A Model for the Interaction of 6-Lauroyl-2-(N,N-dimethylamino)naphthalene with Lipid Environments: Implications for Spectral Properties

Cholesterol as a Subsidiary Component of Sorbitan Surfactant-Based Aggregates: A Study of Formation, Hydrophobicity, and Estimation of Localization of Embedded Molecules

Aggregates of amphiphilic molecules can be used as drug carriers, for which the properties can be modified by mixing with other molecules such as cholesterol. It is important to understand the effects of such additives on the properties because they directly define the material functions. In this work, we investigated the effect of cholesterol on the formation and hydrophobicity of aggregates of sorbitan surfactants. As cholesterol changed its formation from micelles to vesicles, an increase in hydrophobicity was seen, particularly in the middle regions compared with the shallow and deep regions. We show that this gradual hydrophobicity is related to the localization of the embedded molecules. 4-Hydroxy-TEMPO and 4-carboxy-TEMPO were preferentially localized in the shallow region of the aggregates, whereas 4-PhCO₂-TEMPO was preferentially localized in the deep region of the vesicle. The localization of molecules depends on their chemical structure. However, the localization of 4-PhCO₂-TEMPO in micelles was not observed, despite the similar hydrophobicity in the hydrophobic region within the aggregates. The localization of embedded molecules was related to other properties, such as molecular mobility.

Laurdan in live cell imaging: Effect of acquisition settings, cell culture conditions and data analysis on generalized polarization measurements

Cell function is highly dependent on membrane structure, organization, and fluidity. Therefore, methods to probe the biophysical properties of biological membranes are required. Determination of generalized polarization (GP) values using Laurdan in fluorescence microscopy studies is one of the most widely-used methods to investigate changes in membrane fluidity in vitro and in vivo. In the last couple of decades, there has been a major increase in the number of studies using Laurdan GP, where several different methodological approaches are used. Such differences interfere with data interpretation inasmuch as it is difficult to validate if Laurdan GP variations actually reflect changes in membrane organization or arise from biased experimental approaches. To address this, we evaluated the influence of different methodological details of experimental data acquisition and analysis on Laurdan GP. Our results showed that absolute GP values are highly dependent on several of the parameters analyzed, showing that incorrect data can result from technical and methodological inconsistencies. Considering these differences, we further analyzed the impact of cell variability on GP determination, focusing on basic cell culture conditions, such as cell confluency, number of passages and media composition. Our results show that GP values can report alterations in the biophysical properties of cell membranes caused by cellular adaptation to the culture conditions. In summary, this study provides thorough analysis of the factors that can lead to Laurdan GP variability and suggests approaches to improve data quality, which would generate more precise interpretation and comparison within individual studies and among the literature on Laurdan GP.

Cholesterol-containing lipid nanodiscs promote an α-synuclein binding mode that accelerates oligomerization

Dysregulation of the biosynthesis of cholesterol and other lipids has been implicated in many neurological diseases, including Parkinson's disease. Misfolding of α-synuclein (α-Syn), the main actor in Parkinson's disease, is associated with changes in a lipid environment. However, the exact molecular mechanisms underlying cholesterol effect on α-Syn binding to lipids as well as α-Syn oligomerization and fibrillation remain elusive, as does the relative importance of cholesterol compared to other factors. We probed the interactions and fibrillation behaviour of α-Syn using styrene-maleic acid nanodiscs, containing zwitterionic and anionic lipid model systems with and without cholesterol. Surface plasmon resonance and thioflavin T fluorescence assays were employed to monitor α-Syn binding, as well as fibrillation in the absence and presence of membrane models. 1 H-15 N-correlated NMR was used to monitor the fold of α-Syn in response to nanodisc binding, determining individual residue apparent affinities for the nanodisc-contained bilayers. The addition of cholesterol inhibited α-Syn interaction with lipid bilayers and, however, significantly promoted α-Syn fibrillation, with a more than a 20-fold reduction of lag times before fibrillation onset. When α-Syn bilayer interactions were analysed at an individual residue level by solution-state NMR, we observed two different effects of cholesterol. In nanodiscs made of DOPC, the addition of cholesterol modulated the NAC part of α-Syn, leading to stronger interaction of this region with the lipid bilayer. In contrast, in the nanodiscs comprising DOPC, DOPE and DOPG, the NAC part was mostly unaffected by the presence of cholesterol, while the binding of the N and the C termini was both inhibited.

Membrane properties of amacrocyclic tetraether bisphosphatidylcholine lipid: Effect of a single membrane-spanning polymethylene cross-linkage between two head groups of ditetradecylphosphatidylcholine membrane

The plasma membranes of archaea are abundant in macrocyclic tetraether lipids that contain a single or double long transmembrane hydrocarbon chains connecting the two glycerol backbones at both ends. In this study, a novel amacrocyclic bisphosphatidylcholine lipid bearing a single membrane-spanning octacosamethylene chain, 1,1'-O-octacosamethylene-2,2'-di-O-tetradecyl-bis-(sn-glycero)-3,3'-diphosphocholine (AC-(di-O-C14PC)₂), was synthesized to elucidate effects of the interlayer cross-linkage on membrane properties based on comparison with its corresponding diether phosphatidylcholine, 1,2-di-O-tetradecyl-sn-glycero-3-phosphocholine (DTPC), that forms bilayer membrane. Several physicochemical techniques demonstrated that while AC-(di-O-C14PC)₂ monolayer, which adopts a particularly high-ordered structure in the gel phase, shows remarkably high thermotropic transition temperature compared to DTPC bilayer, the fluidity of both phospholipids above the transition temperature is comparable. Nonetheless, the fluorescent dye leakage from inside the AC-(di-O-C14PC)₂ vesicles in the fluid phase is highly suppressed. The origin of the membrane properties characteristic of AC-(di-O-C14PC)₂ monolayer is discussed in terms of the single long transmembrane hydrophobic linkage and the diffusional motion of the lipid molecules.

Changes Caused by Liposomes to the Belousov-Zhabotinsky Reaction

The Belousov-Zhabotinsky (BZ) reaction has been applied to give autonomous dynamic behaviors to artificial systems. This reaction is conducted in an aqueous system, but it produces some hydrophobic intermediates, such as bromine. On the basis of previous works about reactions in the lipid bilayer, we investigated how liposome membranes (water-oil interface) affect the BZ reaction. Herein diacylglycerophosphocholine (PC) molecules with a variety of hydrocarbon tails were selected as components of liposomes, and the BZ reaction in the presence of the liposomes was characterized. As a result, membrane fluidity was the main characteristic leading to changes in the reaction behavior. The decrease of the frequency of oscillations was relevant to membrane fluidity, suggesting the interaction of bromine species in the hydrophobic site of the liposomes. In addition, the heterogeneous membrane (s_o+l_d) of DMPC showed a fast decrease in the amplitude of oscillations. Conclusively, characteristics of the hydrophobic environment play a role in the reaction.

Spectral Imaging for Microbubble Characterization

Microbubbles stabilized by an outer lipid shell have been studied extensively for both diagnostic and therapeutic applications. The shell composition can significantly influence microbubble behavior, but performing quantitative measurements of shell properties is challenging. The aim of this study is to investigate the use of spectral imaging to characterize the surface properties of a range of microbubble formulations representing both commercial and research agents. A lipophilic dye, C-laurdan, whose fluorescence emission varies according to the properties of the local environment, was used to compare the degree and uniformity of the lipid order in the microbubble shell, and these measurements were compared with the acoustic response and stability of the different formulations. Spectral imaging was found to be suitable for performing rapid and hence relatively high throughput measurements of microbubble surface properties. Interestingly, despite significant differences in lipid molecule size and charge, all of the different formulations exhibited highly ordered lipid shells. Measurements of liposomes with the same composition and the debris generated by destroying lipid microbubbles with ultrasound (US) showed that these exhibited a lower and more varied lipid order than intact microbubbles. This suggests that the high lipid order of microbubbles is due primarily to compression of the shell as a result of surface tension and is only minimally affected by composition. This also explains the similarity in acoustic response observed between the formulations, because microbubble dynamics are determined by the diameter and shell viscoelastic properties that are themselves a function of the lipid order. Within each population, there was considerable variability in the lipid order and response between individual microbubbles, suggesting the need for improved manufacturing techniques. In addition, the difference in the lipid order between the shell and lipid debris may be important for therapeutic applications in which shedding of the shell material is exploited, for example, drug delivery.

The Combination of Whole Cell Lipidomics Analysis and Single Cell Confocal Imaging of Fluidity and Micropolarity Provides Insight into Stress-Induced Lipid Turnover in Subcellular Organelles of Pancreatic Beta Cells

Modern omics techniques reveal molecular structures and cellular networks of tissues and cells in unprecedented detail. Recent advances in single cell analysis have further revolutionized all disciplines in cellular and molecular biology. These methods have also been employed in current investigations on the structure and function of insulin secreting beta cells under normal and pathological conditions that lead to an impaired glucose tolerance and type 2 diabetes. Proteomic and transcriptomic analyses have pointed to significant alterations in protein expression and function in beta cells exposed to diabetes like conditions (e.g., high glucose and/or saturated fatty acids levels). These nutritional overload stressful conditions are often defined as glucolipotoxic due to the progressive damage they cause to the cells. Our recent studies on the rat insulinoma-derived INS-1E beta cell line point to differential effects of such conditions in the phospholipid bilayers in beta cells. This review focuses on confocal microscopy-based detection of these profound alterations in the plasma membrane and membranes of insulin granules and lipid droplets in single beta cells under such nutritional load conditions.

Effect of dehydrocholic acid conjugated with a hydrocarbon on a lipid bilayer composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine

The effects of bile acids, dehydrocholic acid (DHA) and DHA conjugated with a hydrocarbon (6-aminohexanoate; 6A-DHA) were evaluated using a lipid bilayer composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). DOPC formed a homogenous thin membrane in presence or absence of the DHA, while 20 mol% 6A-DHA induced phase separation on the DOPC thin membrane. It was observed formation of a stomatocyte-like liposomes when these membranes were suspended in a basic solvent. Generally, liposome formation can be prevented by some bile acids. It was found that DHA and 6A-DHA did not disrupt liposome formation, while DHA and 6A-DHA perturbed the liposomal membrane, resulting in increased local-fluidity due to the bent structure of DHA and 6A-DHA. DHA and 6A-DHA showed completely different effects on the hydrophobicity of the boundary surface of DOPC liposome membranes. The steroidal backbone of DHA was found to prevent the insertion of water molecules into the liposomal membrane, whereas 6A-DHA did not show the same behavior which was attributed to its conjugated hydrocarbon.

Characterization of M-laurdan, a versatile probe to explore order in lipid membranes

Microdomains corresponding to localized partition of lipids between ordered and less ordered environments are the subject of intensive investigations, because of their putative participation in modulating cellular responses. One popular approach in the field consists in labelling membranes with solvatochromic fluorescent probes such as laurdan and C-laurdan. In this report, we describe a high-yield procedure for the synthesis of laurdan, C-laurdan and two new fluorophores, called MoC-laurdan and M-laurdan, as well as their extensive photophysical characterization. We find that the latter probe, M-laurdan, is particularly suited to discriminate lipid phases independently of the chemical nature of the lipids, as measured by both fluorescence Generalized Polarization (GP) and anisotropy in large unilamellar vesicles made of various lipid compositions. In addition, staining of live cells with M-laurdan shows a good stability over time without any apparent toxicity, as well as a wider distribution in the various cell compartments than the other probes.

Nanotomography of Cell Surfaces with Evanescent Fields

The technique of variable-angle total internal reflection fluorescence microscopy (TIRFM) and its application to nanotomography of cell surfaces are described. Present applications include (1) 3D imaging of chromosomes in their metaphase to demonstrate axial resolution in the nanometre range, (2) measurements of cell-substrate topology, which upon cholesterol depletion shows some loosening of cell-substrate contacts, and (3) measurements of cell topology upon photodynamic therapy (PDT), which demonstrate cell swelling and maintenance of focal contacts. The potential of the method for in vitro diagnostics, but also some requirements and limitations are discussed.

Visualizing lipid structure and raft domains in living cells with two-photon microscopy

The lateral organization of cellular membranes is formed by the clustering of specific lipids, such as cholesterol and sphingolipids, into highly condensed domains (termed lipid rafts). Hence such domains are distinct from the remaining membrane by their lipid structure (liquid-ordered vs. -disordered domains). Here, we directly visualize membrane lipid structure of living cells by using two-photon microscopy. In macrophages, liquid-ordered domains are particularly enriched on membrane protrusions (filopodia), adhesion points and cell-cell contacts and cover 10-15% of the cell surface at 37 degrees C. By deconvoluting the images, we demonstrate the existence of phase separation in vivo. We compare the properties of microscopically visible domains (<1 microm2), with those of isolated detergent-resistant membranes and provide evidence that membrane coverage by lipid rafts and their fluidity are principally governed by cholesterol content, thereby providing strong support for the lipid raft hypothesis.