Melting-Temperature-Dependent Interactions of Ergosterol with Unsaturated and Saturated Lipids in Model Membranes

The Chemical Reactivity of Membrane Lipids

It is well-known that aqueous dispersions of phospholipids spontaneously assemble into bilayer structures. These structures have numerous applications across chemistry and materials science and form the fundamental structural unit of the biological membrane. The particular environment of the lipid bilayer, with a water-poor low dielectric core surrounded by a more polar and better hydrated interfacial region, gives the membrane particular biophysical and physicochemical properties and presents a unique environment for chemical reactions to occur. Many different types of molecule spanning a range of sizes, from dissolved gases through small organics to proteins, are able to interact with membranes and promote chemical changes to lipids that subsequently affect the physicochemical properties of the bilayer. This Review describes the chemical reactivity exhibited by lipids in their membrane form, with an emphasis on conditions where the lipids are well hydrated in the form of bilayers. Key topics include the following: lytic reactions of glyceryl esters, including hydrolysis, aminolysis, and transesterification; oxidation reactions of alkenes in unsaturated fatty acids and sterols, including autoxidation and oxidation by singlet oxygen; reactivity of headgroups, particularly with reactive carbonyl species; and E/Z isomerization of alkenes. The consequences of reactivity for biological activity and biophysical properties are also discussed.

Natamycin interferes with ergosterol-dependent lipid phases in model membranes

Natamycin is an antifungal polyene macrolide that is used as a food preservative but also to treat fungal keratitis and other yeast infections. In contrast to other polyene antimycotics, natamycin does not form ion pores in the plasma membrane, but its mode of action is poorly understood. Using nuclear magnetic resonance (NMR) spectroscopy of deuterated sterols, we find that natamycin slows the mobility of ergosterol and cholesterol in liquid-ordered (Lo) membranes to a similar extent. This is supported by molecular dynamics (MD) simulations, which additionally reveal a strong impact of natamycin dimers on sterol dynamics and water permeability. Interference with sterol-dependent lipid packing is also reflected in a natamycin-mediated increase in membrane accessibility for dithionite, particularly in bilayers containing ergosterol. NMR experiments with deuterated sphingomyelin (SM) in sterol-containing membranes reveal that natamycin reduces phase separation and increases lipid exchange in bilayers with ergosterol. In ternary lipid mixtures containing monounsaturated phosphatidylcholine, saturated SM, and either ergosterol or cholesterol, natamycin interferes with phase separation into Lo and liquid-disordered (Ld) domains, as shown by NMR spectroscopy. Employing the intrinsic fluorescence of natamycin in ultraviolet-sensitive microscopy, we can visualize the binding of natamycin to giant unilamellar vesicles (GUVs) and find that it has the highest affinity for the Lo phase in GUVs containing ergosterol. Our results suggest that natamycin specifically interacts with the sterol-induced ordered phase, in which it disrupts lipid packing and increases solvent accessibility. This property is particularly pronounced in ergosterol containing membranes, which could underlie the selective antifungal activity of natamycin.

Charged Lipids Influence Phase Separation in Cell-Sized Liposomes Containing Cholesterol or Ergosterol

Positively charged ion species and charged lipids play specific roles in biochemical processes, especially those involving cell membranes. The cell membrane and phase separation domains are attractive research targets to study signal transduction. The phase separation structure and functions of cell-sized liposomes containing charged lipids and cholesterol have been investigated earlier, and the domain structure has also been studied in a membrane model, containing the yeast sterol ergosterol. The present study investigates phase-separated domain structure alterations in membranes containing charged lipids when cholesterol is substituted with ergosterol. This study finds that ergosterol increases the homogeneity of membranes containing charged lipids. Cholesterol-containing membranes are more sensitive to a charged state, and ergosterol-containing liposomes show lower responses to charged lipids. These findings may improve our understanding of the differences in both yeast and mammalian cells, as well as the interactions of proteins with lipids during signal transduction.

Structural insights into the interaction of antifungal peptides and ergosterol containing fungal membrane

The treatment of invasive drug-resistant and potentially life-threatening fungal infections is limited to few therapeutic options that are usually associated with severe side effects. The development of new effective antimycotics with a more tolerable side effect profile is therefore of utmost clinical importance. Here, we used a combination of complementary in vitro assays and structural analytical methods to analyze the interaction of the de novo antimicrobial peptide VG16KRKP with the sterol moieties of biological cell membranes. We demonstrate that VG16KRKP disturbs the structural integrity of fungal membranes both invitro and in model membrane system containing ergosterol along with phosphatidylethanolamine lipid and exhibits broad-spectrum antifungal activity. As revealed by systematic structure-function analysis of mutated VG16KRKP analogs, a specific pattern of basic and hydrophobic amino acid side chains in the primary peptide sequence determines the selectivity of VG16KRKP for fungal specific membranes.

Phase Separation in Liposomes Determined by Ergosterol and Classified Using Machine Learning

Recent studies indicated that ergosterol (Erg) helps form strongly ordered lipid domains in membranes that depend on their chemical characters. However, direct evidence of concentration-dependent interaction of Erg with lipid membranes has not been reported. We studied the Erg concentration-dependent changes in the phase behaviors of membranes using cell-sized liposomes containing 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). We observed the concentration range of phase separation in ternary membranes was significantly wider when Erg rather than cholesterol (Chol) was used as the sterol component. We used machine learning for the first time to analyze microscopic images of cell-sized liposomes and identify phase-separated structures. The automated method was successful in identifying homogeneous membranes but performance remained data-limited for the identification of phase separation domains characterized by more complex features.

Effect of Cholesterol Versus Ergosterol on DPPC Bilayer Properties: Insights from Atomistic Simulations

Sterols have been ascribed a major role in the organization of biological membranes, in particular for the formation of liquid ordered domains in complex lipid mixtures. Here, we employed molecular dynamics simulations to compare the effects of cholesterol and ergosterol as the major sterol of mammalian and fungal cells, respectively, on binary mixtures with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as a proxy for saturated lipids. In agreement with previous work, we observe that the addition of sterol molecules modifies the order of DPPC both in the gel phase and in the liquid phase. When disentangling the overall tilt angle and the structure of the tail imposed by trans/gauche configurations of torsion angles in the tail, respectively, a more detailed picture of the impact of sterols can be formulated, revealing, for example, an approximate temperature-concentration superposition ranging from the liquid to the gel phase. Furthermore, a new quantitative measure to identify the presence of collective sterol effects is discussed. Moreover, when comparing both types of sterols, addition of cholesterol has a noticeably stronger impact on phospholipid properties than that of ergosterol. The observed differences can be attributed to higher planarity of the cholesterol ring system. This planarity combined with an inherent asymmetry in its molecular interactions leads to better alignment and hence stronger interaction with saturated acyl chains. Our results suggest that the high order demonstrated for ergosterol in fungal plasma membranes must therefore be generated via additional mechanisms.

Systematic Characterization of Nanostructured Lipid Carriers from Cetyl Palmitate/Caprylic Triglyceride/Tween 80 Mixtures in an Aqueous Environment

Nanostructured lipid carriers (NLCs) are gaining attention as the new generation of lipid vehicles. These carriers consist of saturated lipids with small drops of liquid oil dispersed into the inner lipid matrix and are stabilized by a surfactant. Conventionally, NLC-based drug delivery systems have been widely studied, and many researchers are looking into the composition of NLC properties to improve the performance of NLCs. The membrane fluidity and polarity of self-assembling lipids are also essential properties that must be affected by membrane compositions; however, such fundamental characteristics have not been studied yet. In this study, NLCs were prepared from cetyl palmitate (CP), caprylic triglyceride (CaTG), and Tween 80 (T80). Structural properties, such as particle size and ζ-potential of the CP/CaTG/T80 ternary mixtures, were investigated. Then, the systematic characterization of self-assembly properties using fluorescence-based analysis was applied for the first time to the NLC system. As a final step, the ternary diagram was developed based on the self-assembly properties to summarize the possible structures formed at different compositions. The results showed four states: micelle-like, oil-in-water (O/W) emulsion-like, solid lipid nanoparticle-like, and intermediate (solid-liquid coexistence). For the purpose of making the lipid matrix more liquified, the heterogeneous state and the disordered state of the O/W emulsion-like structure might fulfill the criteria of NLCs. Finally, the ternary diagram provides new information about the assembly state of NLC constituents that could become an important reference for developing high-performance NLCs.

Ultrafast excited state dynamics of provitamin D3 and analogs in solution and in lipid bilayers

The photochemical ring-opening reaction of 7-dehydrocholesterol (DHC, provitamin D₃) is responsible for the light-initiated formation of vitamin D₃ in mammalian skin membranes. Visible transient absorption spectroscopy was used to explore the excited state dynamics of DHC and two analogs: ergosterol (provitamin D₂) and DHC acetate free in solution and confined to lipid bilayers chosen to model the biological cell membrane. In solution, the excited state dynamics of the three compounds are nearly identical. However, when confined to lipid bilayers, the heterogeneity of the lipid membrane and packing forces imposed on the molecule by the lipid alter the excited state dynamics of these compounds. When confined to lipid bilayers in liposomes formed using DPPC, two solvation environments are identified. The excited state dynamics for DHC and analogs in fluid-like regions of the liposome membrane undergo internal conversion and ring-opening on 1 ps-2 ps time scales, similar to those observed in isotropic solution. In contrast, the excited state lifetime of a subpopulation in regions of lower fluidity is 7 ps-12 ps. The long decay component is unique to these liposomes and results from the structural properties of the lipid bilayer. Additional measurements in liposomes prepared with lipids having slightly longer or shorter alkane tails support this conclusion. In the lipid environments studied, the longest lifetimes are observed for DHC. The unsaturated sterol tail of ergosterol and the acetate group of DHC acetate disrupt the packing around the molecule and permit faster internal conversion and relaxation back to the ground state.

Exploring the influence of inhaled liposome membrane fluidity on its interaction with pulmonary physiological barriers

Liposome membrane fluidity can influence its interaction with pulmonary physiological barriers, including mucus permeation, macrophage uptake and trachea permeation.

Understanding Interactions of Curcumin with Lipid Bilayers: A Coarse-Grained Molecular Dynamics Study

The interactions of curcumin with various lipid bilayers (POPC, DOPC, oxidized POPC, and oxidized DOPC) and model biomembranes (symmetric bacteria and yeast plasma membranes, as well as asymmetric mammalian plasma membrane) are investigated. A nonlinear thinning effect of curcumin with respect to its concentration is demonstrated in PC membranes and in the yeast. Curcumin induces asymmetry to the symmetric yeast membranes but reduces the degree of asymmetry of the mammalian plasma membranes when the molecule is placed facing the outer leaflets. The molecule is found to diffuse through oxidized PC bilayers, POPC bilayers at a curcumin to lipid ratio C/L = 1/5, yeast membranes at C/L = 1/100, and the mammalian plasma membranes at C/L = 1/5 and when the molecule placed facing the outer leaflets. The results of this work demonstrate that the lipid type, the lipid distribution, and curcumin amount play a critical role in defining the interactions of curcumin with the lipids and their transport behavior through the bilayers.