Phase behavior of aqueous dispersions of mixtures of N-palmitoyl ceramide and cholesterol: a lipid system with ceramide-cholesterol crystalline lamellar phases

Inimitable Impacts of Ceramides on Lipid Rafts Formed in Artificial and Natural Cell Membranes

Ceramide is the simplest precursor of sphingolipids and is involved in a variety of biological functions ranging from apoptosis to the immune responses. Although ceramide is a minor constituent of plasma membranes, it drastically increases upon cellular stimulation. However, the mechanistic link between ceramide generation and signal transduction remains unknown. To address this issue, the effect of ceramide on phospholipid membranes has been examined in numerous studies. One of the most remarkable findings of these studies is that ceramide induces the coalescence of membrane domains termed lipid rafts. Thus, it has been hypothesised that ceramide exerts its biological activity through the structural alteration of lipid rafts. In the present article, we first discuss the characteristic hydrogen bond functionality of ceramides. Then, we showed the impact of ceramide on the structures of artificial and cell membranes, including the coalescence of the pre-existing lipid raft into a large patch called a signal platform. Moreover, we proposed a possible structure of the signal platform, in which sphingomyelin/cholesterol-rich and sphingomyelin/ceramide-rich domains coexist. This structure is considered to be beneficial because membrane proteins and their inhibitors are separately compartmentalised in those domains. Considering the fact that ceramide/cholesterol content regulates the miscibility of those two domains in model membranes, the association and dissociation of membrane proteins and their inhibitors might be controlled by the contents of ceramide and cholesterol in the signal platform.

Phase Separation in Lipid Lamellae Result from Ceramide Conformations and Lateral Packing Structure

Intercellular lipids in the stratum corneum protect the living body from invasion by allergens and pathogens, and also suppresses water evaporation within the body. It is important to understand how differences in the microstructure of intercellular lipids arise. This microstructure is affected by lipid composition. Studies using intercellular lipid models have reported the formation of two phases with different short lamellar periodicities. However, the details of the packing structure characteristics of the two phases observed in these intercellular lipid models are unclear. Our previous report revealed that different short periodicity phases coexist in the N-(α-hydroxyoctadecanoyl)-dihydrosphingosine (CER[ADS]), cholesterol (CHOL), and palmitic acid (PA) complex model. In this study, the characteristics of the packing structure of two phases with different short lamellar periodicities, which were observed in the intercellular lipid model (CER[ADS]/CHOL/PA) that we used previously, were adjusted for models with different lipid compositions. The characteristics of the packed and lamellar structures have been determined by temperature-scanning small-angle X-ray scattering and wide-angle X-ray diffraction measurements simultaneously. These differences in lamellar structure were thought to be caused by differences in ceramides (CER) conformation between the hairpin and the V-shape type. The lamellar structure of the V-shaped CER conformation has a low orthorhombic ratio. The above results suggest that an increase in the ratio of CER with the V-shaped structure causes the lamellar structure to have low orthorhombic ratio, thereby contributing to a decrease in the bilayer's barrier function.

Determination of the influence of C24 D/(2R)- and L/(2S)-isomers of the CER[AP] on the lamellar structure of stratum corneum model systems using neutron diffraction

This study was able to investigate the different influence of the d- and l-ceramide [AP] on the lamellar as well as molecular nanostructure of stratum corneum simulating lipid model mixtures. In this case, neutron diffraction together with specifically deuterated ceramide was used as an effective tool to investigate the lamellar and the molecular nanostructure of the mixtures. It could clearly be demonstrated, that both isomers show distinctly different characteristics, even though the variation between both is only a single differently arranged OH-group. The l-ceramide [AP] promotes a crystalline like phase behaviour even if mixed with ceramide [NP], cholesterol and free fatty acids. The d-ceramide [AP] only shows crystalline-like features if mixed only with cholesterol and free fatty acids but adopts a native-like behaviour if additionally mixed with ceramide [NP]. It furthermore demonstrates that the l-ceramide [AP] should not be used for any applications concerning ceramide substitution. It could however possibly serve its own purpose, if this crystalline like behaviour has some kind of positive influence on the SC or can be utilized for any practical applications. The results obtained in this study demonstrate that the diastereomers of ceramide [AP] are an attractive target for further research because their influence on the lamellar as well as the nanostructure is exceptionally strong. Additionally, the results furthermore show a very strong influence on hydration of the model membrane. With these properties, the d-ceramide [AP] could be effectively used to simulate native like behaviour even in very simple mixtures and could also have a strong impact on the native stratum corneum as well as high relevance for dermal ceramide substitution. The unnatural l-ceramide [AP] on the other hand should be investigated further, to assess its applicability.

Spontaneous formation of two-dimensional and three-dimensional cholesterol crystals in single hydrated lipid bilayers

Grazing incidence x-ray diffraction measurements were performed on single hydrated bilayers and monolayers of Ceramide/Cholesterol/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocyholine at varying concentrations. There are substantial differences in the phase and structure behavior of the crystalline domains formed within the bilayers relative to the corresponding monolayers, due to interactions between the opposing lipid leaflets. Depending on the lipid composition, these interactions lead to phase separation and formation of cholesterol crystals. The cholesterol and ceramide/cholesterol mixed phases were further characterized at 37°C by immunolabeling with specific antibodies recognizing ordered molecular arrays of cholesterol. Previous studies have shown that cholesterol may nucleate in artificial membranes to form thick two-dimensional bilayer crystals. The study herein demonstrates further growth of cholesterol into three-dimensional crystals. We believe that these results may provide further insight into the formation of cholesterol crystals in early stages of atherosclerosis inflammation.

Crystalline lipid domains: characterization by X-ray diffraction and their relation to biology

Biological membranes comprise thousands of different lipids, differing in their alkyl chains, headgroups, and degree of saturation. It is estimated that 5% of the genes in the human genome are responsible for regulating the lipid composition of cell membranes. Conceivably, the functional explanation for this diversity is found, at least in part, in the propensity of lipids to segregate into distinct domains, which are important for cell function. X-ray diffraction has been used increasingly to characterize the packing and phase behavior of lipids in membranes. Crystalline domains have been studied in synthetic membranes using wide- and small-angle X-ray scattering, and grazing incidence X-ray diffraction. Herein we summarize recent results obtained using the various X-ray methods, discuss the correlation between crystalline domains and liquid ordered domains studied with other techniques, and the relevance of crystalline domains to functional lipid domains in biological membranes.

Phase-dependent lateral diffusion of α-tocopherol in DPPC liposomes monitored by fluorescence quenching

The temperature-dependent fluorescence quenching of an amphiphilic palmitoyl derivative of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-L) by α-tocopherol (α-Toc) has been determined in liposomes composed of a saturated lipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The mutual lateral diffusion coefficients (D(L)) were extracted according to a laterally diffusion-controlled dynamic quenching model. Three distinct temperature regimes were identified: one between 65 and 39 °C, where the lateral diffusion coefficients were in the range of 10(-7) cm(2) s(-1) and the lifetime of the probe was monoexponential in the absence of α-Toc, a second one between 39 and 30 °C, where the lateral diffusion coefficients were in the range of 10(-8) cm(2) s(-1) and the lifetime of the probe was biexponential in the absence of α-Toc, and a third one below 30 °C, in which no diffusion was detectable, suggesting D(L) < 10(-9) cm(2)s (-1). These temperature domains were assigned, supported by differential scanning calorimetry (DSC) measurements, to the liquid-crystalline, ripple, and solid-gel phases of DPPC liposomes in the presence of the two additives. The absolute values of the individual lateral diffusion coefficients (taken as (1)/(2) of the D(L) values) of the Fluorazophore-L/α-Toc (ca. 2.5 × 10(-7) cm(2) s(-1) at 52 °C) couple demonstrates that α-Toc does not diffuse at an unexpectedly high rate in comparison to the self-diffusion of DPPC (1.5 × 10(-7) cm(2) s(-1) at 52 °C). However, diffusion in DPPC liposomes is distinctly slower than that in POPC ones (e.g., D(L) = 4.9 × 10(-7) cm(2) s(-1) versus 6.4 × 10(-7) cm(2) s(-1) at 50 °C), with an activation energy of 49 ± 5 kJ mol(-1) (value for POPC: 47 ± 5 kJ mol(-1)), in the temperature range of the liquid-crystalline phase. Diffusion in the ripple phase, that is, below the main phase transition temperature, was found to be non-negligible, with an apparent activation energy of 175 ± 50 kJ mol(-1).