexanol. The sign of the enthalpy change (delta H-delta Ho) is positive and that of the free energy change (delta G-delta Go) is negative, a characteristic of solute-solute hydrophobic interaction. In contrast, DPPC bilayer vesicles exhibit both (delta H-delta Ho) and (delta G-delta Go) greater than 0 in the presence of ethanol in a concentration range where lipid vesicles exist in an interdigitated phase. To support the above distinct thermodynamic observations, fluorescence steady-state polarization (P) measurements were also performed. At the temperature below tm, the value of P decreases as cyclohexanol concentration increases, while a biphasic effect on P was found in the presence of ethanol. These findings support the postulation that the solute-induced interdigitated lipid phase requires the solute molecule to be amphipathic in nature.