Crucial Role of the Double Bond Isomerism in the Steroid B-Ring on the Membrane Properties of Sterols. Grazing Incidence X-Ray Diffraction and Brewster Angle Microscopy Studies

Plant Sterol Clustering Correlates with Membrane Microdomains as Revealed by Optical and Computational Microscopy

Local inhomogeneities in lipid composition play a crucial role in the regulation of signal transduction and membrane traffic. This is particularly the case for plant plasma membrane, which is enriched in specific lipids, such as free and conjugated forms of phytosterols and typical phytosphingolipids. Nevertheless, most evidence for microdomains in cells remains indirect, and the nature of membrane inhomogeneities has been difficult to characterize. We used a new push–pull pyrene probe and fluorescence lifetime imaging microscopy (FLIM) combined with all-atom multiscale molecular dynamics simulations to provide a detailed view on the interaction between phospholipids and phytosterol and the effect of modulating cellular phytosterols on membrane-associated microdomains and phase separation formation. Our understanding of the organization principles of biomembranes is limited mainly by the challenge to measure distributions and interactions of lipids and proteins within the complex environment of living cells. Comparing phospholipids/phytosterol compositions typical of liquid-disordered (Ld) and liquid-ordered (Lo) domains, we furthermore show that phytosterols play crucial roles in membrane homeostasis. The simulation work highlights how state-of-the-art modeling alleviates some of the prior concerns and how unrefuted discoveries can be made through a computational microscope. Altogether, our results support the role of phytosterols in the lateral structuring of the PM of plant cells and suggest that they are key compounds for the formation of plant PM microdomains and the lipid-ordered phase.

A visible-light activated [2 + 2] cycloaddition reaction enables pinpointing carbon-carbon double bonds in lipids

The precise location of C[double bond, length as m-dash]C bonds in bioactive molecules is critical for a deep understanding of the relationship between their structures and biological roles. However, the traditional ultraviolet light-based approaches exhibited great limitations. Here, we discovered a new type of visible-light activated [2 + 2] cycloaddition of carbonyl with C[double bond, length as m-dash]C bonds. We found that carbonyl in anthraquinone showed great reactivities towards C[double bond, length as m-dash]C bonds in lipids to form oxetanes under the irradiation of visible-light. Combined with tandem mass spectrometry, this site-specific dissociation of oxetane enabled precisely locating the C[double bond, length as m-dash]C bonds in various kinds of monounsaturated and polyunsaturated lipids. The proof-of-concept applicability of this new type of [2 + 2] photocycloaddition was validated in the global identification of unsaturated lipids in a complex human serum sample. 86 monounsaturated and polyunsaturated lipids were identified with definitive positions of C[double bond, length as m-dash]C bonds, including phospholipids and fatty acids even with up to 6 C[double bond, length as m-dash]C bonds. This study provides new insights into both the photocycloaddition reactions and the structural lipidomics.

The role of phospholipid composition and ergosterol presence in the adaptation of fungal membranes to harsh environmental conditions-membrane modeling study

Soil fungi play an important role in the environment decomposing dead organic matter and degrading persistent organic pollutants (POP). The presence of hydrophobic POP in the soil and membrane-lytic substances excreted by competing microorganism to the soil solution is the constant threat to these organisms. To survive in the harsh environment and counteract these hazards the fungal cells have to strictly control the composition of the lipids in their cellular membranes. However, in the case of fungal membranes the correlation between their composition and physical properties is not fully understood. In our studies we applied Langmuir monolayers formed by phospholipids typical to fungal membranes and ergosterol as versatile model membranes. These membranes were characterized by the Langmuir technique, Brewster Angle Microscopy and Grazing Incidence X-ray Diffraction, as well as were exposed to the action of phospholipase A2 treated as a model membrane-lytic protein. We started our studies from the equimolar mixture of phosphatidylethanolamine with phosphatidylcholine and doped this matrix with phosphatidylserine (PS) or phosphatidylinositol (PI). It turned out that the membranes with PS were much more condensed at the mesoscale and periodically organized at the molecular level. Starting from these models we derived two families of model fungal membranes adding to these phospholipid matrices ergosterol. It turned out that the level of ergosterol content is of crucial importance for the model membrane structure and its durability. Changing the ergosterol mole ratio from 0 to 0.5 we defined and described in detail four different 2D crystalline phases.

The influence of terpinen-4-ol and eucalyptol - The essential oil components - on fungi and plant sterol monolayers

Antifungal and herbicidal activity of terpenes, being the components of the essential oils, is directly related to the incorporation of these compounds into cellular membranes. Thus, the differences in the lipid composition of various pathogenic membranes may be the factor determining the activity of these molecules. One of the class of lipids, which form the membrane environment are sterols. The aim of this work was to compare the effect of two terpenes: terpinen-4-ol and eucalyptol on the monolayers formed by ergosterol and β - sitosterol, which are the components of fungi and plant membranes, respectively. The modifications in the sterol monolayer properties were investigated in the surface pressure-area measurements and penetration studies as well as in a micrometer scale (Brewster angle microscopy experiments) and in nanoscale (GIXD technique). It was evidenced that although at higher surface pressure the terpene molecules are in part removed from the interface, they are able to substantially modify the condensation, morphology and molecular organization of the sterol film. It was also found that the incorporation of terpenes into sterol films is comparable for both sterols, however, β - sitosterol monolayers properties are affected more strongly than ergosterol films. Finally, the analysis of the results of the studies performed on model membrane systems and the results of antimicrobial studies reported in literature, enabled us to suggest that the activity of terpenes depends on the membrane composition and that the sterol concentration may be important from the point of view of antifungal effect of terpinen-4-ol and eucalyptol.

Phytohormone Behavior in the Model Environment of Plant and Human Lipid Membranes

Interactions between three auxins (indole-3-acetic acid (IAA), 2-naphthoxyacetic acid (BNOA), and 2,4-dichlorophenoxyacetic acid (2,4-D)) and model two-dimensional lipid systems mimicking plant and human cell membranes were investigated in monolayers formed at the air/water solution interface. The analysis was based on the recorded π-A isotherm characteristics complemented with Brewster angle microscopy. The influence of auxins on model membranes was discussed on the basis of condensation changes, modification of mutual lipid-lipid interactions in the mixed films, and morphological alteration of the surface domains on the microscopic scale. It was demonstrated that the lipid composition and mutual proportion of the artificial membranes together with sterol to main the phospholipid ratio play a crucial role in the context of auxin behavior in the membrane-mimicking environment. Apart from specific molecular interactions between studied phytohormones represented by auxins and lipids, the condensation of the investigated monolayers was found to be a regulative factor of model systems' susceptibility toward auxin action. Two effects were recognized: fluidizing of monolayers being in the liquid state (model membranes) and initialization of the three-dimensional structure formation in ordered sterol films at high surface pressure. The influence of auxin molecules on lipid interactions in the monolayer and diminishing of the film condensation was the largest for BNOA, due to the presence of the most bulky nonpolar, aromatic fragment in the molecule. It was also demonstrated that auxins interact with model plant membranes more selectively, stronger, and at markedly lower concentration than with the human membrane models.

Applications of Brewster angle microscopy from biological materials to biological systems

Brewster angle microscopy (BAM) is a powerful technique that allows for real-time visualization of Langmuir monolayers. The lateral organization of these films can be investigated, including phase separation and the formation of domains, which may be of different sizes and shapes depending on the properties of the monolayer. Different molecules or small changes within a molecule such as the molecule's length or presence of a double bond can alter the monolayer's lateral organization that is usually undetected using surface pressure-area isotherms. The effect of such changes can be clearly observed using BAM in real-time, under full hydration, which is an experimental advantage in many cases. While previous BAM reviews focused more on selected compounds or compared the impact of structural variations on the lateral domain formation, this review provided a broader overview of BAM application using biological materials and systems including the visualization of amphiphilic molecules, proteins, drugs, extracts, DNA, and nanoparticles at the air-water interface.

Sterol-Phospholipid Hybrids at the Air/Water Interface: Studies on Properties and Interactions with Parent Lipid Molecules

The two synthetic sterol-phospholipid hybrids DCholPC and PCholPC were investigated in monolayers at the air/water interface. Study was based on π-A isotherm analysis complemented with application of grazing incidence X-ray diffraction. It was found that both compounds are capable of forming stable, highly condensed monolayers of a surface characteristics typical for sterols. GIXD studies show that the crystallographic area for DCholPC monolayer is very similar to that found for cholesterol (37.1 vs 38.0 Å(2)), while for PCholPC (28.8 Å(2)) it is significantly smaller as compared to area for the mixed Chol/DPPC 2/1 monolayer (33.4 Å(2)). In our study the problem of interactions between investigated sterol-phospholipid hybrids and natural membrane lipid components was for the first time analyzed in planar lipid systems. Studies on mixed monolayers showed that both hybrids, similarly to cholesterol, reveal a condensing effect toward DPPC acyl chains; however, DCholPC having two steroid moieties in the molecule was found to be more efficient. On the other hand, the sterol moiety and the hydrocarbon chain of PCholPC molecule are packed in the 2D crystalline phase extremely tight. Our studies showed that the investigated compounds can be applied as biocompatible components of stable liposomes.