Effect of glycerol on the molecular properties of cerebrosides, sulphatides and gangliosides in monolayers

Structure and function of skin barrier lipids: Effects of hydration and natural moisturizers in vitro

Lipid membranes play a crucial role in regulating the body's water balance by adjusting their properties in response to hydration. The intercellular lipid matrix of the stratum corneum (SC), the outermost skin layer, serves as the body's primary defense against environmental factors. Osmolytes, including urocanic acid (UCA) and glycerol, are key components of the natural moisturizing factor that help the SC resist osmotic stress from dry environments. This study examines the effects of UCA and glycerol (each at 5 mol %) on isolated human SC lipids. For this, different techniques were employed, offering complementary information of the system's multiscale characteristics, including humidity-scanning quartz crystal microbalance with dissipation monitoring, infrared spectroscopy, x-ray diffraction, electrical impedance spectroscopy, and studies of water loss and permeability. Our results show that UCA increases water sorption and makes lipid films more liquid-like at high relative humidity, without significantly altering the lipid lamellar structure, chain order, or orthorhombic chain packing. Lipid films containing UCA exhibited higher water loss and significantly higher model drug permeability compared to lipid films without UCA. Further, incorporation of UCA resulted in kinetically faster changes in electrical properties upon contact with aqueous solution compared with control lipids. These observations suggest that UCA reduces lipid cohesion in regions other than the acyl chain-rich leaflets, which may impact SC desquamation. In contrast, glycerol did not influence the hydration or permeability of the SC lipid matrix. However, it increased the proportion of orthorhombic domains at high humidities and slowed the kinetics of the hydration process, as evidenced by slower changes in the dielectric properties of the lipid film. These findings suggest that glycerol enhances lipid cohesion rather than increasing water uptake, which is typically the expected function of humectants. Consequently, UCA and glycerol appear to have distinct roles in maintaining epidermal homeostasis.

The Amphipathic GM1 Molecule Stabilizes Amyloid Aggregates, Preventing their Cytotoxicity

Amyloid aggregates have been demonstrated to exert cytotoxic effects in several diseases. It is widely accepted that the complex and fascinating aggregation pathway involves a series of steps during which many heterogeneous intermediates are generated. This process may be greatly potentiated by the presence of amphipathic components of plasma membrane because they may serve as interaction, condensation, and nucleation points. However, there are few data regarding structural alterations induced by the binding between the amyloid fibrils and membrane components and its direct effects on cell integrity. In this study, we found, by 1-anilinonaphthalene 8-sulfonic acid and transmission electron microscopy/fast Fourier transform, that yeast prion Sup35 oligomers showed higher structural uniformity and altered surface properties when grown in the presence of monosialotetrahexosylganglioside, a component of the cell membrane. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and confocal/sensitized Förster resonance energy transfer analyses revealed that these fibrils showed low cytotoxicity and affinity to plasma membrane. Moreover, time-lapse analysis of Sup35 oligomer fibrillation on cells suggested that the amyloid aggregation process per se exerts cytotoxic effects through the interaction of amyloid intermediates with plasma membrane components. These data provide, to our knowledge, new insights to understand the mechanism of amyloid growth and cytotoxicity in the pathogenesis of amyloid diseases.

Molecular Effects of Glycerol on Lipid Monolayers at the Gas-Liquid Interface: Impact on Microbubble Physical and Mechanical Properties

The production and stability of microbubbles (MBs) is enhanced by increasing the viscosity of both the formation and storage solution, respectively. Glycerol is a good candidate for biomedical applications of MBs, since it is biocompatible, although the exact molecular mechanisms of its action is not fully understood. Here, we investigate the influence glycerol has on lipid-shelled MB properties, using a range of techniques. Population lifetime and single bubble stability were studied using optical microscopy. Bubble stiffness measured by AFM compression is compared with lipid monolayer behavior in a Langmuir-Blodgett trough. We deduce that increasing glycerol concentrations enhances stability of MB populations through a 3-fold mechanism. First, binding of glycerol to lipid headgroups in the interfacial monolayer up to 10% glycerol increases MB stiffness but has limited impact on shell resistance to gas permeation and corresponding MB lifetime. Second, increased solution viscosity above 10% glycerol slows down the kinetics of gas transfer, markedly increasing MB stability. Third, above 10%, glycerol induces water structuring around the lipid monolayer, forming a glassy layer which also increases MB stiffness and resistance to gas loss. At 30% glycerol, the glassy layer is ablated, lowering the MB stiffness, but MB stability is further augmented. Although the molecular interactions of glycerol with the lipid monolayer modulate the MB lipid shell properties, MB lifetime continually increases from 0 to 30% glycerol, indicating that its viscosity is the dominant effect on MB solution stability. This three-fold action and biocompatibility makes glycerol ideal for therapeutic MB formation and storage and gives new insight into the action of glycerol on lipid monolayers at the gas-liquid interface.

Effect of polyols on the DMPC lipid monolayers and bilayers

In this study, the effect of polyols, erythritol, xylitol, mannitol, on a model membrane systems composed of DMPC was investigated using differential scanning calorimetry and Fourier transform infrared spectroscopy. Generally, it is considered that polyols possess strong hydrophilic properties, and either does not interact with the hydrophobic environment at all, or these interactions are very weak. To better understand the mutual interactions between polyols and the lipid system, the Langmuir technique was used to examine the molecular organization of monolayers and to calculate their thickness in the presence of polyols at the subphase. The detailed description of the interactions between polyols and DMPC molecules was complemented by the analysis of the morphology of monolayers with the application of Brewster angle microscopy. From ATR FTIR, the significant spectral shift is observed only for the PO₂^- stretching band, which correlates strongly with the polyol chain-length. The longer the polyol chain, the weaker the observed interactions with lipid molecules. The most important findings, obtained from thickness measurements, reveal that short-chain polyols may prevent the formation of bilayers by the DMPC molecules under high surface pressure. The changes in the organization of DMPC monolayers on the surface, as visualized by Brewster angle microscopy, showed that the domains observed for phospholipid film spread on pure water differ substantially from those containing polyols in the subphase.

The many faces (and phases) of ceramide and sphingomyelin II - binary mixtures

A rather widespread idea on the functional importance of sphingolipids in cell membranes refers to the occurrence of ordered domains enriched in sphingomyelin and ceramide that are largely assumed to exist irrespective of the type of N-acyl chain in the sphingolipid. Ceramides and sphingomyelins are the simplest kind of two-chained sphingolipids and show a variety of species, depending on the fatty acyl chain length, hydroxylation, and unsaturation. Abundant evidences have shown that variations of the N-acyl chain length in ceramides and sphingomyelins markedly affect their phase state, interfacial elasticity, surface topography, electrostatics, and miscibility, and that even the usually conceived "condensed" sphingolipids and many of their mixtures may exhibit liquid-like expanded states. Their lateral miscibility properties are subtlety regulated by those chemical differences. Even between ceramides with different acyl chain length, their partial miscibility is responsible for a rich two-dimensional structural variety that impacts on the membrane properties at the mesoscale level. In this review, we will discuss the miscibility properties of ceramide, sphingomyelin, and glycosphingolipids that differ in their N-acyl or oligosaccharide chains. This work is a second part that accompanies a previous overview of the properties of membranes formed by pure ceramides or sphingomyelins, which is also included in this Special Issue.

Carbohydrate conformation and lipid condensation in monolayers containing glycosphingolipid Gb3: influence of acyl chain structure

Globotriaosylceramide (Gb3), a glycosphingolipid found in the plasma membrane of animal cells, is the endocytic receptor of the bacterial Shiga toxin. Using x-ray reflectivity (XR) and grazing incidence x-ray diffraction (GIXD), lipid monolayers containing Gb3 were investigated at the air-water interface. XR probed Gb3 carbohydrate conformation normal to the interface, whereas GIXD precisely characterized Gb3's influence on acyl chain in-plane packing and area per molecule (APM). Two phospholipids, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), were used to study Gb3 packing in different lipid environments. Furthermore, the impact on monolayer structure of a naturally extracted Gb3 mixture was compared to synthetic Gb3 species with uniquely defined acyl chain structures. XR results showed that lipid environment and Gb3 acyl chain structure impact carbohydrate conformation with greater solvent accessibility observed for smaller phospholipid headgroups and long Gb3 acyl chains. In general, GIXD showed that Gb3 condensed phospholipid packing resulting in smaller APM than predicted by ideal mixing. Gb3's capacity to condense APM was larger for DSPC monolayers and exhibited different dependencies on acyl chain structure depending on the lipid environment. The interplay between Gb3-induced changes in lipid packing and the lipid environment's impact on carbohydrate conformation has broad implications for glycosphingolipid macromolecule recognition and ligand binding.

Glycerol-induced membrane stiffening: the role of viscous fluid adlayers

Lipid interfaces, ranging from cell membranes to thin surfactant layers that stabilize lung alveoli, are integral to living systems. Such interfaces are often subjected to mechanical forces, and because of their membrane-like geometry, they can easily deform by bending into localized folds. In this work, we explore the role of small molecules (i.e., glycerol) on the mechanical stability of model lung surfactant monolayers. We demonstrate that the presence of glycerol increases local monolayer bending stiffness by orders of magnitude. Our x-ray and neutron reflectivity measurements indicate that water is preferentially depleted, or glycerol is preferentially enriched, at the lipid headgroup/solvent interface, and that this glycerol-enriched layer extends O(10Å) beneath the monolayer with an adsorption free energy of -2.5 to -4.6 kJ/mol. The dramatic change in membrane bending stiffness in the presence of the sugar adlayer is understood in terms of two models: 1), lipid antiplasticization by glycerol; and 2), a continuum mechanical model of the viscous adlayer.

Equivalent aqueous phase modulation of domain segregation in myelin monolayers and bilayer vesicles

Purified myelin can be spread as monomolecular films at the air/aqueous interface. These films were visualized by fluorescence and Brewster angle microscopy, showing phase coexistence at low and medium surface pressures (<20-30 mN/m). Beyond this threshold, the film becomes homogeneous or not, depending on the aqueous subphase composition. Pure water as well as sucrose, glycerol, dimethylsulfoxide, and dimethylformamide solutions (20% in water) produced monolayers that become homogeneous at high surface pressures; on the other hand, the presence of salts (NaCl, CaCl(2)) in Ringer's and physiological solution leads to phase domain microheterogeneity over the whole compression isotherm. These results show that surface heterogeneity is favored by the ionic milieu. The modulation of the phase-mixing behavior in monolayers is paralleled by the behavior of multilamellar vesicles as determined by small-angle and wide-angle x-ray scattering. The correspondence of the behavior of monolayers and multilayers is achieved only at high surface pressures near the equilibrium adsorption surface pressure; at lower surface pressures, the correspondence breaks down. The equilibrium surface tension on all subphases corresponds to that of the air/alkane interface (27 mN/m), independently on the surface tension of the clean subphase.

Composition-driven surface domain structuring mediated by sphingolipids and membrane-active proteins. Above the nano- but under the micro-scale: mesoscopic biochemical/structural cross-talk in biomembranes

Biomembranes contain a wide variety of lipids and proteins within an essentially two-dimensional structure. The coexistence of such a large number of molecular species causes local tensions that frequently relax into a phase or compositional immiscibility along the lateral and transverse planes of the interface. As a consequence, a substantial microheterogeneity of the surface topography develops and that depends not only on the lipid-protein composition, but also on the lateral and transverse tensions generated as a consequence of molecular interactions. The presence of proteins, and immiscibility among lipids, constitute major perturbing factors for the membrane sculpturing both in terms of its surface topography and dynamics. In this work, we will summarize some recent evidences for the involvement of membrane-associated, both extrinsic and amphitropic, proteins as well as membrane-active phosphohydrolytic enzymes and sphingolipids in driving lateral segregation of phase domains thus determining long-range surface topography.

Biophysics of sphingolipids II. Glycosphingolipids: An assortment of multiple structural information transducers at the membrane surface

Glycosphingolipids are ubiquitous components of animal cell membranes. They are constituted by the basic structure of ceramide with its hydroxyl group linked to single carbohydrates or oligosaccharide chains of different complexity. The combination of the properties of their hydrocarbon moiety with those derived from the variety and complexity of their hydrophilic polar head groups confers to these lipids an extraordinary capacity for molecular-to-supramolecular transduction across the lateral/transverse planes in biomembranes and beyond. In our opinion, most of the advances made over the last decade on the biophysical behavior of glycosphingolipids can be organized into three related aspects of increasing structural complexity: (1) intrinsic codes: local molecular interactions of glycosphingolipids translated into structural self-organization. (2) Surface topography: projection of molecular shape and miscibility of glycosphingolipids into formation of coexisting membrane domains. (3) Beyond the membrane interface: glycosphingolipid as modulators of structural topology, bilayer recombination and surface biocatalysis.

Favorable and unfavorable lateral interactions of ceramide, neutral glycosphingolipids and gangliosides in mixed monolayers

Interactions among four natural neutral sphingolipids (ceramide, glucosyl-ceramide, lactosyl-ceramide and asialo-GM1) and six gangliosides (GM3, GM2, GM1, GD3, GD1a and GT1b) were studied in binary Langmuir monolayers at the air-buffer interface in terms of their molecular packing, compressibility, dipole potential and mixing behavior. The changes of surface organization can be grouped into three sets: (a) binary films of neutral GSLs, and of the latter with ceramide, exhibit thermodynamically unfavorable mixing with mean molecular area expansions and dipole moment hyperpolarization; (b) mixed monolayers of ceramide, or of GlcCer, and gangliosides occur with thermodynamically favorable interactions leading to mean molecular area condensation and depolarisation; (c) binary mixtures of LacCer or Gg4Cer with gangliosides, and all ganglioside species among them, revealed molecular immiscibility characterized by additive mean molecular area and dipole potential, with composition-independent constant collapse pressure. These results disclose basic tendencies of GSLs to molecularly mix or demix, leading to their surface segregation, which may underlay vectorial separation of their specific biosynthetic pathways.

Ganglioside GD3 and GD3-lactone mediated regulation of the intermolecular organization in mixed monolayers with dipalmitoylphosphatidylcholine

The interactions of dpPC with ganglioside GD3 and two lactones. GD3LacI or GD3LacII, in lipid monolayers occur with reduced, unaltered, or increased molecular area and surface potential/molecule, respectively. dpPC is fully miscible with GD3 and GD3LacI but films with GD3LacII show immiscibility above 75 mol% lactone. At low proportions of GD3 in mixtures with dpPC, GD3 undergoes condensation and depolarization; dpPC is depolarized and its molecular area is reduced above 50 mol% GD3. GD3LacI forms ideally mixed films with dpPC. Mixtures of dpPC with GD3LacII at mole fractions below 0.3 show increased mean molecular area and surface potential/molecule mostly due to lactone alterations. Between mole fractions of 0.3 and 0.75 the surface parameters of dpPC are altered, and above these proportions both lipids are immiscible. Defined variations of molecular properties induced by ganglioside lactonization are selectively transduced to changes of the intermolecular organization and surface electrostatics in mixed interfaces with dpPC. Thus, changes in the relative proportions of a ganglioside and its lactone forms may act as sensitive biotransducers for membrane-mediated cellular functions, without the need for metabolically altering the concentration of gangliosides.

Molecular parameters of semisynthetic derivatives of gangliosides and sphingosine in monolayers at the air-water interface

The molecular parameters (molecular area, surface potential, collapse pressure, dipole moment contributions) of semisynthetic derivatives of ganglioside GM1 and of sphingosine were studied in lipid monolayers at the air-NaCl (145 mM, pH 5.6) interface at 22 +/- 0.3 degrees C. The chemical modifications included alterations of the fatty acyl chain moiety linked to the 2-amino position of the sphingosine (Sph) base. The compounds studied were PKS-1 (N-acetyl Sph), PKS-2 (N-chloroacetyl Sph), PKS-3 (N-dichloroacetyl Sph), PKS-4 (N-trichloroacetyl Sph), Lyso-GM1 (ganglioside GM1 lacking the N-linked fatty acyl chain and the N-acetyl group on the sialic acid), Liga-4 (N-acetyl, lyso[NeuAc]GM1) and Liga-20 (N-dichloroacetyl, lyso[NeuAc]GM1). Relatively small modifications of the chemical structure of sphingolipids introduce dramatic consequences on their surface molecular properties. The absence of the long chain fatty acyl moiety and of the N-acetyl group on the neuraminic acid in Lyso-GM1 leads to a more condensed behavior and to an increase of the collapse pressure compared with GM1. The acetylation or chloroacetylation at the 2-amino position in Liga-4 and Liga-20 induce an expansion of the surface pressure-area isotherm and a decrease of the collapse pressure. The limiting molecular areas of GM1 derivatives, taken at the collapse pressure point, are consistent with the oligosaccharide chain being oriented approximately perpendicularly to the interface. Sphingosine shows a liquid expanded isotherm. The acetylation and successive chlorination of the acetyl residue at the 2-amino position of Sph cause a progressive increase in the limiting molecular area. The variation of the resultant dipole moment under compression, calculated from the surface potential values, suggests the reorientation of selective groups within these molecules that depend on the degree of intermolecular packing. Thermodynamic-geometric correlations on the basis of the molecular parameters of these derivatives suggest that small alterations of the substituent group at the 2-amino position of Sph could have large and amplified consequences on the type, curvature and stability of the possible self-aggregated structure that these lipids may form in aqueous medium.

Modulation by glycosphingolipids of membrane-membrane interactions induced by myelin basic protein and melittin

The effect of glycosphingolipids (GSLs) with oligosaccharide chains of different length and charge on membrane-membrane interactions induced by myelin basic protein (MBP) or melittin (Mel) was comparatively investigated with small unilamellar vesicles. MBP induces a fast vesicle aggregation and close membrane apposition. Merging of lipid bilayers and vesicle fusion induced by MBP are slower and less extensive processes compared to membrane apposition. The changes of membrane permeability concomitant to these phenomena are small. The Trp region of MBP remains in a rather polar environment when interacting with vesicles; its accessibility to NO3- or acrylamide quenching depends on the type of GSLs in the membrane. The Trp region of Mel is inserted more deeply into the lipid bilayer and its accessibility to the aqueous quenchers is less dependent on variations of the oligosaccharide chain of the GSLs. Mel induces a faster and more extensive membrane apposition and bilayer merging than does MBP. Extensive vesicle disruption occurs in the presence of Mel. Negatively charged GSLs facilitate membrane proximity and vesicle aggregation but an increase of the oligosaccharide chain length of either neutral or acidic GSLs decreases the interaction among vesicles that are induced by either protein. This effect is independent of the different mode of insertion of MBP and Mel into the membrane. Our results suggest that the modulation by the oligosaccharide chain on the protein-induced interactions between bilayers containing GSLs is probably exerted beyond the level of local molecular interactions between the basic proteins and the lipids.

Effect of sulfatide and gangliosides on phospholipase C and phospholipase A2 activity. A monolayer study

The effect of sulfatide and gangliosides GM1, GD1a and GT1b on the activity of phospholipase C from Clostridium perfringens on dilauroylphosphatidylcholine and of porcine pancreatic phospholipase A2 on dilauroylphosphatidic acid was studied in lipid monolayers containing different proportions of glycolipids under zero-order kinetics at various constant surface pressures. The presence of sulfatide in the monolayer increases the activity of phospholipase C at high surface pressures. Gangliosides shift the cut-off pressure to lower values and inhibit the action of phospholipase C. In mixed monolayers with dilauroylphosphatidic acid, sulfatide at a molar fraction of 0.5 increases the activity of phospholipase A2 at surface pressures below 18 mN/m and shows an inhibitory effect at higher pressures. Ganglioside GM1 at a molar fraction of 0.25 completely inhibits the enzyme above 20 mN/m and markedly reduces its activity at lower pressures. Gangliosides GD1a and GT1b abolish the enzyme activity at all pressures at molar fractions of 0.25 and 0.15, respectively. The modified velocity of the enzymatic reaction in the presence of glycosphingolipids is not due to an irreversible alteration of the catalytic activity.

Thermodynamic-geometric correlations for the morphology of self-assembled structures of glycosphingolipids and their mixtures with dipalmitoylphosphatidylcholine

The morphology of aqueous dispersions of five neutral glycosphingolipids (GalCer, GlcCer, LacCer, asialo-GM2, asialo-GM1), sulfatide, and five gangliosides (GM3, GM2, GM1, GD1a and GT1b) and their mixtures with dipalmitoylphosphatidylcholine was studied by negative staining electron microscopy. The morphological features are interpreted on the basis of thermodynamic and geometric constraints previously studied in these systems (Maggio, B (1985) Biochim. Biophys. Acta 815, 245-258). The correlation between the theoretical predictions and the experimental findings are in reasonable agreement. Small changes in the molecular parameters of the individual glycosphingolipids or in their proportion in mixtures with dipalmitoylphosphatidylcholine bring about remarkable variations on the type of structure formed, its radius of curvature and thermodynamic stability.

Interaction of 1-anilinonaphthalene 8-sulfonic acid with interfaces containing cerebrosides, sulfatides and gangliosides

The fluorescence lifetime, quantum yield and emission spectra of 1-anilinonaphthalene 8-sulfonic acid (ANS) associated with interfaces of pure dipalmitoylphosphatidylcholine or its mixtures with phosphatidylserine, galactosylceramide, sulfatide or gangliosides GM1 and GD1a were studied at low and high ionic strength. Modification of the molecular organization of the lipid interfaces in the presence of the probe was also studied with mixed lipid monolayers. ANS has little affect on the intermolecular packing of the lipids but influences their surface potential, consistent with a location of ANS in the polar head group region of the interface. ANS senses a more polar microenvironment when associated with interfaces containing anionic glycosphingolipids at low ionic strength but, except for interfaces containing phosphatidylserine, it detects approximately the same polarity for neutral or anionic interfaces in 0.25 M NaCl.